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Keyhole Unit of the Pick-Sloan Missouri Basin Program, consisting of Keyhole Dam and Reservoir, is on the Belle Fourche River about 17 miles northeast of Moorcroft, Wyoming. Keyhole Reservoir is a multipurpose facility that provides storage for irrigation, flood control, fish and wildlife conservation, recreation, sediment control, and municipal and industrial water supply.
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Keyhole Reservoir provides a supplemental water supply to the 57,068-acre Belle Fourche Project located about 146 miles downstream in western South Dakota. Water for the project is released into the Belle Fourche River and then diverted for project purposes. Supplemental water also is furnished to the Belle Fourche-Wyoming Water Association for approximately 7,523 acres of privately developed land and associated facilities around the reservoir and downstream from the dam for a distance of about 25 miles. Contractual arrangements for furnishing water to a housing development near the reservoir are pending.
The apportionment of inflows to Keyhole Reservoir is set forth in the provisions of the Belle Fourche River Compact negotiated by the States of Wyoming and South Dakota and confirmed by the Congress in 1944 (58 Stat. 94).
The compact provides that the unappropriated flow of the Belle Fourche River, as of the date of the compact, shall be allocated 10 percent to Wyoming and 90 percent to South Dakota, provided that Wyoming shall have unrestricted use for domestic and stock water purposes. Wyoming can purchase 10 percent of the storage capacity of Keyhole Reservoir to regulate its portion of the unappropriated water.
The primary feature of the unit is Keyhole Dam across the Belle Fourche River, which forms a reservoir with a total capacity of 334,200 acre-feet and water surface of 13,700 acres.
Keyhole Dam is a zoned earthfill structure with a maximum structural height of 168 feet, and a crest length of 3,420 feet, including the 2,120-foot dike extension of the right bank.
The spillway is a vertical slot-type uncontrolled concrete crested structure with a concrete-lined open channel located in the dike near the right abutment of the dam. The spillway has a crest length of 19.25 feet topped by a concrete bridgedeck on the crest of the dam. Spillway discharge capacity is 11,000 cubic feet per second.
The outlet works consists of a concrete-lined horseshoe shaped tunnel 9.5 feet wide by 8.25 feet high through the left abutment. Release capacities of about 1,480 cubic feet per second are controlled by two vertical lift, high-pressure hydraulic slide gates.
Keyhole Dam and Reservoir are operated and maintained by the Bureau of Reclamation.
French trappers settled in the vicinity of Belle Fourche and, reportedly, engaged in fur trading with the Indians as early as 1854. Settlement in the area began with the gold rush to the Black Hills in 1876. Livestock became the principal industry in the general area. The Chicago and Northwestern Railroad reached the city of Belle Fourche in 1891 and, for the remainder of the 19th century, the city was considered the largest original shipping point for livestock in the United States.
Belle Fourche Project lands which are provided water service directly from the Belle Fourche River and then to the Inlet Canal without advantage of regulatory storage had been plagued with repeated and prolonged water shortages since the project was first irrigated in 1908. The intense and extended drought of the 1930`s further emphasized the inadequate water storage and supply facilities.
Several investigations of storage possibilities on the main stem of the Belle Fourche River and its tributaries were made during 1917 through 1941 by the Bureau of Reclamation and the Corps of Engineers. These studies showed that the Keyhole site was the most favorable for construction of a dam to provide improved water supplies to the project. Subsequently, it was included in the Missouri River Basin Project plan.
Keyhole Unit was authorized by the Flood Control Act of 1944 (Public Law 534, 78th Congress) which approved the general comprehensive plans set forth in Senate Document 191 and House Document 475 as revised and coordinated by Senate Document 247, 78th Congress. Initial funds for construction were provided by the Second Supplemental Appropriation Act, 1948 (Public Law 299, 80th Congress).
Construction of Keyhole Dam began on June 29, 1950 , and was completed on October 25, 1952. Impoundment of water began in March 1952.
Supplemental supply of stored water is provided to the Belle Fourche Project and the Belle Fourche-Wyoming Water Association which have 57,068 acres and 6,157 acres of irrigated land, respectively. Under present conditions, the integrated irrigation-dryland farms produce alfalfa, corn, small grains, pasture, and livestock. The principal products are used to provide a stable feed supply for stock-cow herds and sheep and fattening of cattle and lambs.
Numerous facilities associated with outdoor recreation provided at Keyhole Reservoir include picnic grounds, campgrounds, boat ramp developments, swimming beach, and scenic overlooks. The recreation areas are administered by the Wyoming Recreation Commission. The warm-water fishery at the reservoir is maintained by the Wyoming Game and Fish Department.
Flood control benefits in Keyhole Reservoir are provided by use of an exclusive flood control capacity of 140,462 acre-feet and a surcharge capacity of 294,810 acre-feet for a total flood capacity of 435,810 acre-feet and, as of 1998, the reservoir has reduced flood damages by about $3.6 million.