Wasco Dam is a 59-foot-high zoned earthfill structure that contains 57,000 cubic yards of material. The crest is 20 feet wide and 415 feet long. The reservoir behind the dam has a total capacity of 13,100 acre-feet (active 11,900 acre-feet) and a surface area of 557 acres. The outlet works consists of 20-foot-wide approach and outlet channels, a submerged vertical intake structure, a single rectangular 4- by 5-foot conduit, a gate chamber with two 3-foot-square slide gates and two overflow weirs, and a 56-foot-long stilling basin. The slide gates are operated manually by two lifts on top of the gate chamber structure at the crest of the dam. The emergency spillway, crossing the left abutment, consists of an unlined channel with a base width of 30 feet. A concrete overflow grade wall is in the spillway 10 feet upstream of the axis of the dam.
In October 1997, a restriction limiting reservoir operation to elevation 3505.0 feet was imposed due to seepage on the face of the dam which occurs above this elevation. This 9-foot restriction reduces the available reservoir capacity to 7,000 acre-feet.
Wasco Dam is in a narrow valley with steep slide slopes. The left abutment juts out into the valley and at a height of 50 feet above the stream flattens off into a bench. Ice or glacial deposits, causing ice or glacial melt water to discharge once blocked the stream channel over a bench a few hundred feet behind the left abutment. A channel developed which extends from this rock bench to a point in the present channel a few hundred feet downstream from the dam site. Bedrock at the lowest point of the old channel is about 15 feet above the reservoir flow line. Overburden at the flow line on the right abutment consisted of 28 feet of gravel with excess sand and silt. Boulders were hard, fresh andesite 2 to 3 feet in diameter. Much of the over burden between flow line and the valley flow was undoubtedly quite permeable.