Niobrara (nĪəbrârˈə) [key], river, c.430 mi (690 km) long, rising in the High Plains, E Wyo., and flowing E across N Nebraska to the Missouri River on Nebraska's northeast border. The Mirage Flats irrigation project uses water impounded by Box Butte Dam (completed 1946). A scenic 76-mi (122-km) portion of the river comprises Niobrara National Scenic River (see National Parks and Monuments, table).
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