The Osage River is a 276-mile-long (444 km) tributary of the Missouri River in central Missouri in the United States. The Osage River is the 8th largest river in Missouri. The river drains a mostly rural area of 15,300 square miles (40,000 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 easterly, then northeasterly for the final 80 miles (130 km) where it joins the Missouri River. It is impounded in two major locations. Most of the river has been converted into a chain of two reservoirs, the Harry S. Truman Reservoir and the Lake of the Ozarks.