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The Angostura Unit of the Pick-Sloan Missouri Basin Program is in the Great Plains region at the southeast edge of the Black Hills in southwestern South Dakota. Angostura Dam and Reservoir, located on the Cheyenne River about 9 miles southeast of Hot Springs, South Dakota, provides multipurpose benefits, including irrigation, flood control, fish and wildlife conservation, recreation, and sediment control. The unit lies within Custer and Fall River Counties of South Dakota.
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The primary function of the Angostura Unit is to impound and deliver a full supply of irrigation water for production of forage and grain crops. The unit lands, consisting of 12,218 acres extending along the Cheyenne River approximately 24 miles downstream from the dam, are served by the Angostura Canal. The canal has a design capacity of 290 cubic feet per second. With a 29.7-mile-long alignment along the southerly edge of the unit lands, the canal crosses the Cheyenne River through a 9,800-foot-long inverted siphon to the north side of the river. Unit lands are served by 39 miles of laterals and 34 miles of open and closed drains.
The prime features of the unit are Angostura Dam across the Cheyenne River, and Angostura Reservoir.
The dam is a composite type, consisting of a concrete gravity structure and an earth embankment. The concrete portion of the dam comprises a gated spillway section located in the river channel and two nonoverflow sections, one extending to the left abutment and one abutting the earth embankment extending to the right abutment. The dam has a crest length of 2,030 feet; the concrete section is 970 feet long and the earth embankment 1,060 feet long, with a structural height of 193 feet and a hydraulic height of 136 feet above the riverbed.
The spillway is an overflow section in the concrete portion of the dam, controlled by five 50- by 30-foot radial gates. Discharge capacity is 247,000 cubic feet per second. The river outlet works consists of a 4.5-foot diameter steel conduit through the concrete dam section, controlled by one 4-foot-square high-pressure slide gate in the valve house at the downstream end. Discharge capacity is 590 cubic feet per second. The main canal outlet works for irrigation water delivery consists of a 6-foot-diameter steel conduit through the concrete dam terminating in a valve house, a stilling basin, and canal headworks at the downstream end. Releases into the 290-cubic- foot-per-second-capacity canal are controlled by two 3.5-foot-square high-pressure slide gates in the valve house.
Angostura Unit, including Angostura Dam and Reservoir and the associated project irrigation facilities, has been operated and maintained by the Angostura Irrigation District, since January 1, 1968.
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Dry farming by settlers in the Angostura Unit area began about 1880. The gold rush to the Black Hills in 1876, followed by several years of adequate precipitation and accompanying good crop production, stimulated development in the area. Severe drought conditions and heavy infestations of grasshoppers in the late 1880`s contributed to mortgage foreclosures, resulting in fewer landowners with larger land holdings.
Irrigation potential along the Cheyenne River in South Dakota prompted a field reconnaissance of the Angostura Unit by South Dakota in 1913. On the basis of the reconnaissance report and local organizational support, the State legislature appropriated funds for one-half the cost of a survey by the Reclamation Service made in 1917-1918. A Bureau of Reclamation report in 1939 cited 16,200 acres of land suitable for irrigation development. Continuing local interest for development of the unit area for resettlement and rehabilitation of distressed farmers prompted authorization of the Angostura Unit under the Water Conservation and Utilization Act of August 11, 1939.
The Angostura Unit was included in Senate Document 191, 78th Congress, 2nd session, and was reauthorized by the Flood Control Act of 1944, Public Law 534.
Construction of Angostura Dam began on August 23, 1946, and was completed on December 7, 1949. The first delivery of irrigation water was made in 1953.
A full supply of irrigation water is provided to the 12,218 acres of irrigable land. Alfalfa and corn are the principal crops, along with wheat, barley, oats, pasture, and forage.
Activities associated with outdoor recreation around Angostura Reservoir include picnic sites, campgrounds, marinas, swimming beaches, and areas for seasonal use cabins. All recreation areas and facilities, including the fishery in the reservoir, are administered by the South Dakota Department of Game, Fish, and Parks.
For specific information about recreational activities click on the name below.
There is no exclusive flood capacity in Angostura Reservoir; however, flood control benefits are provided by the use of conservation capacity, as available, and the surcharge capacity of 56,360 acre-feet above the top of the radial spillway gates. As of 1998, Angostura Reservoir has prevented $21,000 in flood damages.