East Fork San Juan River Landslide
On May 14, 2008, the Colorado Geological Survey (CGS) was contacted about a significant landslide event on US Forest Service property in Archuleta County. The slide is named the East Fork Slide, in reference to its location 2 miles up the East Fork of the San Juan River from the point where it joins the West Fork along US Hwy 160. This location is 12 miles northeast (upstream) from Pagosa Springs.
The active slide comprised about 35 acres, but was just one within a larger complex of older earthflow-type landslides on the hill covering approximately 350 acres. The older landslide goes uphill toward a 300 ft high head scarp of volcanic bedrock. The toe of the active landslide was pinching the East Fork of the San Juan River, creating the possibility of blocking the river channel for up to 600 ft in length. After reviewing all of the data it seemed clear that this partial reactivation of an older, large landslide complex was a threat to the East Fork of the San Juan River.
Excel Energy maintains a major natural gas pipeline in the East Fork Valley. This pipeline was compromised during the initial landslide movement. An Excel Energy geotechnical geologist monitored the landslide for additional movement with a series of 50 lath markers across the toe of the landslide. They noted that the slide moved 50 ft horizontally in two weeks time. Excel responded by excavating their 8-inch pipeline and temporarily replacing it with an above ground flexible 4-inch line that could be moved uphill daily until the slide activity ended.
A copy of the May 18, 2008 East Fork Landslide Report is downloadable here.