Conconully Dam is a hydraulic earthfill structure that when completed in 1910 was 70 feet high, and contained 359,000 cubic yards of fill. In 1920, the dam was raised 2.5 feet. Total capacity of the reservoir is 13,000 acre-feet (active 13,000 acre-feet). During 1968-69, the crest of the dam was repaired with new embankment materials and riprap. The old open-chute concrete spillway that had an inadequate capacity of 6,000 cubic feet per second was replaced with a concrete-baffled apron spillway that has a capacity of 11,580 cubic feet per second.
Conconully is located in Okanogan Highlands a mountainous district bordered by the Columbia Plateau to the south and the Cascade Mountains to the west, and includes the Colville Mountains which apparently represent a continuation of the higher mountains in British Columbia. The damsite is located in the southern end of the glaciated valley. The effect of the glacial ice was to scour out a depression below the surrounding area much like an enclosed valley. Granite rock generally forms the right wall of the valley; however, at the damsite elevation there exists a short ridge of metamorphic sedimentary rock. The left abutment and left valley wall are formed in metamorphic sedimentary rock. The valley floor is composed of deposited glacial and alluvial materials such as silt, clay, sand and gravel, as well as boulders and blocks of rock. The spillway is located in the rock ridge of the right abutment. The outlet works was excavated through the metamorphic sedimentary rock comprising the left abutment. The rock is gneiss; light to medium gray on a fresh surface it weathers and oxidizes to a grayish brown. The rock is hard where unweathered and moderately jointed.