The Wabash River is a tributary of the Ohio River, located in the Midwestern United States. It is the largest northern tributary of the Ohio and is approximately 503 miles long. The Wabash River forms the state boundary between Illinois and Indiana and flows through the states of Indiana, Ohio, and Illinois. The Wabash River is an important source of water for many cities and towns along its course, including Terre Haute, Vincennes, and Lafayette.