OHIO RIVER, the principal eastern tributary of the Missis sippi river, U.S.A. It is formed by the confluence of the Alle gheny and Monongahela rivers at Pittsburgh (Pa.), and flows north-west nearly to the west border of Pennsylvania, south south-west between Ohio and West Virginia, west by north be tween Ohio and Kentucky, and west-south-west between Indiana and Illinois on the north and Kentucky on the south. It is the largest of all the tributaries of the Mississippi in respect to the amount of water discharged (an average of about 158,000cuit. per sec.), is first in importance as a highway of commerce, and in length (967m.) as well as in the area of its drainage basin (ap proximately 21o,000sq.m.) it is exceeded only by the Missouri. The slope of the river at low water ranges from 'ft. or more per mile in the upper section to about 0.75ft. per mile in the middle section and 0.29ft. per mile in the lower section, and the total fall is approximately 50oft. The greatest falls are at Louisville, where the river within a distance of 2.25m. descends 23.9ft. over an irregular mass of limestone. The ordinary width of the upper half of the river is uniform, from 1,200 to 1,5ooft. Islands are numerous and vary in size from an acre or less to 5,000ac.
Besides its parent streams, the Allegheny and the Monon gahela, the Ohio has numerous large branches. On the north it receives the waters of the Muskingum, Scioto, Miami and Wabash rivers, and on the south those of the Kanawha, Big Sandy, Licking, Kentucky, Green, Cumberland and Tennessee.
The drainage basin of the Ohio, in which the annual rainfall averages about 43in., is especially in the southern part of the river, of the "quick-spilling" kind, and as the swift mountain streams in that section are filled, the Ohio rises very suddenly and not infrequently attains a height of 3o to soft. or more above low water level spreads out 10 to 15 times its usual width, sub merges the bottom lands, and causes great damage. In March 1913, an especially destructive flood reached the highest stages on record on that part of the river between Wheeling and Cin cinnati and on the tributaries entering from the north.
Robert Cavelier, Sieur de La Salle, asserted that he discovered the Ohio and descended it until his course was obstructed by a fall (thought to be the falls at Louisville) ; this was probably in 1670, but until the middle of the next century, when its strategic importance in the struggle of the French and the Eng lish for the possession of the interior of the continent became fully recognized, little was generally known of it. By the treaty of 1763 ending the Seven Years' War the English finally gained undisputed control of the territory along its banks. By the treaty of 1783 the entire Ohio country became a part of the United States and by the famous Ordinance of 1787 the north side was opened to settlement. Most of the settlers entered the region by
the headwaters of the Ohio and carried much of their market pro duce down the Ohio and Mississippi to New Orleans.
Until the successful navigation of the river by steamboats a considerable portion of the imports was carried overland from Philadelphia or Baltimore to Pittsburgh. The first steamboat on the Ohio was the "New Orleans," which was built in 1811 by Nicholas J. Roosevelt and sailed from Pittsburgh to New Orleans in the same year, but it remained for Capt. Henry M. Shreve (1785-1854) to demonstrate with the "Washington," which he built in 1816, the success of this kind of navigation on the river. From 1820 to the Civil War the steamboat on the system of inland waterways of which the Ohio was a part was a dominant factor in the industrial life of the Middle West. Cincinnati, Louisville and Pittsburgh on its banks were extensively engaged in building these vessels. Until the Erie canal was opened in 1825 the Ohio river was the chief commercial highway between the East and the West.
The Federal Government in the year 1827 undertook to remove the snags and to increase the depth of water on the bars by the construction of contraction works, such as dikes and wing dams, and appropriations for these purposes as well as for dredging were continued until 1844 and resumed in 1866; but as the channel obtained was less than 3ft. in 1870, locks with mov able dams—that is, dams that can be thrown down on the ap proach of a flood—were then advocated, and five years later Congress made an appropriation for constructing such a dam, the Davis island dam immediately below Pittsburgh, as an experi ment. This was opened in 1885 and was a recognized success. As a result of the activity of the Ohio Valley Improvement Association, Congress authorized the secretary of war to ap point a board of engineers to report on the merits of a channel 9f t. in depth. The board reported in 1908 in favour of such a channel.
In 1910 Congress adopted the report and provision was made for a comprehensive project for canalizing the Ohio. Only slight changes have since been authorized. As it stood in 1926, the project provided for 5o locks and dams, all of the movable type, except the one immediately below Pittsburgh (Pa.), which is a fixed dam, completed in 1921. Of these dams 42 were completed and in operation, and the other eight were under construction.
The total tonnage shipped via the Ohio river in 1925 was tons, of which 43.5% was sand and gravel and was coal.
See the board of engineers' Report of Examination of Ohio River with a view to obtaining channel depths of 6 and pft. respectively (Washington, 1908) ; and A. B. Hulbert, Waterways of Westward Expansion (Cleveland, 1903) and The Ohio River, a Course of Empire (1906).