Gibson Dam, the principal structure of Sun River Project, is on the Sun River, 70 miles west of Great Falls, Montana. It is a concrete arch dam 199 feet high and contains 167,500 cubic yards of concrete. A drop inlet spillway located just upstream from the north end of the dam has a capacity of 30,000 cubic feet per second. The entrance to the shaft and 29.5-foot diameter tunnel is controlled by six 34- by 12-foot radial gates. Outlet structures for the dam are two 72-inch diameter semisteel-lined conduits through the base of the dam. The maximum capacity of the outlets is 3,050 feet per second. Gibson Reservoir has a total capacity of 99,100 acre-feet.
Crystalline limestone in regular beds which strike crosswise of river. Special treatment: cement-grout curtain beneath upstream cutoff trench; spring and left abutment grouted.