Ochoco Dam, a hydraulic-fill structure on Ochoco Creek 6 miles east of Prineville, was constructed immediately after World War I as a part of the Veterans Farm Settlement Program undertaken by the State of Oregon. The left abutment is an alluvial fan, and the right abutment is a slide mass consisting of fine earth and rock. The original dam was about 126 feet high and 1,000 feet long, with an average crest width of 15 feet. The dam leaked badly through the main section, with heavy leakage at or through the right abutment. Since the dam was a constant hazard to life and property in the valley and the city of Prineville, some rehabilitation was required. The dam was rehabilitated by the Bureau of Reclamation in 1949 and the reservoir capacity was increased at that time. The dam provides flood control of Ochoco Creek in addition to storing water for irrigation. As repaired and reconstructed by the Bureau of Reclamation, the dam is 152 feet high with a crest length of 1,300 feet. The spillway is an open concrete chute at the south end of the dam.
Landslide (Qls) - This unit is a chaotic mixture of welded rhyolitic blocks up to 20 feet in diameter in a matrix of predominantly granular soil. It extends about one mile right and downstream of the centerline of the dam and several miles upstream. It has high permeability with repeated development of sinkholes along the reservoir shore. This unit is connected with most of the major seepage problems at the dam. It forms the right abutment of the dam and the foundation of the outlet works conduit. This unit contains a Basal Zone (Qlsb) with a significantly high proportion of clay fines and lower permeability. The Basal Zone is the failure surface upon which the landslide rides.
Alluvial Fan/Talus Complex (Qaf) - This unit is a crudely stratified mixture of hard angular cobbles and boulders in a matrix of gravelly silt and clay. It is generally semipervious but can transmit significant seepage in some areas. It forms most of the foundation of the spillway and most of the left abutment of the dam.
Slopewash (Qsw) - This unit is composed of local residual and accumulated deposits of angular cobbles and boulders with a matrix of gravel, sand, silt and clay on the left abutment.
Alluvium (Qal) - This unit is composed of gravel and sand deposits of Ochoco Creek with some interbeds of silt and clay and occasional cobbles. The permeabilities range from about 200 to 7,000 ft/yr. Upper and lower zones form local aquifers with the lower zone being artesian to the upper zone.
Fluviolacustine Sediments (Qfs) - This unit is predominantly fine-grained silt and clay with minor amounts of sand and gravel. The unit behaves as an aquitard between the upper and lower alluvium. This unit forms the foundation for the maximum section of the dam.
Mud/debris Flow (Qtmf) - This unit is about 75 percent cobbles and boulders to a maximum dimension of 12 feet in a matrix of dense sandy silt. It was found exposed on the left abutment during excavation for the spillway stilling basin.
John Day Formation (Tjdt) - This is the local bedrock. It is moderately hard to hard lapilli tuff and is generally lightly weathered but with zones of intense weathering. It is moderately to intensely fractured. Unconfined compression tests ranging from 130 to 3,000 psi.