Bull Lake Reservoir is located on Bull Lake Creek about 40 miles northwest of Riverton, Wyoming. It lies in a glaciated valley that formerly held a smaller natural lake. The dam and reservoir overlie a series of morainal and lacustrine deposits. Surrounding valley walls are covered with a veneer of glacial debris overlying rocks ranging in age from Mississippian near the mountain front to Tertiary near the dam. Landslides have developed an the slopes in both the glacial deposits and in the underlying bedrock.
Near the left abutment of the dam boulders from a Pleistocene till deposit occasionally fall into the parking area. They are more of a nuisance than a hazard. Along the south side of the reservoir near the dam wave cut banks that continually ravel material into the reservoir have formed in a similar till deposit. No significant changes from previous visits were noted at either site.
A large slide which involves bedrock is located on the south valley wall about four miles upstream from the dam. It consists of a series of interconnecting rotational slumps that include the Frontier and Cody Formations of Cretaceous age. These formations are composed of dark gray to black, incompetent marine shales that dip 10 to 15 degrees to the northeast. The direction of movement of the slide is to the northwest, or normal to bedding dip. The unstable area occurs on a 5-to-I slope, is 2,500 feet wide, 3,000 feet long, and probably exceeds 20 feet in thickness. Volume of the mass is probably greater than 5 million cubic yards. Similar landslides occur around other Bureau impoundments in Wyoming and Montana. Documented movement of the slide masses is limited to intermittent creep that is usually activated.by increasing moisture conditions or lowering reservoir water levels. Rapid movement of the slides has not been recorded at any of the sites.