The Wabash River is a 475 mi (765 km) long river in the eastern Unite States that flows southwest from northwest Ohio near St. Henry, Ohio across northern Indiana to Illinois where it forms the southern Illinois-Indiana border before draining into the Ohio River, of which it is the largest northern tributary. From the dam near Huntington to its terminus at the Ohio River, the Wabash flows freely for 661 kilometers (411 miles) which makes it the longest stretch of free-flowing river in the United States east of the Mississippi River.