Cheyenne, river, 527 mi (848 km) long, rising in E Wyo. and flowing NE to the Missouri River near Pierre, S.Dak. The Cheyenne basin is part of the Missouri River basin project. The U.S. Bureau of Reclamation has established a project on the Belle Fourche River, the Cheyenne's main tributary; the Rapid Valley irrigation project in the Cheyenne valley; and the Angostura Dam, for irrigation, hydroelectric power, and flood control, on the Cheyenne itself.
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