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Stock groundwater well, Lost Creek Basin, Weld and Adams County, Colorado, August 2009. Photo credit: Colorado Geological Survey

ON-010 Colorado Groundwater Atlas

[The Atlas is, of course, live and online—you’re reading it here and now—but it is also under review until May 01, 2020. We are looking for your critical appraisals, leads on new data, and other input. Use this form.]

Groundwater continues to gain recognition as a critical natural resource issue in Colorado. The state, through the Colorado Water Conservation Board (CWCB), is developing and implementing a general water plan with its supporting Analysis and Technical Update. At the same time the Colorado Department of Public Health and Environment (CDPHE) is developing a statewide groundwater protection plan that will identify aquifer vulnerability to pollution. The CGS hydrogeology team was brought in to augment the science and develop a comprehensive online portal to the geoscience behind these efforts.

The CDPHE determined that an ever-expanding digital environment makes it necessary that information and data are available to a wide public audience. This effort compiles existing material in an easily-accessible digital format from many sources and builds on our award-winning 2003 Ground Water Atlas of Colorado. It follows much the original structure, adding new data and revising information based on expanding scientific knowledge of Colorado’s complex geologic setting. Not only has the technology for conveying information changed dramatically since 2003, but scientific knowledge—both geologic and hydrologic—has expanded significantly.

Citation: Barkmann, Peter E., Lauren D. Broes, Martin J. Palkovic, John C. Hopkins, Kenneth Swift Bird, Lesley A. Sebol, and F. Scot Fitzgerald. “ON-010 Colorado Groundwater Atlas.” Geohydrology. ON-010 Colorado Groundwater Atlas, 08 January 2020. https://coloradogeologicalsurvey.org/water/colorado-groundwater-atlas/


Based on: Topper, R., K. L. Spray, W. H. Bellis, J. L. Hamilton, and P. E. Barkmann. SP-53 Ground Water Atlas of Colorado. Special Publications, SP-53. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 2003.

Supersedes the later online version: Topper, R., K. L. Spray, W. H. Bellis, J. L. Hamilton, and P. E. Barkmann. ON-06-01 Ground Water Atlas of Colorado. Online Publications, ON-06-01. Denver, CO: Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources, 2006.

Please refer to the bibliography and to relevant GIS meta-data for specific information sources.

The structure of our new online Groundwater Atlas is as follows: here on the main CGS website we include a Table of Contents that links all the various sections and platforms. A master GIS map frames the various basins and aquifers along with their geologic settings. Also included in the GIS space is the full set of our extensive GIS data that inform groundwater issues.

All content in this website will be subject to ongoing updates as new information and data become available. Groundwater in Colorado is an ever-changing discipline as many agencies, public entities, and academic institutions are constantly gathering new information to better understand and manage this critical resource. As a web-based information source, this Atlas provides a flexible platform where data can be added as it becomes available. The CGS plans to identify, process, and present pertinent additional data so that the users have access to the most useful information on a timely basis.

We consider you, the users, to be our best source for feedback and potential information that may not be readily available through normal literature search platforms. If you have access to, or know about, new data on any of the aquifers, basins, or regions that you feel would enhance the content of this atlas, please contact us.

The following sections introduce the general state-wide situation regarding groundwater and answer the basic questions: What is groundwater? Why is it important to the citizens of the state? How does Colorado water law affects the ‘ownership’ and use of groundwater? What is the state doing to administer and manage the availability and quality of the groundwater supply?

Afternoon thunderstorm, Great Sand Dunes National Park, Colorado, May ©2011 hopkins/neoscenes.
Afternoon thunderstorm, Great Sand Dunes National Park, Colorado, May ©2011 hopkins/neoscenes.

(01.01) Sustainable use of water is a priority for Colorado today. The state’s rapid and continued growth and its popularity as a tourist destination has focused attention on the natural resources required to sustain its wildlife, communities, businesses, and recreational sites. Demands on Colorado’s water supply are dominated by agricultural, domestic, and industrial use but also increasingly include requirements for recreation and ecosystems. With its ready access and storage capability, both historic and current, surface water provides the bulk of the water supply across the state. Over-appropriation of this resource, combined with increased demand and a lack of suitable or acceptable future storage reservoir sites, has directed attention to groundwater.

Water is essential for life on this planet. It is all around us, in vapor form in the atmosphere, in solid form as ice and snow, and in its liquid form as precipitation, streams and rivers, ponds lakes and oceans. Some water inevitably seeps into the ground where it becomes groundwater. Underground, it fills and moves through open spaces, or pores, between the grains of soil or rock. Grains of soil and rock may be loose, or unconsolidated in geologic materials near the ground surface. Alternatively, the grains may be tightly compacted or cemented together by other minerals, forming solid bedrock. Many bedrock formations have little or no pore space between grains and must have fractures, or open cavities to hold and transmit water.

An aquifer is a geologic material that holds groundwater that can be released to springs and wells. Aquifers in Colorado can be classified by whether they are made up of loose unconsolidated materials, consolidated bedrock, or fractured crystalline bedrock. They are found throughout the state in its many river basins and geologic settings with the type depending on its setting and geologic history.

The use of groundwater in Colorado for public supply and domestic and industrial purposes began before 1900. Groundwater resources currently supply approximately 18 percent of the state’s needs and groundwater development is continuing at a fast pace. Historic patterns of groundwater development reflect the state’s climate and population distribution patterns. Future patterns of groundwater use will undoubtedly have to respond to evolving climate and population changes.

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Groundwater seep from Weber sandstone, Jones Canyon, Dinosaur National Monument, Colorado, May ©2019 hopkins/neoscenes
Groundwater seep from Weber sandstone, Jones Canyon, Dinosaur National Monument, Colorado, May ©2019 hopkins/neoscenes

(02.01) Groundwater is water present beneath the surface of the earth where it fills open pore spaces between soil particles, rock grains in sedimentary rocks or open fractures in crystalline rocks (Figure 02-01). Because groundwater is hidden from view, many people think of its occurrence in the form of underground lakes, streams, and veins. Only in very special geologic settings, such as cavernous limestone and lava tubes in certain types of volcanic rocks, does groundwater occur as viewed in this common perception.

The ultimate source of groundwater is precipitation in the form of rain, snow, sleet, or hail. Precipitation that does not immediately evaporate or flow away as surface water infiltrates into the ground. Rate and efficiency of water infiltration through soil and rock, commonly referred to as percolation, depends on many factors, including land use, soil physical properties and moisture content. It also depends on the intensity and duration of precipitation. Precipitation that falls as snow or hail may melt slowly, allowing more to infiltrate than if it fell as intense rain. When rainfall is intense, exceeding the rate of infiltration, water accumulates on the surface and runs off downhill as overland flow.

As gravity pulls the water through the pore spaces or fractures in the soils, sediments, or rocks some may be taken up by plant roots. What is left continues to infiltrate until it reaches a depth where all of the available pore space is completely filled with water. At this depth (Figure 02-02) the material is considered saturated, and if it is permeable enough, it may be considered an aquifer. Water that infiltrates to the aquifer is considered recharge. An aquifer is a groundwater reservoir composed of geologic materials that are saturated with water and sufficiently permeable to yield water in a usable quantity to wells and springs. Sand and gravel deposits, sandstone, limestone, and fractured, crystalline rocks are examples of geologic formations that form aquifers.

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Water is a limited, but essential, resource. Accordingly, Colorado has developed over time a comprehensive system for allocating and administering its use. Surface water allocations are driven by the doctrine of prior appropriation, or “first in time, first in right”. Generally, tributary groundwater is considered directly tied to the surface water system and rules and regulations follow the same doctrine. The State Engineer for the State of Colorado (SEO) is authorized by statute to administer the waters of the state. This authority includes responsibility for the administration and distribution of the state’s waters, the promulgation of rules and regulations, the collection of data on water supply, the compliance of interstate compact commitments, and the enforcement of the laws imposed by statute and by order of the courts. To accomplish these responsibilities, the state is divided into seven water divisions with boundaries that correspond to the major river watershed boundaries (Figure 03-01). Each water division is staffed with its own division engineers and water commissioners. The State Engineer’s Office in Denver oversees the seven water divisions.

Groundwater use in Colorado is increasing, both in absolute withdrawals and in relation to surface-water uses. In spite of its significance as a resource, the legal rules governing the allocation, use, and protection of groundwater are of relatively recent origin and continue to evolve as our understanding and knowledge of the dynamics of this resource improve. The primary objectives of groundwater law are to:

  • Regulate the rate of groundwater depletion in aquifers for which recharge is insufficient to meet demands;
  • Regulate the rate of depletion in aquifers that contribute to the supply and flow of adjacent surface water;
  • Protect aquifers from pollution; and
  • Regulate the extraction or injection of groundwater in areas prone to geologic instability.

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(04.01) Managed Aquifer Recharge (MAR) is the process of enhancing recharge of groundwater to an aquifer. It may be implemented for a variety of objectives and through many methods depending on objectives and aquifer characteristics. Artificial recharge is a term commonly used for the practice in the past, but MAR is gaining preference because it implies a broader spectrum of applications. Enhanced recharge can potentially be applied to any type of aquifer with the main limitations being (1) obtaining suitable source water, (2) cost of implementation, and (3) regulatory, legal, or other institutional considerations. Development of MAR has been expanding globally over the past 20 years and MAR is gaining recognition in Colorado as an option for water management.

Objectives

The objectives (Figure 04-01) of most MAR applications fall into one, or a combination, of the following categories:

  • Manage water supply, including short-term water supply regulation, seasonal storage, long-term storage (drought mitigation), emergency supply, and conjunctive use;
  • Meet legal obligations, such as providing augmentation water, supplementing downstream water rights, or facilitating compliance with interstate agreements;
  • Manage water quality through the improvement of surface water or groundwater quality or treated wastewater disposal;
  • Restore aquifers by restoring groundwater levels, limiting aquifer compaction and surface subsidence resulting from excessive groundwater withdrawals, or mitigating saltwater intrusion;
  • Protect the environment by maintaining wetland hydrology, enhancing endangered species habitat, or controlling the migration of groundwater contamination.

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(05.01) Colorado’s Water Plan is focused on achieving the right balance of water resource management strategies. It recognizes that water is important for all sectors and regions in Colorado, and greatly affects Coloradans’ livelihoods. It stresses collaboration between stakeholders with a variety of interests: agriculture, municipal, industrial, recreational, and environmental. It also strives for collaboration between different regions and river basins.

Colorado’s Water Plan takes into account the state’s history, legal system, policy structure (which includes local, state, and federal laws, institutions, and stakeholders), and institutional arrangements that influence decisions about available water resources. Colorado’s Water Plan affirms the private ownership of water rights under the state’s prior appropriation system. Furthermore, this plan supports the authorities and responsibilities of local governments and water providers established by state law. It recognizes the limited statutory role of state agencies in decisions regarding the allocation and reallocation of water to various beneficial uses, and the overlay of federal regulatory and permitting processes that pervade water resources management decisions in Colorado. Thus, the plan advocates for cooperation among parties so that no one governmental agency, water provider, or private party is compelled to go it alone and make unilateral decisions. This plan is a framework to guide future decision making and to address water challenges with a collaborative, balanced, and solutions-oriented approach. The State recognizes that Coloradans have accomplished innovative and creative work, and acknowledges that there is still much work to do.

Colorado strives for vibrant and sustainable cities, viable and productive agriculture, a robust recreation and tourism industry, and a thriving natural environment. The goals of the Colorado Water Plan are to meet the water supply gap, defend Colorado’s compact entitlements, improve regulatory processes, and explore financial incentives—all while honoring Colorado’s water values and ensuring that the state’s most valuable resource is protected and available for generations to come.

Legislation in 2005, known as the Colorado Water for the 21st Century Act, created a grassroots process for multiple stakeholders to determine water needs and propose solutions to meet those needs. This process included establishment of nine regional roundtables based on river basins (Figure 05-01) and a statewide Interbasin Compact Committee to facilitate cooperation between individual basins. Implementation of the Colorado Water Plan will originate mainly through the basin roundtables through their individual basin implementation plans (BIPs).

Technical resources essential to implementing the Colorado Water Plan are found in the Analysis and Technical Update. This process provides data and analytical tools for determining water needs, water supplies, and gaps to be used by basin roundtables in their planning activities. The process incorporates water supply and demand conditions considering potential effects of population growth, climate change, and other factors. Aquifer data available through this atlas can be a powerful tool for analyzing groundwater supply and underground water storage options.

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(06.01) A comprehensive approach to protecting surface and groundwater quality was enacted in the 1970s with the passage of the Federal Clean Water Act (CWA) and Safe Drinking Water Act (SDWA). Many states, including Colorado, manage and enforce these laws at the state level. The Colorado Department of Public Health and Environment is the lead agency responsible for the administration, management, and enforcement of water quality regulations. The focus of the CWA was the regulation of “end of the pipe” discharges to address pollution in streams, rivers, and lakes. The SDWA is administered by the EPA and provides national standards to protect the public from harmful effects of some contaminants in our drinking water.

Groundwater quality in Colorado is addressed in the Colorado Water Quality Control Act. Water-quality data from public water systems supplied by groundwater indicate the most common contaminants in Colorado are nitrate, fluoride, selenium, iron, manganese, alpha radiation (radon), and uranium. Sources may be natural or may result from either planned or unplanned discharges and can be categorized as waste- and non-waste derived.

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(07.01) Colorado has many thermal springs and wells that have water temperatures measurably above the land surface temperatures. There are many different definitions for the temperature of a hot or a warm spring, but, in practice, application of the definitions depends on the surface temperature and use of the water. The generic term thermal water is therefore used here.

Temperature increases with increasing depth in the earth. Therefore, in general, the deeper the aquifer, the hotter the groundwater in the aquifer. In some areas more heat is escaping from the earth: areas with active volcanoes are an example. In those areas temperature increase with depth more rapidly than in areas without volcanoes. Temperatures increase more slowly in areas where the rocks allow the heat to pass easily (high thermal conductivity) and more rapidly where the rocks resist the flow of heat (low thermal conductivity). If there is no water flow, the temperature increases at depth in proportion to the depth and the magnitude of the heat flowing through the rocks and in inverse proportion to the thermal conductivity of the rocks. In Colorado the heat flow is generally low in the eastern plains and moderately high west of the mountain front. There is one particular rock layer that has a low thermal conductivity, the Pierre Shale, also known as the Mancos Shale. It typically has a thermal conductivity less than half of the rocks that lie beneath it. In the higher heat flow areas in western Colorado, temperatures increase rapidly in the Pierre Shale, 40°C/km (2.2°F/100 feet) or higher, and then much less rapidly in underlying layers, 20°C/km (1.1°F/100 feet). Thermal wells may be pumping water from depth at the ambient temperature of the water under these regional temperature conditions.

Thermal springs and many thermal wells produce water from systems where the thermal water is unusually shallow. None of Colorado’s thermal springs and wells have water chemistry that would be indicative of having interacted with a volcanic source. The alternative source of heat is deep circulation to depths where temperatures are higher and return to the surface where they discharge in springs or through wells. For many of the thermal springs, the recharge areas are difficult to constrain, but from the chemistry of the waters their depths of circulation are estimated to be a minimum of 2 km (6,500 feet). The waters appear to rise to the surface through relatively well-defined fractures at a speed of a few mm/s (around 1/8 inch/sec). At much slower speeds the water cools on its way to the surface. At much faster speeds water flow cools the rocks at depth.

Many thermal spring/well systems have unique characteristics. At Pagosa Springs a portion of the thermal water spreads laterally in the Dakota aquifer with the remainder rising to the surface spring. This has resulted in a circular thermal anomaly centered on the surface spring. Water from thermal springs at Glenwood Springs is saline because the groundwater circulates through the Eagle evaporites at depth. Discharge from these springs is very high and surface temperatures are thought to be the same as the maximum temperature at depth because flow through the system is rapid. In the Mt. Princeton/Hortense thermal spring system water chemistry indicates a much hotter geothermal resources at depth and water is thought to rise at least in part by thermal buoyancy. At the northern end of the San Luis Valley, a number of artesian wells are thermal wells, but a plot of the temperatures of the wells against their depths indicates that their temperatures are normal for the depths of the wells and that there is no heat source associated with the wells.

Thermal waters in Colorado have been used since prehistoric times for spiritual and healing purposes and they continue to be used for spas, bathing, geothermal greenhouses, aquaculture, domestic and commercial heating and hot water, and other direct-heat uses. Pagosa Springs has a geothermal district heating system. There is plenty of room for expansion of these green, sustainable, carbon-free direct-uses of geothermal energy. Studies indicate that geothermal electricity may be generated from some of the higher temperature resources. However, the development of these resources awaits economic investment and incentives equivalent to those given to solar and wind.

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(08.01) There are six classes of underground injection wells (I through VI), as described below. In Colorado all classes are administered by the US EPA, except for Class II – administered by the COGCC.

  • Class I is for industrial and municipal waste disposal wells, which are used to inject hazardous and non-hazardous wastes into deep, confined rock formations.
  • Class II is for oil and gas related injection wells, which are used only to inject fluids associated with oil and natural gas production.
  • Class III is for injection wells for solution mining, which are used to inject fluids to dissolve and extract minerals.
  • Class IV is for shallow hazardous and radioactive injection wells, which are shallow wells used to dispose hazardous or radioactive wastes into or above a geologic formation that contains an underground source of drinking water (USDW).
  • Class V is for injection of non-hazardous fluids into or above underground sources of drinking water. Most Class V wells are shallow disposal systems that depend on gravity to drain fluids directly in the ground. More sophisticated Class V wells may rely on gravity or use pressure systems for fluid injection, including systems used to inject and store water for later reuse.
  • Class VI is for wells used to inject carbon dioxide (CO2) into deep rock formations. This long-term underground storage is called geologic sequestration (GS). Geologic sequestration refers to technologies to reduce CO2 emissions to the atmosphere and mitigate climate change.

Class IV and V injection wells are the only permit types for groundwater aquifers containing an underground source of drinking water. It is recognized that this disposal can pose a threat to groundwater quality if not managed properly.

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(09.01) An aquifer is a geologic material with pore spaces saturated with water and that is permeable enough to yield water to springs and wells. At the highest level aquifers in Colorado fall into three primary groups, based on geologic material type: (1) unconsolidated granular deposits, (2) sedimentary bedrock formations, and 3) fractured crystalline bedrock. Differences in structural characteristics of these primary types can lead to very different ways that water moves through and is stored within them (Figure 09-01). (Refer to Groundwater Basics for more details on the dynamics of subsurface water movement.)

Unconsolidated granular deposits include sand, gravel, silt, and clay where the particles are not bound together and pore spaces between the open pore spaces between them can hold water. When the pore spaces are large enough, water can move through them with ease. These materials have been deposited by water, wind, and gravity.

Sedimentary bedrock formations include sandstone, conglomerate, siltstone, and shale (or mudstone), where the particles are bound together to varying degrees. This group also includes many types of carbonate formations, typically limestone and dolomite. Sedimentary bedrock formations can be semi- to well consolidated due to compaction or the presence of other minerals that bind the particles together. Pore space can be diminished or blocked because of the consolidation so that there is less capacity to hold and transmit water.

Fractured crystalline bedrock includes many types of rock with the most common being igneous and metamorphic rocks, such as granite, gneiss, and schist. It can also include many volcanic rocks, such as basalt and welded tuff. Mineral grains in these rocks interlock with little to no pore space that could hold or transmit water. The rocks must be fractured, and the fractures must be wide enough to allow water to move through to make an aquifer.

Not all geologic formations fit neatly into these classifications. Volcanic rocks can include flows that are clearly crystalline in nature that are interbedded with unconsolidated volcanic ash or unconsolidated to consolidated water- and debris flow- deposited sediments. Some sedimentary formations are so well consolidated that they behave very similarly to igneous and metamorphic rocks that must be fractured to form aquifers. Many carbonate sedimentary rocks have a crystalline structure that lacks primary porosity and permeability. Similarly, in some sandstone formations the pore spaces are completely filled with mineral “cement” and the rock, often called quartzite, cannot hold or transmit water. Like igneous and metamorphic rock, these sedimentary rocks require fractures or cavities formed by dissolution to allow the transmission of water.

Unconfined aquifers are aquifers that are not completely saturated with water and there is connection to the atmosphere (Figure 09-02). The upper portion of the aquifer, where the pore spaces are only partially filled, is referred to as the unsaturated zone.

Confined or artesian aquifers are completely saturated, permeable geologic units overlain by low permeability confining layers that prevent the free movement of air and water between the layers. The water is thus confined under pressure and if tapped by a well rises to a level above the top of the aquifer, but not necessarily above the land surface. A perched aquifer represents a limited unconfined aquifer with an underlying confining layer that lies above and is separated from the regional water table by an unsaturated zone.

Confining units are geologic units that have permeability low enough to inhibit the flow of water. The most common confining units are shale and mudstone formations that are made up of very small particles of clay. These materials may have porosity, but the pore spaces are so small that water can not move freely through them. The pore spaces may not be well connected and capillary forces hold the water in place.

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Most people are familiar with Colorado’s major rivers, all of which originate within the State. The Continental Divide splits the state with river basins heading east to the Atlantic, south to the Gulf of Mexico, and west to the Pacific. Sand and gravel deposited by the many rivers and their tributaries in these river basins form a network of unconsolidated aquifers that have historically been a critical source of groundwater supplying the State’s vital agricultural economy. Because the major alluvial aquifers in Colorado are all associated with active rivers and streams they are grouped by the major river basins. Generally, these river basins determine the boundaries of the Colorado Division of Water Resources administrative divisions (Figure 10-01).

Nearly every river, stream, arroyo or water course of any size has some alluvium associated with it. Flowing water transports the sediments down from upstream areas either as suspended sediment or bed load in the channels. As the water slows down, or the river changes course, the material settles out as alluvium. Sand and gravel may predominate, but layers of silt and clay are common. Organic material such as wood debris may also be present.

Groundwater in alluvial aquifers fills pore spaces between loose grains of sediment and is typically connected directly with the surface water in the stream (see Groundwater Basics). Many perennial streams only carry water during Spring runoff or storm events, yet the alluvium beneath the dry bed may carry groundwater year round. Alluvial aquifers rarely extend deeper than 100 feet and are limited to areas along the rivers that may extend from hundreds of feet to many miles wide. Although often limited in extent, alluvial aquifers may be the most productive aquifer type in the state with yields commonly over 1000 gpm.

Alluvial deposits associated with modern river systems were deposited during the Quaternary Period, or the last 2.4 million years, making them young geologic formations (see Geologic Evolution of Colorado). During this period of time the rivers have generally carved downward through the older bedrock as the landscape evolved. In some areas, where bedrock is easily eroded, the rivers moved from side as they cut downward to side forming broad valleys. In others, where bedrock is harder and more resistant to erosion, the valleys are narrow, or even deep canyons.

The pace of downcutting has not always been the same, nor was the sediment load carried by the streams constant as time passed. Repeating periods of glacial advance followed by periods of retreat made for a cycle of downcutting followed by alluvial deposition over and over again. This cycle of downcutting followed by deposition, overprinted on a gradual overall trend of downcutting, left behind series of terraces climbing higher in the landscape away from the modern river.

River terraces that become progressively older as you climb away from the river can be seen along almost any river in Colorado. Typically, each is mantled by alluvial sand, gravel, clay, and silt with a layer of soil developed on the surface. Older terraces, or those highest in the landscape, can have well developed soil profiles and calcium carbonate “caliche” hard-pan deposits can be common. The number and ages of individual terraces found above rivers throughout the state can vary depending on bedrock conditions and the histories of incision. In some areas at least seven terraces, dating back through the middle Pleistocene, have been identified. Naming conventions for the terrace systems vary from region to region.

Older terraces, high above the modern rivers and streams, are often carved on bedrock (Figure 10-02). On these older terraces, alluvial deposits may not be directly connected, hydraulically, to the modern rivers below. Depending on the type and permeability of the underlying bedrock, groundwater fed by local precipitation, irrigation, or other human activities can form localized perched groundwater systems. Occasionally, that perched groundwater is sufficient to supply domestic, stock, and irrigation wells.

Quaternary deposits from other processes may be associated with the alluvium deposited directly from rivers and streams. These may include wind-blown sand and silt, landslide deposits, glacial drift, local debris fan deposits, alluvial fan deposits from small tributaries. They can overlap with the alluvial deposits of the trunk stream, potentially expanding the extent of the main alluvial aquifer. For example, windblown sand deposits extend for miles beyond both the South Platte and Arkansas Rivers. These vast dune deposits can mask the extent of alluvium beneath, but can also be very effective at capturing precipitation for recharge with very high permeability and having limited soil development.

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The South Platte River basin drains an 18,924 square mile area in the northeastern quarter of Colorado (Figure 10-01-01). The basin along with the Laramie River and Republican/Arikaree Rivers defines Water Division 1, with the divisional office in Greeley. As of early 2001, there were nearly 12,000 alluvial wells of record in the South Platte River basin.

Originating high in the Rocky Mountains, the mainstem of the South Platte River and its many tributaries descend through high, glaciated mountain valleys before incising deep canyons through the foothills. Well known tributaries include the Big Thompson, Cache la Poudre, and St. Vrain Rivers, and Boulder, Clear, and Cherry Creeks. From Colorado, the South Platte River continues east to join the North Platte River at North Platte, Nebraska and then on to join the Missouri River south of Omaha. Nine trans-mountain diversions import over 400,000 acre-feet of water annually into the South Platte River watershed from west-slope basins, exceeding the imports into all other basins in the state. Over 1.1 million acre-feet of water are stored in 22 reservoirs within the basin.

According to the 2000 Census, approximately three million people, or 70 percent of the state’s population, live within the basin along the Front Range urban corridor. Aside from the urban and growing suburban land use, the region is an important agricultural area with over 1 million acres of irrigated cropland.

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The Republican/Arikaree River basin in eastern Colorado encompasses an area of 8,775 square miles and comprises four major streams: the South Fork Republican River, the Arikaree, the North Fork Republican River, and Frenchman Creek. Management of the waters in the Republican and Arikaree River basins are under the jurisdiction of Colorado Water Division 1, with offices located in Greeley. As of February 2001, only 67 wells were recorded within the confines of the mapped alluvium of the Republican River and its tributaries. The majority of the alluvial wells are located on the South Fork Republican River and the Arikaree River. The occurrence and distribution of alluvium along the Republican and Arikaree Rivers are shown in yellow on Figure 10-02-01.

As is characteristic of the Eastern Plains, there is little variation in elevation in the Republican River basin. Elevation ranges from about 5,000 feet at the headwaters to about 3,500 feet where the streams exit the state. Northeastern Yuma County from Wray northward is composed of sand hills, with no surface drainage except on the periphery. The sand hills extend into southeastern Phillips County and westward into eastern Washington County. The landscape is characterized by flat to gently rolling grasslands. Agriculture is the dominant land use in the basin. Population in the area is sparse, with 3 or fewer persons per square mile in Lincoln and Washington Counties, and 4 to 9 persons per square mile in Kit Carson, Phillips, and Yuma Counties.

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The Arkansas River basin drains a 28,273 square mile area in the southeastern quarter of Colorado (Figure 10-03-01). The basin defines Water Division 2 with the divisional office in Pueblo. As of early 2001, there were over 5,450 alluvial wells of record in the Arkansas River basin.

The Arkansas River originates high in the Rocky Mountains near Leadville, Colorado, flowing south-southeast through the mountains before it turns east and enters the plains near Pueblo. Well known tributaries include the Purgatoire, Huerfano, Cucharas, and Apishapa Rivers, along with Fountain and Big Sandy Creeks. Total annual flow in the Arkansas River within Colorado is approximately one million acre-feet. There are ten major trans-basin diversions of surface water into the upper Arkansas River that provide an additional source of recharge to the underlying alluvial aquifer. The diversions consist of tunnels and ditches that routed 144,288 acre-feet of water into the basin in 1998. Over 1.8 million acre-feet of water are stored in 19 reservoirs within the basin.

Current land use in the upper Arkansas River valley is primarily recreation and tourism with limited agriculture and industry. Land use in the lower Arkansas River valley is heavily agricultural, with both surface and groundwater being utilized to grow a significant amount of farm crops, including the famous Rocky Ford cantaloupes. The alluvial aquifer is an important source of groundwater in the plains.

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The Colorado portion of the Rio Grande Basin, located in south-central Colorado, encompasses approximately 7,500 square miles and constitutes Colorado Water Division 3 (Figure 10-04-01). Administration of water in the Rio Grande Basin is governed not only by Colorado state law, but also as stipulated by the Rio Grande Compact with New Mexico and Texas.

San Luis Valley spans much of the Rio Grande River Basin and is an open, almost treeless, intermontane basin bounded by the foothills of the San Juan Mountains on the west and the Sangre de Cristo Range on the east. The Rio Grande and Conejos River originate in the eastern San Juan Mountains and are the dominant watersheds in the river basin. Many other tributaries to the Rio Grande River flow out of the San Juan Mountains as well as the Sangre de Cristo Range to join the Rio Grande before it flows south into New Mexico at the head of the Rio Grande Gorge.

The 3,200 square mile San Luis Valley encompasses the counties of Saguache, Rio Grande, Alamosa, Conejos, and Costilla. The San Luis Valley represents a significant groundwater resource that is recognized nationally. The overall Rio Grande watershed also includes Mineral and Hinsdale counties, as well as a small area in the southeastern corner of Archuleta County.

Total population in the San Luis Valley is about 45,000 and the population in the entire Rio Grande Basin is only slightly higher, as few people reside in the mountainous areas. The San Luis Valley is an open, relatively treeless, flat valley floor with some hills in the southern end, whereas the remaining part of the Rio Grande watershed, the mountainous areas outside the central valley, are generally heavily vegetated, rugged, and steep. Irrigation is widely practiced in the valley, with the major crops being potatoes, barley, vegetables, and alfalfa.

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The Gunnison River basin of southwestern Colorado encompasses approximately 8,000 square miles (Figure 10-05-01), extending from the Continental Divide in the east to Grand Junction in the west where the Gunnison joins the Colorado River. It passes out of the Mountainous region into the Colorado Plateaus region before joining the Colorado River. The watershed comprises portions of seven counties: Mesa, Delta, Montrose, Ouray, Hinsdale, Gunnison, and Saguache. Management of the waters in the Gunnison River basin is under the jurisdiction of Colorado Water Division 4, with its divisional office located in Montrose. As of February 2001, there were approximately 1,844 alluvial wells of record in the Gunnison River basin.

Elevations in the basin are greater than 13,000 feet in the headwater areas along the Continental Divide, and less than 4,600 feet at the confluence of the Gunnison with the Colorado River. Principal tributaries of the Gunnison River include the Uncompahgre, North Fork Gunnison, Lake Fork Gunnison, East, and Taylor Rivers; and Ohio, Tomichi, and Cochetopa Creeks. Surface water is the main water resource in the basin. The historic average annual discharge of the Gunnison River into the Colorado River at Grand Junction is approximately 1.9 million acre-feet. Diversions for irrigation use were also of this magnitude and represented 99 percent of the total surface-water diversions.

Alluvial groundwater, although relatively insignificant in terms of total volume withdrawn, is important for irrigation, public and domestic water supply, and livestock uses. The principal agricultural areas in the Gunnison River basin include the Gunnison area, East River valley, North Fork Gunnison River valley, Delta area, Orchard City-Cedaredge area, and the Uncompahgre River valley. The estimated population of the Gunnison River basin, based on the 2000 Census for the five principal counties, is approximately 80,000 persons.

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The Colorado River basin watershed encompasses an area of approximately 9,830 square miles (Figure 10-06-01). The basin defines Water Division 5 with the divisional office in Glenwood Springs. As of early 2001, there were approximately 1,370 alluvial wells of record in the Colorado River basin. The headwaters of the main stem are within Rocky Mountain National Park in eastern Grand County. The Colorado River flows southwest some 230 miles through Grand, Eagle, Garfield, and Mesa counties and exits the state at the Utah border. Principal tributaries are the Fraser, Blue, Eagle, and Roaring Fork Rivers. Surface water is the principal water resource in the basin. The Colorado River represents the largest surface-water outflow in the state with over 4.6 million acre-feet of water leaving annually. Agriculture dominates the use of the basin’s water resources, with diversions of approximately 2.2 million acre-feet for the irrigation of 300,000 acres. Between 450,000 and 600,000 acre-feet of water are also diverted annually from the basin to eastern Colorado.

Alluvial groundwater resources are used for public water supply and agricultural irrigation, and represent an important resource in rural areas for domestic supplies. The principal agricultural area is the Grand Valley from Palisade to Fruita; other agricultural areas include Plateau Creek (Collbran area), Colorado River Valley from New Castle to Parachute, and the Roaring Fork Valley. The estimated population of the Colorado River basin, based on the 2000 census of the six major counties within the basin, is slightly over 253,000 people.

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The Yampa River basin is located in north-central and northwestern Colorado (Figure 10-07-01). With its tributaries, the Yampa River drains all of Routt, most of Moffat, northeastern Rio Blanco, and a small portion of northeastern Garfield counties. Based on the 2000 census, the Yampa River basin contains a population of approximately 33,000. The Colorado portion of the drainage basin encompasses an area of approximately 6,765 square miles, and is a part of Water Division 6 with the divisional office in Steamboat Springs.

The headwaters of the main stem are within the Park Range in western Routt County. The Yampa River drains the Colorado portion of the Wyoming Basin. The Yampa is a tributary of the Green River and their confluence is near the western border of Moffat County in Dinosaur National Monument. The historical average annual outflow for the Yampa River leaving Colorado is approximately 1.66 million acre-feet, or about 15 percent of the total stream flow that leaves Colorado. Principal tributaries of the Yampa are the Little Snake River, Williams Fork River, Fortification Creek, and Elk River.

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The White River flows from east to west parallel to and south of the Yampa River in northwestern Colorado (Figure 10-08-01). The White River basin drains approximately 3,770 square miles encompassing nearly all of Rio Blanco County, a small portion of southern Moffat County, and small areas of northern Garfield County. The White River basin is sparsely populated and contains a population of approximately 6,000 people. The drainage basin is part of Colorado Water Division 6 with the office of the Division Engineer in Steamboat Springs. However, the Water Judge and Clerk assigned to the White River are a part of the Water Division 5 administration.

The White River drains part of the Colorado Plateau, Wyoming Basin, and Southern Rocky Mountains. Like the Yampa, the White River is a tributary of the Green River and the confluence with the Green is south of Ouray, Utah. The historical average annual outflow for the White River leaving Colorado is approximately 590,100 acre-feet, or about 5% of the total streamflow that leaves Colorado. Principal tributaries of the White River are Douglas Creek, Yellow Creek, Piceance Creek, and the North and South Forks of the White River.

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The North Park River Basin encompasses headwaters of both the North Platte and Laramie Rivers. The North Platte River originates in North Park and flows north into Wyoming. North Park is a broad, intermontane valley of flat to rolling topography nested between the Park Range to the west, the Medicine Bow Range to the east and the Rabbit Ears Range to the south. It encompasses about 1,190 square miles of Jackson County (Figure 10-09-01). According to the 2000 Census, Jackson County recorded a population of approximately 1600, with agriculture being the dominant business. The Laramie River lies on the east side of the Medicine Bow Range in a sparsely populated valley in Larimer County. This river also flows north into Wyoming to join the North Platte River nearly 140 miles to the northeast close to the border between Wyoming and Nebraska. From there the North Platte flows eastward and is part of the Mississippi River drainage system.

North Park, and the North Platte River, fall under the administration of Water Division 6, with offices located in Steamboat Springs. The Laramie River, on the other hand, falls under the administration of Water Division, with offices located in Greeley.

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The San Juan River originates along the Continental Divide in southern Colorado, just north of the town of Pagosa Springs. The watershed (Figure 10-10-01) encompasses about 26,000 square miles of Colorado, New Mexico, and Arizona before merging with Lake Powell and the Colorado River system in Utah. The entire San Juan River system in Colorado is located within Water Management Division 7, with the division office in Durango.

In Colorado, the San Juan River system is fed by a series of sub-parallel rivers that drain the San Juan and La Plata Mountains. These rivers, from east to west, include the Piedra, Los Pinos, Florida, Animas, La Plata, and Mancos Rivers. The Mancos and Mc Elmo Rivers head west-southwest to join the San Juan in New Mexico and Utah respectively. Altitudes in the San Juan River system range from greater than 13,000 feet near the headwaters of the San Juan and Piedra Rivers to nearly 4,500 feet where the Mancos River exits the state just east of Four Corners. The San Juan River contributes about 10 percent of the total flow in the Colorado River Basin. The average annual flow exiting the state through the San Juan River represents approximately 7 percent of the state’s total annual stream outflows.

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The Dolores and San Miguel Rivers originate in the Rico, La Plata, and San Juan Mountains of southwest Colorado (Figure 10-11-01). The Dolores River basin is about 95 miles long from northwest to southeast and encompasses an area of just over 5,300 square miles, including parts of Montezuma, Dolores, San Miguel, Montrose, and Mesa counties. The San Miguel River basin is tributary to the Dolores River, is 68 miles long, and encompasses about 1,600 square miles, including portions of San Miguel and Montrose counties. The Dolores River basin falls within both Water Management Divisions 4 and 7. Altitudes in the Dolores and San Miguel River basins range from about 14,200 feet near the Dolores River headwaters to 4,100 feet at their combined confluence with the Colorado River in Utah.

Although restricted in extent, the alluvium is an important aquifer to those people who utilize it for domestic, stock, and minor irrigation use. Population is sparse in the Dolores and San Miguel River basins, with the 2000 Census indicating that approximately 65,700 people live in the combined Dolores, San Miguel, Montezuma, and Montrose Counties.

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Sedimentary bedrock aquifers are composed primarily of consolidated or semi-consolidated granular materials deposited in river, wind, shoreline, or deep-sea environments. These originated as unconsolidated sediments, but over time were consolidated by compaction or cementation as minerals formed in the pore spaces. The most common cement minerals are silica, calcite or other carbonates, and iron oxides. Because of consolidation, well productivity in sedimentary bedrock aquifers tends to be lower than in unconsolidated alluvium. Mean well yield from sedimentary bedrock aquifers is generally less than 30 gpm, and rates over 500 gpm only occur under rare conditions. However, these aquifers can extend over much larger areas than alluvial aquifers.

Sedimentary bedrock formations may be quite variable in composition and the way different types of sediment are layered. The layering affects how well water is stored and can move through the formations and determines what type of hydrogeologic unit a formation may be. Some bedrock formations may be made up entirely of a well-sorted and permeable material, such as wind blown sandstone, that can form a fairly uniform aquifer through which water can move easily. Other geologic formations may consist of strata of varying size and composition in multiple layers with confining units separating aquifers where water may move easily in a horizontal direction but not so vertically. Some may have individual permeable bodies limited lateral extent imbedded in impermeable material. An example of the later are lens-shaped deposits of sand deposited by meandering streams surrounded by mudstone deposited in marshes and floodplains.

Sedimentary bedrock aquifer distribution, depth, and thickness is quite variable depending on geologic setting. Sedimentary formations in the state can be grouped into aquifer systems with common characteristics based on geologic setting (Figure 11-01). Several aquifer systems are regional, where multiple geologic formations can be wide-spread with only minor variation in characteristics. Structural deformation of the formations across a region may be present, but structural style tends to consist of a limited number of large scale, well defined features. What formations are present at, or near the surface depends on structural and topographic setting across the region. Other aquifer systems are defined by distinct, and often very deep, structural basins. There may be formations unique to individual basins with characteristics that might change dramatically across the basin. Specific formations may be present at, or near the surface in one location, but at a very great depth in a relatively short distance.

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Colorado’s aquifers can be grouped into four main regions: Colorado Piedmont, High Plains, Colorado Plateaus, and Mountainous regions. Regional aquifer systems are those that are found spanned across large parts, or all, of any of these regions. Some aquifers, for example the Dakota Aquifer, are found in each of the regions. Some geologic strata with similar characteristics and geologic origins can be found in different regions, yet they have different names. Much of the Pierre Shale of the Colorado Piedmont region is stratigraphically equivalent to the Mancos Shale of the Colorado Plateaus region; the two formations and they share sedimentary characteristics. Hydrologically, both the Pierre Shale and Mancos Shale form regional confining units.

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The Colorado Piedmont region covers over 30,000 square miles of eastern Colorado and is characterized by broad open valleys separated by low hills. It extends from the abrupt face of the Rocky Mountains east to the escarpment of the High Plains (Figure 11a-01-01). Topographic expression within the Colorado Piedmont tends to be gentle, interrupted by local features with higher relief where geologic formations resistant to erosion form mesas and buttes. Elevation and topographic relief increase closer to the west edge of the region closer to the Rocky Mountain foothills. The South Platte and Arkansas River Basins carve their way from west to east across the Colorado Piedmont region.

Over 80% of Colorado’s population lives within the Colorado Piedmont region and most of that is in the Front Range urban corridor from Fort Collins to Pueblo. The region supports a robust economy driven by agriculture, transportation, industry, service, and construction. It also serves a vital transportation hub.

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The High Plains aquifer is an extensive regional aquifer that underlies approximately 174,000 square miles of the Great Plains states extending from South Dakota on the north to Texas and New Mexico on the south (Figure 11a-02-01). The aquifer is of significant economic importance as it provides groundwater to approximately 20 percent of the irrigated cropland in the United States.

In Colorado, the High Plains aquifer is present beneath all or parts of Weld, Logan, Sedgwick, Phillips, Washington, Yuma, Lincoln, Kit Carson, Cheyenne, Kiowa, Prowers, Las Animas, and Baca counties (Figure 11a-02-02). Land use in eastern Colorado is almost exclusively agricultural with most groundwater withdrawal used for irrigation and crop-related purposes. The topography is characterized by flat to gently rolling terrain that is bisected by mostly eastward-flowing rivers and streams.

The population in eastern Colorado remains low, with population densities rarely exceeding four to nine residents per square mile. Colorado counties containing the High Plains aquifer are included in the Office of the State Engineer Water Divisions 1 and 2. Most of the aquifer is a part of Colorado’s Northern or Southern High Plains Designated Ground-Water Basins and their associated groundwater management districts. Virtually thousands of wells (15,600 wells of record as of February 2001) are completed in the High Plains aquifer.

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Western Colorado covers portions of the Colorado Plateau and Wyoming Basin Plateau region (Figure 11a-03-01), an area characterized by deep canyons and high plateaus. Geologic formations at the surface in this area are dominated by Pennsylvanian through Quaternary sedimentary deposits with scattered Neogene volcanic flows and intrusions. Upper Cretaceous marine Mancos Shale, the western equivalent of the Pierre Shale, dominates much of this region. The deeper Dakota Group underlies large areas and can be an important regional aquifer. Deformation of the sedimentary formations in this area include broad uplifts and large-scale folds formed during the Ancestral Rocky Mountains and Laramide events.

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Large quantities of groundwater occur in deep basins formed during several of the major geologic events that define the evolution of the geologic landscape of Colorado. These basins typically contain multiple geologic formations made up of sand, gravel, and mudstone shed off of nearby uplifts. Stratigraphic setting, or the way different layers are stacked, can be complex with some formations changing characteristics rather quickly with distance away from an active rangefront. Deposition close to the active range front may be dominated by high-energy alluvial fans where the deposits can be mostly coarse grained sandstone and conglomerate. Sorting may be very poor near the range front. Away from the active uplifts, stream transport tended to slow down and grain size decreases, but sorting increases. Fine-grained shale and mudstones can be dominant with greater distance from the rangefront.

In Colorado three episodes of basin development created important hydrogeologic settings. The earliest was the Pennsylvanian to Permian Ancestral Rocky Mountain event. Next came the Upper Cretaceous to Paleogene Laramide mountain building event which created the foundation for many of the mountain ranges present today. Basins formed during these two events form large-scale aquifer systems recognized today. The third, and most recent, episode of basin development is the development of deep basins during extension of the Rio Grande Rift. These basins form many of the deep valleys included in the Mountainous region.

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The oldest major basin-forming episode in the progression of events for Colorado that is significant for groundwater resources is the Pennsylvanian-Permian ancestral Rocky Mountain event. This major tectonic event occurred between approximately 250 and 320 million years ago when uplift occurred along two northwest-trending belts, the Ancestral Front Range-Apashipa range on the east and the Uncompaghre-San Juan uplift on the west (Figure 11b-01-01). A deep basin, the Central Colorado Trough separated the two uplifts. Boundaries of these features are often difficult to define because later tectonic events modified defining structural elements and overprinted new ones.

Sediments eroded from the uplifts filled basins flanking the uplifts, the Paradox Basin to the west, Ancestral Denver Basin to the east, and the Central Colorado Trough in between. Total thickness of the sediments that accumulated can be over 10,000 feet in places. The stratigraphy represents transition from active mountain fronts, dominated by alluvial fan systems, basinward to quieter depositional settings that, at times, were open to the ocean and flooded with seawater. Sediments can include sandstone, siltstone, conglomerate, shale, and limestone with proportions and bedding relationships that change with distance from the adjacent rangefront. At times circulation with the open ocean was restricted and evaporation of ponded water in the basins led to the buildup of thick evaporite deposits, chiefly salt and gypsum.

Older Paleozoic sedimentary formations blanketed the region prior to the Ancestral Rocky Mountain event. These formations, dominated by limestone and dolomite with lesser amounts of quartz sandstone and shale, were eroded off of the uplifts but remain within the basins. Because they are mainly restricted to the Ancestral Rocky Mountains basins, they can be considered part of those basins.

Later tectonic events, primarily the Laramide mountain building event, brought parts of the old basins up to be exposed at the surface. Sedimentary formations from the Ancestral Rocky Mountains event include the colorful Maroon and Minturn Formations of the Eagle Basin, the Fountain Formation of the Ancestral Denver Basin, and the Hermosa Formation of the Paradox Basin. Formations also include thick accumulations of salt and gypsum like the Eagle Valley Evaporite Member exposed in the Eagle and Roaring Fork Valleys in the Colorado River Basin. Not only do salt and gypsum impair water quality locally, but their dissolution by groundwater can cause the ground to collapse in areas where they occur.

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The Central Colorado Trough (Figure 11b-01-01-01) covers a wide, northwest-trending swath across central Colorado. Remnants of it are found throughout the Mountainous region with its greatest exposure is recognized as the Eagle Basin. The Eagle Basin represents that part of the Central Colorado Trough that was uplifted during the Laramide event along the White River and Sawatch uplifts. The Central Colorado Trough continues to the northwest with part deep within the Sand Wash Basin and part exposed in the Axial Basin-Uinta uplifts. Fragments of the Central Colorado Trough can also be found southeast in the Mountainous region as structural blocks within valleys and high in the mountain ranges.

Eagle Basin underlies approximately 1,500 square miles in north-central Colorado along the western flank of the Continental Divide. It lies primarily in Eagle County, but also extends into Routt, Rio Blanco, Garfield, Pitkin, and Grande counties. The Eagle, Roaring Fork, and Colorado Rivers form the primary surface water drainages in the basin, with the headwaters of those rivers located along the Continental Divide. The Eagle River generally follows the axis of the basin to the town of Wolcott, where it turns west and meets with the Colorado River at Dotsero. The bulk of the Eagle Basin is included in Water Division 5.

Land use in Eagle County is primarily recreation and tourism with limited ranching outside the Interstate 70 corridor. Recreational opportunities and the proximity of major ski resorts have resulted in tremendous development along the I-70 corridor between Vail and Edwards. According to the 2000 Census, 41,659 people permanently reside in Eagle County, with at least one-quarter of these located in the resort towns of Vail and Avon. Over 2,600 non-alluvial wells are permitted in the Eagle Basin. Most of these water supply wells are clustered near the population centers along the Interstate 70 and Roaring Fork River corridors.

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The Paradox Basin, as recognized today, is an elliptical-shaped structural depression located within the Colorado Plateau physiographic province in southwestern Colorado and southeastern Utah. At the time of the Ancestral Rocky Mountains event the Paradox Basin extended to the southeast where the Laramide San Juan structural basin later developed. In Colorado, the Paradox Basin is bounded on the northeast side by the Uncompahgre uplift (Figure 11b-01-02-01), and it extends beyond the borders into Utah to the west and New Mexico to the south. The Laramide San Juan Dome separates today’s surface expression of the basin from the San Juan Basin, where the Paradox Basin units continue at great depths. The basin covers approximately 14,000 square miles of which, 40 percent or, 5,600 square miles are located in Colorado.

The terrain is mostly composed of high plateaus with deeply incised canyons and dry arroyos. The San Miguel and Dolores Rivers, which are part of the Colorado River system, form the main surface-water drainage system in the Colorado portion of the Paradox Basin. Administratively, the basin lies in both Water Divisions 7 and 4. In Colorado, the Paradox Basin is primarily located in Dolores, San Miguel, and western Montrose counties. Few people live in the Paradox Basin, with the 2000 Census recording only 1,844 people in Dolores County and 6,594 in San Miguel County. Agriculture has historically been an important industry in this region. According to the Colorado Division of Water Resources (DWR) well permit database, less than 2,000 bedrock wells are located in the basin, with most clustered around the population centers and river courses.

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The Ancestral Denver Basin is a large structural basin formed during the Ancestral Rocky Mountains that is in the South Platte and Arkansas River Basins (Figure 11b-01-03-01). It flanked the east side of the Ancestral Front Range uplift and in places now underlies the Denver and Cheyenne Laramide Basins that formed later. It also lies within the Colorado Piedmont region, and High Plains regions. With this placement it is not commonly recognized, yet the strata preserved within it share characteristics with other Ancestral Rocky Mountains basins to justify including it as a specific hydrogeologic basin.

The basin is bound on the west by the Front Range, the southwest by the Apishapa-Sierra Grande uplift, and it extends to the east beyond the stateline without a defining boundary. Formations deposited while it was active are exposed along the hogback and include the prominent Lyons Sandstone and Fountain Formation.

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The second episode of basin development with relevance to groundwater resources is the Late Cretaceous-Paleogene Laramide mountain-building event that occurred between approximately 45 and 70 million years ago. Seven major basins formed during this event, including the Denver Basin, which often contain coal, oil, and gas resources as well as large quantities of groundwater. These basins formed in between a series of uplifting fault-bound blocks cored with older sedimentary formations and Precambrian crystalline bedrock. The region had been part of the Upper Cretaceous Interior Seaway which retreaded as the uplifts began, gradually fragmenting the large downwarp that accommodated the seaway. Sedimentary formations in each basin record an upward succession from marine shoreline environments to coastal plain and delta environments and then on to very active fluvial basin environments (Figure 11b-02). Complexity in the stratigraphy increases upward in the active basin environments which represent lateral transition from high energy fluvial systems near the active range fronts to lower energy river and marshland areas in the basin centers. Coal formed in many of the lower energy basin interiors and many of the porous sandstones are important petroleum reservoirs in the deeper parts of several of the basins.

At times, the subsiding basins filled with intermountain lakes that resulted in accumulations of shale, carbonates, and evaporite deposits. The largest of these lakes were Lake Uinta in the Piceance Basin and Lake Gosiute in the Sand Wash Basin. Oil shale, which is a lake deposit rich in kerogen makes up part of these deposits and is recognized as a major potential source of energy.

Not only do the Laramide basins share a general succession of sedimentary formations related to the basin evolution, but they also share a general spoon-shaped geometry. Each formed near an active uplift with its deepest part close to the uplift, giving the basin an asymmetrical profile with depth. At the edge nearest the active uplift, which is often defined by a major fault or fault zone, the sedimentary formations within the basin rise to the surface with steep dips. Sometimes the beds can be overturned near the basin-bounding faults. On the opposite side, away from the uplift, the same formations come back to the surface with much more gentle dips. The spoon-shape places the youngest aquifers at the surface in the basin center ringed by successively older aquifers away from the center.

Laramide differ in other ways. Some are more structurally deformed than others and can have faults and folds in their interiors. The types of rocks exposed in the bounding uplifts can also differ so that the compositions of the sedimentary formations can be markedly different.

In this atlas Laramide basin outlines, as seen in the web-maps, are based on the extents of the lowermost shoreline bedrock formations above the main Interior Seaway shale body. On the east side of Colorado this includes shoreline retreat deposits of the Fox Hills and Trinidad Sandstones. On the west side of the state, this is the Mesaverde Group.

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The Denver Basin is a Laramide structural basin that encompasses much of the Front Range urban corridor metropolitan area from Denver to Colorado Springs. It is entirely within the Colorado Piedmont region of the Great Plains physiographic province. The center of the basin lies just west of Parker, where the lowermost of four aquifers, the Laramie-Fox Hills Aquifer, is approximately 3,000 feet deep. The administrative groundwater portion of the basin underlies a 6,700 square mile area extending into Weld County on the north, El Paso County on the south, Jefferson County on the west, and the eastern portions of Adams, Arapahoe, and Elbert Counties on the east (Figure 11b-02-01-01). The basin straddles the boundary between Water Divisions 1 and 2.

The northern part of the Denver Basin extends into an area of known oil and gas resources in the deep part of the basin with the Wattenberg Field being the largest recognized producing field. Producing formations are mainly in the Upper to Lower Cretaceous sandstones beneath the Pierre Shale. As much as 7,000 feet of nearly impermeable Pierre Shale separates the main producing zones from the Denver Basin bedrock aquifers.

The Denver Basin is an important nonrenewable source of groundwater for municipal, industrial, agricultural and domestic uses. The eight-county Denver metropolitan area contains 56% of Colorado’s population, or slightly over 2.4 million people according to the 2000 census. The lack of available surface-water rights and accelerated urban growth has resulted in extensive development of the Denver Basin aquifers as both primary and supplemental sources of water supply. As of February 2001, approximately 33,700 wells of record have been completed in the sedimentary rock aquifers of the Denver Basin.

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The Cheyenne Basin is part of a greater Laramide basin known as the Denver-Cheyenne Basin. This larger basin is also called the Denver Julesburg Basin, or DJ Basin in the oil and gas community. The greater structural basin is divided into two sub-basins, the Denver Basin on the south and the Cheyenne basin on the north. A subtle, northeast-trending structural divide known as the Greeley Arch separates the two (Figure 11b-02-02-01). Cheyenne Basin continues north into Wyoming.

Cheyenne Basin in underlies over 2,000 square miles in the northern part of Weld County and parts of Larimer and Morgan Counties. The area is primarily agricultural and industrial with Greeley as its main urban center. It is at the northern end of the Front Range Urban corridor and underlies an area experiencing rapid growth. Until recently most groundwater use has been from the alluvial aquifers for agriculture; however, recent growth patterns are increasing need for bedrock groundwater to meet domestic and other demands.

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The Raton Basin is a north-northwest trending structural basin located in south-central Colorado within Las Animas and Huerfano counties. The basin extends from the vicinity of the Huerfano River valley in its northwest corner southward into New Mexico (Figure 11b-02-03-01). The Colorado portion of the basin is bounded on the west by the Culebra Range, on the east by the Park Plateau, on the north by the extent of the Poison Canyon Formation, and on the south by the state line. The basin is located within the jurisdiction of Water Division 2.

Topography ranges from fairly flat along the Cucharas River west of Walsenburg to very steep and rugged in the vicinity of the igneous stocks of the Spanish Peaks and Mount Mestas. The Park Plateau, which covers most of the area south of the Spanish Peaks, is highly dissected with elevations over 8,400 feet at the upper plateau surface.

Las Animas and Huerfano Counties are sparsely populated. Land use is primarily forest with some agricultural use. The major industry in the basin is coal mining. Currently, coal-bed methane production is very active in the basin. As of February 2001, almost 1,370 permitted water wells of record are present in the Colorado portion of the Raton Basin.

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The San Juan Basin is an asymmetric Laramide structural basin that encompasses a surface area of approximately 21,600 square miles in Colorado, New Mexico, Utah, and Arizona. The basin occupies the eastern third of the Colorado Plateau physiographic province and is characterized by rugged terrain. Landscape features include mesas, terraces, escarpments, canyons, dry washes (arroyos), and mountains.

In Colorado, the San Juan Basin extends into Archuleta, La Plata, and Montezuma counties (Figure 11b-02-04-01). Much of the land located in the Colorado portion of the basin belongs to the Southern Ute Indian Reservation. Land use in southwestern Colorado is highly variable including irrigated-agriculture, ranches and residential lots clustered around the main community centers. The San Juan Basin is a significant producer of coal, oil, and natural gas. As of February 2001, almost 5,000 permitted water wells of record are present in the Colorado portion of the San Juan Basin.

Several major rivers, including the San Juan, Los Pinos, Florida, Animas, and La Plata, flow through Colorado’s portion of the San Juan Basin. All of the rivers flow southward into New Mexico and are part of the Colorado River system via the San Juan River. The entirety of the Colorado portion of the basin is included in Water Division 7.

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The Piceance Basin is an elongated structural depression trending northwest-southeast in western Colorado (Figure 11b-02-05-01). It is an extension of the greater Uinta Basin that continues to the northwest into Utah. The Douglas Creek Arch is a structural high that separates the Piceance Basin from the Uinta Basin even though many of the bedrock formations carry over from one basin to the other. The basin is more than 100 miles long and has an average width of over 60 miles, encompassing an area of approximately 7,110 square miles. The Piceance structural basin encompasses varying portions of Moffat, Rio Blanco, Garfield, Mesa, Pitkin, Delta, Gunnison, and Montrose counties.

Being part of the Colorado Plateau physiographic province, the Piceance Basin is characterized by a series of high plateaus and deep valleys. Downcutting of the Colorado River has divided the Piceance Basin into a northern and southern province. The southern province is marked by two significant erosional remnants, Grand Mesa and Battlement Mesa. The northern province, that portion of the Piceance Basin between the Colorado and White Rivers, still retains basin-like features with rocks dipping inward from the margins toward the deepest part of the basin at the northern end.

The Piceance Basin is part of the upper Colorado River basin that includes the area drained by the Colorado River and its tributaries upstream from Lee’s Ferry, Arizona. The principal rivers that drain the Piceance Basin are the Colorado, Gunnison, North Fork Gunnison, and White. Three Colorado Water Divisions administer the water in the Piceance Basin. Water Division 6 manages the north portion of the basin drained by the White River and its tributaries. Water Division 5 manages the central part of the basin drained by the Colorado River and its tributaries. Water Division 4 manages the southern portion of the basin from the crest of Grand Mesa south to about the Gunnison River.

There are no significant population centers within the Piceance Basin. Well permit records with the Division of Water Resources indicate that approximately 2,200 water supply wells have been drilled as of early 2001.

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The Sand Wash Basin of northwestern Colorado straddles the Wyoming state line between the Park Range on the east and the Uinta uplift on the west. It is located within the northeastern half of Moffat County and western two-thirds of Routt County, as well as a small section of northeastern Rio Blanco County and the northern tip of Garfield County. The Sand Wash Basin is part of the Wyoming Basin physiographic province and the Colorado portion encompasses approximately 4,760 square miles (Figure 11b-02-06-01).

Most of the basin is a rolling plain with elevations above 6,000 feet, although the eastern part of the basin grades into foothills and mountains with elevations above 10,000 feet. The Yampa River and its tributary, the Little Snake River, flow into the Green River, and represent the principal surface drainage systems for the basin. Administration of water within the Sand Wash Basin is under the jurisdiction of Water Division 6, located in Steamboat Springs. This portion of Colorado is sparsely populated.

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The South Park Basin is a structural basin bounded by the Front Range uplift on the east and the Sawatch uplift on the west (Figure 11b-02-07-01). The structural basin lies entirely within Park County.

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The Colorado Headwaters Basin is a structural basin bounded by the Front Range\Medicine Bow uplift on the east and the Park Range uplift on the west (Figure 11b-02-08-01). It is better recognized as the Middle and North Parks; however, as a Laramide basin it can be considered as a single structural basin. The north part of the structural basin lies within Jackson County and the south part lies within Grand County. The Rabbit Ears Mountains separates the two into the topographic North and Middle Parks.

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The Southern Rocky Mountains bisect the state, covering nearly one third of its area. This mountainous region (Figure 12.01) has the greatest topographic relief with 54 summits exceeding 14,000 feet, the venerable 14ers of the state. It is also the most geologically complex region in the state. Within this region are deep valleys formed by Neogene faulting that are filled with sedimentary formations; complexly deformed sedimentary formations of many ages; and areas of intense volcanic activity covered with lava flows, ash deposits, and sediments shed off of eroding volcanoes.

Fractured crystalline bedrock, the third common aquifer type in Colorado, is at or near the surface over large portions of this region where it forms the only available aquifer. Rocks forming this bedrock consist of interlocking mineral crystals with no interconnected pore space to create porosity or permeability. Groundwater only occurs and moves through where fractures and faults crosscut the rock. Because of this, groundwater production is generally low as compared to the sedimentary aquifers. Mean well yields are generally less than 10 gpm and rates over 50 gpm are rare. Production and depth to groundwater can also be variable from well to well.

Crystalline bedrock includes Precambrian igneous bodies that intruded in molten form deep within the earth at high temperatures. The most commonly recognized rock is granite, but there are many other varieties. Precambrian metamorphic rocks also make up a large portion of these rocks and include gneiss and schist of varying composition and texture. These are rocks that were originally sedimentary or volcanic in origin but were changed in form during deep burial under pressure and high temperature.

Volcanic and igneous intrusive rocks make up the remainder of the crystalline bedrock within the Mountainous region. These rocks include lava flows and ash deposits that welded under their own heat into very dense rock after coming to rest. Other rocks formed from explosive eruptions and erosion of the volcanoes in this region are often interlayered with the crystalline flows. They can be very similar to sedimentary bedrocks in that they consist of consolidated, semi-consolidated, and even unconsolidated material derived directly from the volcanic centers. However, these rocks are granular and often have better connected pore spaces between grains.

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Mountainous Valleys

South Park is a north-south trending, elongated, intermontane basin located in Park County in central Colorado. The basin is approximately 45 miles long and 20 miles wide, encompassing some 893 square miles. South Park is a high-altitude, relatively flat, generally treeless plain located between the Mosquito Range and the Front Range (Figure 12a-01-01). It is traversed by several ridges that rise hundreds of feet above the floor of the Park and extend for a number of miles down the center of the basin (e.g. Red Hill and Reinecker Ridge).

South Park sources the South and Middle Forks of the South Platte River and other tributaries include Tarryall and Jefferson Creeks. Tarryall Creek and its tributaries drain the northern portion of the Park, while the South Platte River and its tributaries drain the remainder of the Park. Important water storage reservoirs include Antero, Elevenmile Canyon, Spinney Mountain and Taryall. South Park falls into the management of Water Division 1 with offices located in Greeley. South Park encompasses about 40 percent of Park County’s total area, whose population density in 2000 was about seven persons per square mile.

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The Colorado portion of the Rio Grande Basin, located in south-central Colorado, encompasses approximately 7,500 square miles and constitutes Colorado Water Division 3. The Rio Grande and Conejos River originate in the eastern San Juan Mountains and are the dominant watersheds in the basin. These rivers flow through the San Luis Valley, which is an open, almost treeless, intermontane basin bounded by the foothills of the San Juan Mountains on the west and the Sangre de Cristo Range on the east. The 3,200 square mile San Luis Valley encompasses the counties of Saguache, Rio Grande, Alamosa, Conejos, and Costilla (Figure 12a-02-01). The San Luis Valley represents a significant groundwater resource that is recognized nationally.

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Upper Arkansas Valley is a narrow, north-south trending, intermontane basin encompassing Lake and Chaffee counties in central Colorado (Figure 12a-03-01). It is bounded on the west by the lofty Sawatch Range, home to 14 of Colorado’s peaks over 14,000 feet high, and on the east by the somewhat lower Mosquito Range. The basin is approximately 76 miles long and up to 30 miles wide at its widest, encompassing some 1,500 square miles. Watersheds in the surrounding mountain ranges bring the area to over 20 miles wide in some places. Low hilly areas in the valley are studded with piñon and juniper forests and the rich farmland characterizes the valley bottoms. The area has a rich mining and agricultural history and is becoming a year-round recreation focal point for the state.

With headwaters at the north end of Lake County, the Arkansas River flows south through the Upper Arkansas Valley to exit through Bighorn Sheep Canyon on its way to the spectacular Royal Gorge. The valley falls into the management of Water Division 2 with offices located in Pueblo.

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The Wet Mountain Valley and Huerfano Park are located in south-central Colorado in southwestern Fremont, western Custer, and northwestern Huerfano counties between the Sangre de Cristo Mountains on the west and the Wet Mountains on the east (Figure 12a-04-01). Both these intermontane basins contain thick Tertiary-age, basin-fill sediments. These two areas fall within and are under the administration of Water Division 2 of the Colorado Division of Water Resources.

The Promontory Divide forms the boundary between the Wet Mountain Valley and Huerfano Park, as well as Custer and Huerfano counties. The two areas are discussed together because their basins are adjoining and their geology and hydrology are similar. The basin-fill deposits of the Wet Mountain Valley cover an area of approximately 230 square miles. Huerfano Park lies directly southeast of the Wet Mountain Valley and is underlain by many of the same aquifers. Land use in this area is primarily agriculture, rangeland, or forest. The area is sparsely populated with an average population density of approximately five residents per square mile.

Surface water streams and creeks in the Wet Mountain Valley flow north to the Arkansas River, while those in Huerfano Park generally flow south toward the Huerfano River which is itself tributary to the Arkansas.

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Blue River Valley is a narrow, north-south trending, intermontane basin encompassing Summit County and extending north into Grand County as far as Kremmling in central Colorado (Figure 12a-05-01). It is bounded on the west by the Gore and Ten-mile Ranges and on the east by the Front Range and Williams Fork Range. The basin is approximately 45 miles long and 8 to 25 miles wide, encompassing some 700 square miles. Watersheds in the surrounding mountain ranges bring the area to over 25 miles wide in its southern end, narrowing to about 8 miles in its northern end. It is a high, generally forested valley rich mining history. It has become a center for winter sports recreation with four major ski resorts, while also being a year-round destination for many mountain activities.

Its namesake river Blue River Valley is a major headwater tributary to the Colorado River and water use is administered through Division 5 with offices on Glenwood Springs. The valley is home to two major water storage reservoirs, Green Mountain Reservoir and Lake Dillon. Transmountain diversions export water from Dillon Reservoir to the South Platte watershed through the 23-mile long Roberts Tunnel that bores up to 4,000 feet beneath the Colorado Front Range.

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North Park and Middle Park are intermontane basins located in north-central Colorado lying between the Front Range on the east and the Park Range to the west (Figure 12a-06-01). These basins are in the Southern Rocky Mountains physiographic province and represent a single synclinal structural basin some 70 miles long and about 35 miles wide in Jackson and Grand counties. Although the Parks are considered a single structural basin, they are separate topographic, surface drainage, and groundwater basins, since the Continental Divide, in the form of the Rabbit Ears Range, bisects the syncline.

North Park encompasses about 1,190 square miles of Jackson County, and is a broad, intermontane valley of flat to rolling topography that is drained by the North Platte River and its principal tributaries. The North Platte flows northward into Wyoming and is part of the Mississippi River drainage system. According to the 2000 Census, Jackson County recorded a population of approximately 1600, with agriculture being the dominant business. North Park falls under the administration of Water Division 6, with offices located in Steamboat Springs.

Middle Park Basin is located in Grand County and encompasses about 1,030 square miles of complex valleys and sub-basins that are traversed by belts of overthrust faulting. The 2000 Census population for Grand County was approximately 12,500, with tourism accounting for nearly 66 percent of the employment. Middle Park is drained by the Colorado River and its tributaries. Groundwater and surface water in Middle Park are administered in Water Division 5, with offices located in Glenwood Springs.

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Through geologic time Colorado has been the site of sometimes intense volcanism. As a result, volcanic rocks of varying ages can be found in many parts of the state (Figure 12b-01). Perhaps the most significant volcanic event in the State, from the perspective of current hydrogeologic setting, occurred in the Paleogene. This event began about 37 million years ago and lasted until about 24 million years ago and from it are several large volcanic fields. This episode was characterized by an early phase of intermediate, or andesitic, volcanism with eruptions from large stratovolcanoes. Little evidence remains of the actual volcanic centers, but the rocks from them are very common. This phase was followed by more silicic, or rhyolitic, volcanism with the eruptions from huge “super volcanoes” and the development of caldera complexes. Volcanism continued after this but changed character to widespread mafic, or basaltic, eruptions from cinder cones and fissures. The most recent eruption of this later type was from the Dotsero Crater in Eagle County only about 4,150 years ago.

Volcanic fields all lie within the Mountainous region and include the San Juan Volcanic field, one of the largest in the world, that covers parts of ten counties in the southern part of the state. Other smaller fields include the Central Colorado volcanic field, the Flat Tops, and many other smaller outliers of volcanic rocks. Many form highlands mostly in public lands, while the larger fields extend across lowland areas now experiencing rapid growth dependent on groundwater.

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Colorado’s crystalline rocks represent a unique and expansive aquifer system. The crystalline rocks are Precambrian aged (950 to 1800 million years old) igneous and metamorphic rocks; composed mostly of granites, gneisses, and schists. They also include Late Cretaceous and Paleogene igneous intrusive rocks. These rock types occupy approximately 19 percent of the state’s total surface area, and represent the fractured, crystalline-rock aquifers that supply much of the domestic water supply needs in the mountainous portion of Colorado (Figure 12c-01). Unlike sedimentary rock aquifers, igneous and metamorphic crystalline rocks have no primary porosity; water is stored in fractures within the rocks.

Crystalline rocks are exposed at the surface throughout the Mountainous region. The general physiographic features of this area include high peaks, great relief, rugged terrain, steep slopes, shallow soils, and extensive areas of exposed bedrock. These rocks are exposed at the surface to some extent in all seven Water Divisions.

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Over geologic time the landscape of Colorado has continuously evolved. Rocks and sediments in Colorado tell a complex tale of this evolution stretching back nearly three billion years. Geologic time is divided into Eons which are then subdivided into Periods as shown in Table 13-01. The order shown is from youngest on top to oldest on the bottom, just as the rocks are normally found layered with with youngest rocks at the surface.

Even though much has happened through such vast amounts of time, it is possible to group the evolution of the geologic landscape into major events. Eight major geologic events have left characteristic marks on the bedrock stratigraphy of the region (see the primary stratigraphy chart for the state, copies of the chart are available). These major events, also listed in Table 13-01, are the basis for grouping rocks and sediments shown in a generalized geologic map (Figure 13-01). Each of the eight major events formed rocks and sediments with unique sets of characteristics that determine aquifer type and groundwater conditions.

Events are based on periods of relative stability or great tectonic upheaval. Periods of relative stability are those where regional deformation by tectonic forces is minor. There may have been prolonged times when older rock formations were exposed for erosion and the development of deep weathered surfaces. At times there was widespread deposition of sediments with similar characteristics across the region. Environments could have been either marine, with widespread carbonate reef development, or non-marine with broad fluvial plains or vast dunefields of wind-blown sand. Periods of great tectonic upheaval were those when the landscape changed dramatically because of deep crustal downwarps or the development of deep basins separated by uplifts. Material eroding off of the active uplifts filled the adjacent deepening basins. Faults and folds deformed older rocks and features. Widespread volcanism often accompanied the deformation.

Tectonic forces changed over time, both in intensity and direction. And through time newer features overlapped older. Features formed during episodes of tectonic upheaval from one event may have formed where features from an earlier event formed. The newer features may be similar to older; a new basin overprints an older one with the same basic geometry. Conversely younger features may cross-cut older features with a different geometry. In some cases a younger feature may have formed with a completely opposite sense of deformation than an older feature it overlapped; an uplift brings up what was once a basin. This progression of changing forces and environments created a very complex three-dimensional architecture of geologic formations: basins overlap basins, uplifts overlap basins, basins conceal uplifts, and so on (Figure 13-02).

A brief summary of the major events follows in order from oldest to youngest to convey the progression through time.

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During the Precambrian, nearly 2 billion years ago, the North American Continent was expanding as plate tectonics merged slivers of island arc complexes with the main core. This process built a basement of igneous and metamorphic rocks upon which all younger sediments accumulated. There were likely many different periods of tectonic upheaval separated by periods of relative stability. However, because the rocks formed over this long period of time are generally similar in their physical characteristics, they are considered a product of one event. The Proterozoic crystalline rocks formed during this period of continental accretion underlie the entire state but rise to the surface only in the cores of mountain ranges that uplifted much later during subsequent tectonic events. In these uplifted areas, fractured and faulted igneous and metamorphic rocks form the crystalline bedrock aquifer that in many places is the sole groundwater source. The oldest known Precambrian rocks in Colorado are represented by meta-sediments exposed in the Uinta Mountains in the northwest corner of the state.

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During the Cambrian through Mississippian Periods, from approximately 540 to 320 million years ago, the region of the continent where Colorado now sits was relatively stable. Periodically, marine advances flooded the area with shallow seawater. This environment and the evolution of shellfish resulted in the deposition of sandstone, limestone, and dolomite. The resulting formations flank many of the younger uplifts and can be local aquifers. Primary porosity and permeability may be limited in many of these formations but secondary porosity and permeability from fracturing and dissolution of the carbonates can lead to very productive aquifers. Vast ancient cave systems in the Mississippian Leadville Limestone is an excellent example of this type of secondary porosity and permeability.

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In early Pennsylvanian time, about 320 million years ago, this region underwent a period of tectonism that created a series of mountain uplifts referred to as the ancestral Rocky Mountains. The uplifts brought up the deep Proterozoic crystalline basement rocks while eroding sediments accumulated along their flanks. These sediments include sandstone, conglomerate, shale and limestone that can be thousands of feet thick in places. A deep trough between uplifts cut across Colorado that was frequently flooded with seawater. Restricted flow with open ocean lead to accumulation of thick deposits of evaporite minerals, chiefly halite (salt) and gypsum. The coarser-grained sediments that often flank modern mountain ranges, or span plateau regions, can be local sedimentary bedrock aquifers. However, where present near the surface, the thick accumulations of evaporite minerals can lead to dissolution features, convoluted geologic structure, and degradation of groundwater.

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Tectonic stability returned to the region in the Permian Period, about 270 million years ago, and the ancestral Rocky Mountain landscape was reduced to a nearly flat surface. The Paleozoic era began with an explosion of life and ended with a dramatic mass extinction of marine invertebrates. It is estimated that 85 percent of all marine species and 70 percent of all terrestrial species went extinct in less than 1 million years (Bowring and others, 1998). At the end of this era, Colorado was a relatively flat, low-lying region with an arid or semi-arid climate. From late Permian through Early Cretaceous time, alluvial, lacustrine and aeolian sediments intermittently accumulated on this surface. The coarser-grained sediments that often flank modern mountain ranges, or span plateau regions, can be local sedimentary bedrock aquifers.

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Tectonic forces at the western edge of the continent beginning about 100 million years ago in the Cretaceous caused the entire region where Colorado sits to subside. This sag allowed seawater to inundate the region to form what is called the Cretaceous Interior Seaway. The Cretaceous sea shoreline moved westward over the entire state depositing shoreline beach, barrier island, and deltaic sediments. As the shoreline continued to transgress westward, thick layers of marine shales were deposited. Marine shale and limestone, some rich in organic material, accumulated to thousands of feet. Sandstone, shale and coal accumulated in the bounding shoreline and delta settings. Not only do the sediments that accumulated during this event account for most of the state’s fossil fuel resources, but they also form regional sedimentary bedrock aquifers. Since marine shale makes up a large portion of these sediments permeability can be very low and water quality can be very poor. Groundwater constituents of concern frequently found in these sediments include sodium, chloride, sulfate, selenium, and uranium.

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Late in the Cretaceous, or about 70 million years ago, the style of deformation in the region changed. This became one of the most significant events in Colorado’s geologic history, the Laramide mountain-building event and the rise of the modern day Rocky Mountains. The seas of the western interior seaway retreated for the last time with emerging mountain ranges and terrestrial basins dominating the Colorado landscape through the ensuing Cenozoic era. As with the Paleozoic, the Mesozoic era ended with a mass extinction, this time of the dinosaurs. What was once a broad downwarp began to fragment into a series of basement-cored uplifts separated by deep subsiding basins driven by compressive tectonic forces. The Laramide mountain-building event continued into the Paleogene period and produced tens of thousands of feet of uplift with associated downwarp creating ranges and basins throughout Colorado. The subsiding basins pulled the older formations to great depths while sediments derived from erosion of the uplifts filled in above. Aerial extent of the Laramide basins in Colorado is expansive across seven major structural features. These large basins hold energy resources in the deeply buried older formations and groundwater resources in the coarse-grained sediments that accumulated while the basins formed.

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At the end of the Laramide Orogeny, a period of erosion is recorded in the rock strata by thick sequences of alluvial and colluvial sediments in the valleys and basins between mountain ranges. Tectonic forces slowly changed from compressive to extensional late in the Paleogene Period, about 30 to 25 million years ago. Widespread volcanism accompanied the transition as ash, lava flows, and debris from multiple volcanic centers blanketed much of the state. The San Juan Mountains represent one of the larger volcanic centers, containing several large calderas or basin-shaped volcanic depressions. Flows, welded tuffs, and intrusive bodies from this episode form part of the crystalline bedrock aquifer system. Gradually, extensional forces uplifted and deformed the region to produce Colorado’s present topography of block-faulted mountains and basins, plateaus, and high plains. Fault-bound sediment-filled grabens formed during this event drop the older geologic formations to great depths. These grabens have different characteristics from the earlier Laramide basins in both structural architecture and sediment type. Sediments filling the basins includes sand and gravel along with volcanic ash-rich mudstones. At the same time blankets of sediments similar to those filling the grabens, spread downslope from the central highlands.

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The landscape as we know it today evolved during this last phase of tectonic activity as surface drainage developed into the major river basins of the Platte River, Colorado River, and Rio Grande with their many tributaries. The Quaternary period, representing the last 2 million years of geologic history, has been dubbed the “Ice Age,” as great continental glaciers covered most of Canada and much of the northern United States during that time. The ice sheets did not extend into Colorado, but the cooler climate produced alpine glaciers in many of the mountain ranges. The glacial meltwater accelerated ongoing erosional downcutting leaving terrace deposits in several mountain valleys. With the end of the glaciation, about 10,000 years ago, wind-related (or eolian) deposits covered large parts of the plains of eastern Colorado. Sediments carried by these rivers form the unconsolidated alluvial aquifers.

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