Osage River, a river in Kansas and Missouri. It is formed by the junction of the Little Osage and Marais des Cygnes rivers in east-central Kansas. From its source it flows generally eastward, and empties into the Missouri River near Jefferson City, Missouri. The Osage and Marais des Cygnes system is 500 miles (800 km) long. The waters of the Osage are impounded by Bagnell Dam, forming Lake of the Ozarks in central Missouri.