Monongahela River Locks Map

Prior to early Pleistocene regional glaciation, more than 780,000 years ago, the ancestral Monongahela River (a. k. a. the Pittsburgh River) flowed northward from present-day north-central West Virginia, across western Pennsylvania and northwestern Ohio, and into the Saint Lawrence River watershed. One (or more) extensive ice sheet advance dammed the old north-flowing drainage and created a vast lake—known as Lake Monongahela—stretching from an unknown point north of present-day Beaver, Pennsylvania for ~200 miles (320 km) south to Weston, West Virginia. A river-lake with many narrow bays, its maximum water surface rose to 1,100 feet (340 m) above sea level. Over 200 feet (61 m) deep in places, its southwestward overflow gradually incised old drainage divides and contributed to the geological development of the present-day upper Ohio River Valley.

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