The Ohio River forms in Allegheny County from the meeting of the Allegheny and Monongahela rivers at Pittsburgh, the largest city on the Ohio River. From Allegheny, the Ohio flows west into the neighboring Beaver County before leaving Pennsylvania for Ohio and West Virginia.
The Ohio River forms the border of 14 counties in Ohio. From farthest east to farthest west, the Ohio touches the following counties: Columbiana, Jefferson, Belmont, Monroe, Washington, Athens, Meigs, Gallia, Lawrence, Scioto, Adams, Brown, Clermont and Hamilton County. Hamilton County is home to Cincinnati, the second largest city on the Ohio.
The Ohio River separates Ohio from West Virginia, and it forms the border of 12 counties. Beginning with Hancock, the county farthest north, the river continues past the following: Brooke, Ohio, Marshall, Wetzel, Tyler, Pleasants, Wood, Jackson, Mason, Cabell and Wayne counties.
Kentucky has 25 counties with a border formed by the Ohio River, the largest number of counties touching the river. Starting with Boyd County, the river runs above Greenup, Lewis, Mason, Bracken, Pendleton, Campbell, Kenton, Boone, Gallatin, Carroll, Trimble, Oldham, Jefferson, Hardin, Meade, Breckinridge, Hancock, Daviess, Henderson, Union, Crittenden, Livingston, McCracken and Ballard. Louisville, in Jefferson County, is the third largest city on the river and the largest in Kentucky.
The Ohio River meets Indiana at its southeast border with Ohio, and the river flows across ll of Indiana's southern counties, separating Indiana from Kentucky. Starting with Dearborn County on the far southeast, the Ohio River flows across Ohio, Switzerland, Jefferson, Clark, Floyd, Harrison, Crawford, Perry, Spencer, Warrick, Vanderburgh and Posey counties.
Illinois is the last state to touch the Ohio. The Ohio River flows across the southeast part of the state, separating it from Kentucky. It forms a border of Gallatin County and continues through the following counties: Hardin, Pope, Massac, Pulaski and Alexander. At Alexander County, the Ohio flows into the Mississippi River.
