The Chariton River is a 218-mile-long (351 km) tributary to the Missouri River in southeast Iowa and northeast Missouri. The river forms in southeastern Clarke County, Iowa. It is dammed at 11,000-acre (45 km2) Rathbun Reservoir in Appanoose County, Iowa and then flows 30 miles (48 km) before entering Missouri where it forms the boundary between Putnam and Schuyler counties. It enters the Missouri River in Chariton County near Keytesville. 112 miles (180 km) are in Missouri and 106 miles (171 km) are in Iowa. The river has been called Missouri's "Grand Divide" because streams west of the Chariton flow into the Missouri and streams east of it flow into the Mississippi River.