According to Wikipedia:

http://en.wikipedia.org/wiki/Ohio_river#River_depth

Due to the damming, along with glacier formations and migrations in the latter part of the second Ice Age, the rivers depth increases nearly fivefold over about 100 miles, coming to a maximum depth of 168 feet just west of Louisville, Kentucky.

The 50 miles around Louisville represent the deepest area of the river with an average depth of approximately 132 feet, allowing for much larger vessels to traverse the river.