The Cossatot River is an 89-mile-long (143 km) river in Howard, Polk and Sevier counties in the U.S. state of Arkansas. The Cossatot begins in the Ouachita Mountains southeast of Mena, Arkansas. The river flows through the Ouachita National Forest and then in a generally southerly direction until it empties into Gillham Lake. After exiting Gillham Lake the river becomes leisurely until it joins with the Little River at the site where it empties into Millwood Lake. The World War II fleet oiler USS Cossatot (AO-77) is named after this river.