The Ohio River Story - Part II (GM-24)
Before the glacial advances that formed the Ohio River took place, the Kentucky River flowed northeastward along what is now the Indiana-Kentucky border and into west-central Ohio, where it joined the westward-flowing Teays River. But about a million years ago, a tongue of ice advanced far enough south to clock this drainage way. Water from the Kentucky River was ponded until it found its way across a divide near Madison and it a small tributary of the Salt River. This stream, greatly augmented my meltwater from the glacier, rapidly cut down this divide and so helped to create the Ohio River as we now know it.

Our Hoosier State Beneath Us: Geomorphology