The Osage River is a tributary of the Missouri River, 360 mi (579 km) long, in central Missouri in the United States. The Osage River is one of the larger rivers in Missouri. The river drains a mostly rural area of 15,300 sq mi (39,600 km²). The watershed includes an area of east central Kansas and a large portion of west central and central Missouri where it drains northwest areas of the Ozark Plateau. The river flows generally southeasterly in Kansas before turning easterly in Missouri then northeasterly for the final 80 miles where it joins the Missouri River. 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.