Osage, river, c.360 mi (580 km) long, formed by the confluence of the Marais des Cygnes and the Little Osage rivers, W Mo. It flows NE to join the Missouri River near Jefferson City. Bagnell Dam (completed 1931) across the Osage River impounds the Lake of the Ozarks and also provides hydroelectricity. The power produced there is consumed mainly by St. Louis. The Osage River basin project provides for flood control, hydroelectric power, and recreational facilities.