Little Missouri.1. River, c.145 mi (230 km) long, rising in the Ouachita Mts., SW Ark., and flowing generally SE to join the Ouachita River N of Camden. North of Murfreesboro is Narrows Dam (1950), which impounds Lake Greeson. 2. River, c.560 mi (900 km) long, rising in NE Wyo. and flowing NE into Garrison Reservoir on the Missouri River, W N.Dak. It flows through Theodore Roosevelt National Memorial Park.