Rye Patch Dam lies within a valley cut by the Humboldt River. The materials forming the valley slopes at the damsite are variably consolidated Lahontan and pre-Lahontan lacustrine and fluviatile deposits consisting of very thinly-bedded to thickly-bedded gravel, sand, silt, and clay. Nearly continuous outcrops of the deposits occur in the valley slopes.
The deposits are nearly horizontal except for occasional local dips of 20 to 30 degrees south, southwest, and northwest along undulating erosional surfaces. Locally, Holocene age slopewash (Qs) forms a 1- to 10-foot-thick clay and silt deposit on the lower half of the Humboldt River valley slopes.
The Lahontan deposits lie above elevation 4160 on the left abutment of the dam and above elevation 4200 on the right abutment of the dam and are not part of the abutments. The pre-Lahontan deposits (Qpl), which form the right and left abutments, include the Paiute, Rye Patch, and Lovelock Formations.
A volcanic ash bed and a calcium-carbonated-cemented gravel bed (5 feet thick) crop out in the upper Rye Patch Formation.
Since Lake Lahontan receded from the Rye Patch Dam area about 10,000 years ago, the Humboldt River has eroded the present valley and deposited up to 40 feet of unconsolidated alluvial silt and sand (Qa). This alluvium has been divided into six subunits based on physical characteristics. The central portion of the dam is founded on this alluvium.
A small shear zone was revealed during dam construction. This feature was attributed to minor slumping of pre-Lahontan deposits in the valley slopes. No faults are recognized in the foundation or abutments. No landslides or other major surficial slips are known to exist in the reservoir area. No fault displacement of Lahontan or pre-Lahontan deposits are known to exist in the immediate area of the dam.
