The Osage River is a tributary of the Missouri River, 360 mi (579 km) long, in central Missouri in the United States. The largest river entirely in Missouri, it drains a rural area of 15,300 sq mi (39,600 km²) on the north edge of the Ozark Mountains west to east across Missouri, with its watershed stretching into eastern Kansas. It is impounded in two major locations such that most of the river has been converted into a chain of two reservoirs, the Harry S. Truman Reservoir and the Lake of the Ozarks.