Hyrum Dam and Reservoir are located on the Little Bear River just south of Hyrum City. The dam is a rolled earth and rockfill structure containing 352,000 cubic yards of earthfill, 62,000 cubic yards of rockfill, and 13,000 cubic yards of riprap and gravel blanket, for a total of about 430,000 cubic yards of material. The construction period was 1934 to 1935. The date of closure (first storage) was April 1, 1935, and the first water was made available in July 1935. The dam has a maximum base width of 600 feet, a maximum structural height of 116 feet, a hydraulic height of 82 feet (from the streambed to the maximum water surface elevation), a crest width of 35 feet, a crest length of 540 feet, and a crest elevation of 4,680 feet. The maximum water surface elevation of this reservoir is 4,672.0 feet (top of the spillway radial gates). The dam is located near the southwest corner of Hyrum City and creates a reservoir with a total capacity of 18,685 acre-feet, and a surface area of 480 acres. The maximum capacity of the outlet works is 300 cubic feet per second (cfs) at a water surface elevation of 4,666.0 feet. Water flows from the reservoir through the outlet works in a concrete lined pressure tunnel, located in the right abutment, controlled by two sets of 33 inch square high pressure slide gates in the gate chamber. Two steel pipes continue to the outlet well.
Hyrum Reservoir is impounded within and upon Lake Bonneville sediments. These sediments consist of interbedded low plasticity silt and clay, fine to course, subrounded to angular sand, and fine to course subrounded to angular gravel.
These Lake Bonneville sediments are basically flat lying, but are locally crossbeded.