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downstream flooding. Also, soil and debris from land- slides can "bulk" or add volume to otherwise normal stream flow or cause channel blockages and diversions creating flood conditions. Finally, landslides can negate \ the protective functions of a dam by reducing reservoir capacity and creating surge waves that can overtop a dam, resulting in downstream flooding (as mentioned above). In turn, flooding can cause landsliding. Erosion, due to rapidly moving flood waters, often undercuts slopes or cliffs. Once support is removed from the base of saturated slopes, Iandsliding often ensues. Examples of interrelated landslide/flood events in Colorado occurred: . in Larimer County, 1976: the mountain torrent flood in the Big Thompson Canyon; . in Ouray (Ouray County), 1981 and 1982: debris flows in the creeks feeding into the Uncompahgre River (Canyon, Cascade, Portland, etc.) and flooding in the Uncompahgre River itself; . in Telluride (San Miguel County), 1914 and 1969: debris flows in Coronet Creek and flooding in the San Miguel River: 1080 1060 . near Lake City (Hinsdale County): Lake San Cristobal was formed in prehistoric time when a large earthf10w (the Slumgullion) dammed the Lake Fork of the Gunnison River. The landslide dam held, and the lake was formed. Landsliding as Related to Seismic Activity Most of the mountainous areas of Colorado that are vulnerable to landslides have also experienced moderate levels of seismicity in historic times. Recent studies by the Colorado Geological Survey indicate that the poten- tial for earthquakes in the state is even greater than previously suspected. The occurrence of earthquakes in landslide-prone areas greatly increases the likelihood that landslides will occur and amplifies the risk of serious damage to a level considerably higher than if the processes occurred separately. Comparison of Kirkham and Rogers' 1987 map of historic earthquake intensities (Figure 13) with Wiggins' 1978 map of landslide potential (Figure 1) emphasizes the relationship between landslide-prone 3rtdearthquake-prone areas in Colorado. 1040 -Modified Merealli Intensity o , . MMI* VI I.~ MMI V D MMI IV or less '00 , 7 MMI where greater than contour interval 50 , Figure 13. Maximum historical earthquake intensities in Colorado (Kirkham and, Rogers. 1987). Miles 20