Ohio River, the largest branch of the Mississippi river from the east, known to the early French settlers as la telle riviere, and famed for the uniform smoothness of its current as well as for the beauty and fertility of its valley. It is formed in the W. part of Pennsylvania by the junction at Pittsburgh of the Monongahela and Alleghany rivers. By the latter the drainage valley of the Mississippi is extended into the S. W. part of New York, and in Potter co., Pa., reaches a point where over an extent of a few acres it is a mere chance whether the water that falls upon the surface reaches the ocean by the gulf of Mexico, the gulf of St. Lawrence, or Chesapeake bay. The course of the Ohio and of all its tributaries, from their sources W. of the Alle-ghanies to the outlet of the river in the Mississippi, at Cairo, 111., is through a region of stratified rocks, little disturbed from the horizontal position in which they were deposited, and nowhere intruded upon by uplifts of the azoic formations, such as in other regions impart grandeur to the scenery and variety to the valleys of the rivers. Over an area of drainage of the Ohio and its branches estimated at 214,-000 sq. m., the topography is uniform in its principal features, and, though often beautiful, still for the most part tame.

The valleys are depressions below the general summit level of the country; all of them were eroded by currents of water, and the piles of strata presenting no portions that could resist the action of these, the descent of the river beds is gentle, with no sudden breaks or precipitous falls. The banks, however, are often steep, and in many places, especially upon the smaller rivers, the waters have worn a narrow passage between vertical cliffs of limestone to the depth of several hundred feet from their summits. Generally the rivers spread out to considerable width, and in dry seasons become shoal to the serious impediment of navigation. An interesting feature in the banks of the Ohio is the succession of terraces often noticed rising one above another at different elevations, and sometimes spreading out in broad alluvial flats. Though they are often 75 ft. or more above the present mean level of the river, they were evidently formed by fluviatile deposits made in distant periods, when the river flowed at these higher levels. Evidence is altogether wanting to fix the date of these periods.

Upon the lower branches of the river, at the level of present high water, are mounds and earthworks wonderful in their numbers and extent, which were constructed, as far as can be ascertained from various proofs, full 2,000 years since, the fact being thus established that the river must have flowed at its present level at least so far back. The city of Cincinnati stands upon two of these terraces, the upper one 52 ft. above the lower, and this 60 ft. above low water of the river. In the gravel of the upper one have been found the teeth of an extinct species of elephant. Shells which have been found at corresponding elevations are of recent species, such as are still common to the waters in the neighborhood. - The total length of the Ohio river is 975 m.; but from Pittsburgh to the mouth of the river in a straight line it is less than three fifths of that distance. Its course till it passes out of Pennsvlvania is N. N. W. to Beaver, and thence W. S. W. to the line of the state of Ohio. It then flows S. and S. W. between Ohio and West Virginia, passing Wheeling, 86 m. below Pittsburgh. The general course of the river is W. S. W. After passing between Ohio and West Virginia, it borders the whole length of Kentucky, separating that state from Ohio, Indiana, and Illinois on the north.

The width of the upper third of the river, between Pittsburgh and Point Pleasant, is 1,000 ft. at low water and 1,200 ft. at high water, thence gradually increasing till near the mouth, where it is 3,000 ft. Its depth at different seasons is very fluctuating, the range between high and low water being often 50 and sometimes 60 ft., and the usual range throughout the entire river is 45 ft. During portions of the summer and in the autumn, when the water is low, the larger steamboats ascend no further than Wheeling, and even below this point they pass with difli-cultv, or are arrested by the sand bars, which, with the low sandy islands, called towheads, badly obstruct the navigation. At the lowest stage, generally in August and September, the river may be forded at several places above Cincinnati. " In the winter it is often frozen over, and for several weeks floating ice prevents its navigation. The rate of its current varies with the stage of the water from 1 to 3 m. an hour. The only rapids are at Louisville, and these are not insurmountable to all the steamboats. In 2½ m. the fall is about 27 ft.

A canal was long since constructed past these rapids at Louisville, through which steamers of 3,000 tons may pass. - The country bordering the Ohio is for the most part a thriving agricultural region, and many prosperous towns and cities have grown up within the present century on its banks. Manufactures are encouraged by the mines of coal and iron ore that abound in the country traversed by this river and its tributaries, and the products of these add largely to the immense transportation carried on by the boats. The character and extent of these operations are particularly noticed in the descriptions of the several states and large towns on the borders of the river. - The tributaries of the Ohio from both sides are numerous, and many of them are important rivers, as the Muskingum and Miami of Ohio, the Wabash of Indiana and Illinois, and the Sandy, Licking, Kentucky, Cumberland, Green, and Tennessee of Kentucky.