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The Shadehill Unit of the Pick-Sloan Missouri Basin Program which consists of Shadehill Dam and Reservoir is located about 12 miles south of Lemmon, South Dakota in the northwestern part of the state, immediately downstream from the confluence of the North and South Forks of the Grand River. Current benefits provided by the reservoir are flood control, fish and wildlife conservation, recreation, and silt detention. Shadehill Dam and Reservoir was also constructed to provide irrigation benefits but due to the salinity levels of the water, the irrigation distribution system was never constructed. The soil and water were incompatible and there are currently no contracts for water supply from the reservoir. Shadehill Dam and Reservoir lands are owned by the Bureau of Reclamation. Reclamation is responsible for operation and maintenance of the dam, outlet works, and spillway. The administration of the reservoir lands and recreation management programs are contracted to South Dakota Game, Fish & Parks.
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The dam and associated dikes are rolled-fill earth structures. The dam has a crest length of 12,843 feet and a maximum structural height of 145 feet. The two dikes have a total length of 1,360 feet with a maximum structural height of 83 feet. The reservoir has an active capacity of 81,443 acre-feet. The uncontrolled service spillway is a reinforced concrete conduit, 13.5 feet in diameter, with a morning-glory inlet, and was designed to accommodate a maximum flow of 5,700 cubic feet per second. The controlled outlet works are a horseshoe-shaped concrete conduit with a capacity of 600 cubic feet per second. The original outlet works was modified in 1960 to provide a 600-cubic-foot-per-second-capacity outlet extension to the Grand River. This modification provides additional flood control capacity and permits flood control releases to the Grand River when the reservoir water level is below the crest of the spillway. An open-cut unlined emergency spillway with a bottom width of 1,500 feet and capacity of 127,000 cubic feet per second is located immediately upstream from the left abutment. The spillway discharges into Flat Creek, a tributary of the Grand River downstream from Shadehill Dam. The drainage basin area above Shadehill Dam is about 3,120 square miles.
Shadehill Dam and Reservoir are operated and maintained by the Bureau of Reclamation.
The early settlers in this area were cattlemen who established widely separated ranches on the open range. Major settlement took place from 1900 to 1910, after the railroad was extended into the region. Favorable crop yields and prices stimulated farming operations, and most of the lands which appeared suitable for tillage were brought under cultivation. Recurring years of deficient precipitation, combined with submarginal quality of much of the cultivated land, contributed toward the abandonment of much of the crop land for tillage.
In 1938, following urgent requests by local residents as result of several extreme drought years, the Bureau of Reclamation made an inspection of the Shadehill Unit area which led to the initiation of further investigations.
Investigations leading to the development of the Shadehill Unit began in 1931. In the course of studies made by the Corps of Engineers, a topographic survey was made of the reservoir site on the Grand River immediately downstream from the confluence of the North and South Forks. This was followed by a reconnaissance survey by the Bureau of Reclamation in 1938-1939 to determine the quality of available water and conduct limited field investigations. In 1941, while the Indian Service was preparing plans for the construction of Blue Horse Dam, approximately 30 miles downstream from Shadehill Dam, the Bureau of Reclamation was making further studies for a reservoir at the Shadehill site. It was jointly agreed that the Shadehill Unit would be the preferable initial development as it offered a greater probability for rapid conversion to irrigation. The Indian Service would participate in the cost and receive part of the stored water for use on reservation lands.
A water supply and quality study of the Grand River and detailed engineering field surveys of the dam and reservoir site were carried out from 1945 to 1948. Land classification surveys of potentially irrigable lands that could receive irrigation water service from Shadehill Reservoir were also undertaken during this period.
The unit was authorized by the Flood Control Act of December 22, 1944, Public Law 534, which approved the general comprehensive plan set forth in Senate Document 191 and House Document 475, as revised and coordinated by Senate Document 247, 78th Congress, 2nd session.
Construction of Shadehill Dam began April 19, 1949, and was completed August 15, 1951. The first impoundment of water was made on July 1, 1950.
Facilities provided for outdoor recreation around the reservoir include picnic sites, campgrounds, swimming beaches, areas for seasonal use cabins, and a marina. All recreational areas and facilities, including the fishery, are administered by the South Dakota Department of Game, Fish, and Parks.
Flood control benefits are provided by the the exclusive flood control capacity of 217,708 acre-feet and surcharge capacity of 111,203 acre-feet for a total flood capacity and, as of 1998, has reduced downstream flood damages by $8.8 million.