Mann Creek Dam, on Mann Creek about 13 miles northeast of Weiser, Idaho, is a zoned earth and rockfill structure with a crest length of 1,176 feet and height of 148 feet above bedrock. The crest width of 30 feet with guardrails along each shoulder provides a crossing for the relocated Monroe Creek road. A reinforced concrete morning-glory spillway inlet that connects to a reinforced concrete conduit, 11 feet in diameter, terminates in a concrete stilling basin located along the right abutment. The outlet works, located along the left abutment, is also of reinforced concrete. The flow of water is controlled by two 2.25-foot-square high-pressure gates located in the control house and one 2.75-foot-square high-pressure gate located in the gate chamber for emergencies. Mann Creek Reservoir, at the normal water surface elevation of 2,889.0 feet, is 1.8 miles long, has a surface area of 283 acres, and at closure had a total capacity of 13,000 acre-feet (active 11,100 acre-feet). There are separate outlets that branch off the main outlet works pipe to supply water to the Joslyn Ditch and the Mann Creek Ditch.
A 1992 sedimentation survey estimated that Mann Creek Reservoir now has a total capacity of 12,500 acre-feet (active 10,900 acre-feet).
The construction which forms the dam is due to the presence of a succession of basaltic flows, ash lenses, and layers of bentonitic tuff. The basalt flows are considerably harder and more durable than the lake sediments which are found along the valley both upstream and downstream from the damsite. They vary in thickness from 3 feet to 40 feet, dip very slightly downstream, and are composed of dark, fine-grained, dense, fresh, hard basalt. Contacts between the flows are generally well-bonded and quite impermeable. The volcanic tuff, although it has a tendency to slack and swell slightly upon exposure to air and water, is moderately hard and very impermeable in place.
Along the valley floor at the dam, bedrock is concealed by stream deposits varying in thickness from 4-1/2 feet to 14 feet. The overburden consists of a mixture of silt, sand, gravel, and boulders. With depth, the boulders become dominant; several of them are 18 inches to 24 inches in diameter. Most of the over burden is fairly permeable.
The abutments are covered by an irregular mantel of soil and talus, varying in depth from a few inches to possibly 35 feet, consisting of a compact mixture of large basalt blocks, small angular fragments, and clayey silt.
Bedrock in the dam and reservoir area is the Payette formation, an enormously thick series of lakebeds and intercalated volcanic products consisting of basalt, volcanic ash, and tuff. The lakebeds, making up most of the reservoir floor and sides, are thin-bedded, semi consolidated silts, clays, limestones, marls, diatomaceous earth, and sandstones. They are extremely water tight and water losses from the reservoir should be negligible.