O'Sullivan Dam, one of the larger zoned earthfill dams in the United States, is on Crab Creek about 15 miles south of Moses Lake. The 27,800-acre Potholes Reservoir formed by this dam collects return flows from all irrigation in the upper portion of the project for reuse in the southern portion. Active storage capacity of the reservoir is 332,200 acre-feet. A system of wasteways has been built on both the West and East Low Canals to provide operational safety for the canals and a means of delivering water into Potholes Reservoir to supplement the natural and return flows
Bedrock in the area of O'Sullivan Dam consists principally of basalt flows and interflow zones of the same general type as encountered elsewhere in the Columbia basin project. Exposures of Ringold formation sediments occur in Lind Coulee to the east of the dam site area, but not within the actual foundation area of the dam. Overburden within the damsite area consists of a thin, discontinuous cover of windblown silt and sand which, together with sand and gravel, partially fills many of the potholes and channels cut into the basalt flows. Several of the channels in the southwesterly portion of the area contain deposits of sodium sulfate derived from the evaporation of the seasonal lakes. A predominant terrace of sand and gravel overlies the basalt and Ringold sediments to the north. Toward the northwest the surface of the reservoir area is covered by superficial deposits of sand gravel and windblown sand. Beneath these overburden materials are the Ringold sediments and the basalt flows. The basalt flows of the dam site area range from near horizontal to gently warped except for a narrow zone of disturbance known as the Lind Coulee Flexure. This zone was characterized by faulting and sharp folding, trends in a rather irregular fashion in a general east-west direction through the dam site area.