Stormwater runoff is excess water associated with a rain or snow storm event that flows over the land surface and is measurable in a downstream river, stream, ditch, gutter, or pipe. From a regulatory perspective, stormwater is managed through some sort of engineered conveyance and is focused on specific pollutants. Hydrologically, stormwater also includes water that is infiltrated into the subsurface and contributes to increased stream discharge.

Urbanization and development causes changes to the natural hydrologic system in a watershed. Alterations in land use and land cover for agriculture, buildings, roads, and other urban infrastructure result in loss of vegetation and topsoil. These changes and the construction of a drainage network alter the hydrology of the impacted area producing radically different flow regimes than the pre-development hydrology. The developed landscape results in a reduction of infiltration and evapotranspiration functions of the soil and vegetation, such that stormwater flows rapidly across the land surface discharging into streams in short, concentrated bursts of high flows. When combined with pollutant sources, increased stormwater runoff leads to water quality and habitat degradation. Stormwater has been identified as a leading source of pollution for all waterbody types in the United States (EPA, 2007).

Traditional stormwater practices were developed with flood control in mind and promote collection and conveyance of precipitation from all storms away from the site to prevent property flooding. This has the unintended consequences of conveying water from small storms out of the watershed, concentrating pollutants, causing stream channel impacts, and depleting groundwater recharge. Local governments with their dual responsibility of land use planning and stormwater management have direct control over stormwater runoff impacts. CGS studies and investigations identify and document stormwater management technologies and practices that may be implemented locally to protect and conserve water resources through mitigation of detrimental impacts caused by land disturbances and modifications associated with land development.

Read the full report: Managing Stormwater to Protect Water Resources in Mountainous Regions of Colorado