Ohio, a river of the United States, one of the largest branches of the Mississippi. It was named by the French explorers La Belle Riviére (The Beautiful River). It is formed by the union of the Allegheny and Monongahela Rivers at Pittsburg, Pa., and flows southwest 975 miles, joining the Mississippi at Cairo, Illinois. It is from 400 to 3,000 feet wide, spreading out so as to become quite shallow in dry seasons. It is subject to floods from the accumulation of snows in the mountains near its headwaters; there is a series of terraces along the banks, which have evidently been the bed of the river. The boundary between Ohio, Indiana and Illinois on the north and West Virginia and Kentucky to the south is formed by the Ohio. At Louisville are falls, which are passed by means of a ship-canal. The river is navigable for its entire length, and carries enormous fleets of boats laden with coal, besides other products of the regions.