Monongahela River. The Monongahela River, sometimes referred to locally as the Mon, is a 130-mile-long river on the Allegheny Plateau in north-central West Virginia and Southwestern Pennsylvania. The river flows from the confluence of its west and east forks in north-central West Virginia northeasterly into southwestern Pennsylvania, then northerly to Pittsburgh and its confluence with the Allegheny River to form the Ohio River. The river includes a series of locks and dams that makes it navigable. The Unami word Monongahela means falling banks, in reference to the geological instability of the river's banks. Moravian missionary David Zeisberger gave this account of the naming: In the Indian tongue the name of this river was Mechmenawungihilla, which signifies a high bank, which is ever washed out and therefore collapses. The Lenape Language Project renders the word as Mënaonkihëla, translated where banks cave in or erode, from the verbs mënaonkihële the dirt caves off and mënaonke, it has a loose bank. Monongalia County and the town of Monongah, both in West Virginia, are named for the river, as is the city of Monongahela in Pennsylvania. The USGS name for the river is the Monongahela River; there have been numerous alternate names, alternate spellings and misspellings in historical texts. The Monongahela is formed by the confluence of the West Fork River and its east fork, the Tygart Valley River, at Fairmont in north central West Virginia. From there it flows northeasterly to cross the Pennsylvania border just west of north Cheat Lake on its Cheat River tributary. Then it flows northerly across southwestern Pennsylvania, taking a bit of a detour northeast 10 miles south of Pittsburgh to approach Pittsburgh from the southeast and its confluence with the Allegheny River to form the Forks of the Ohio at The Point of Point State Park in Downtown Pittsburgh. Prior to early Pleistocene regional glaciation, more than 780,000 years ago, the ancestral Monongahela River flowed northward from present-day north-central West Virginia, across western Pennsylvania and northwestern Ohio, and into the Saint Lawrence River watershed. One 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 south to Weston, West Virginia. A river-lake with many narrow bays, its maximum water surface rose to 1,100 feet above sea level. Over 200 feet 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. Via the Ohio River, the river is part of the Mississippi watershed which drains to the Gulf of Mexico on the Atlantic Ocean. The river's length is 130 miles, its drainage basin is 7,340 sq.mi. and the average 30-year discharge at Elizabeth, Pennsylvania, is 9,109 cfs. It falls 3,831 ft. in elevation from its highest source to its mouth on the Ohio River. It falls 280 feet from its forks to its mouth, a stretch made navigable by locks. The mean depth is about 20 ft. In southwestern Pennsylvania, the Monongahela is met by two major tributaries: the Cheat River, which joins at Point Marion, and the Youghiogheny River, which joins at McKeesport. Major tributaries include: Becks Run, Big Sandy Creek, Buffalo Creek, Cheat River, Crooked Run, Deckers Creek, Dunkard Creek, Lick Run, Middle Fork River, Paw Paw Creek, Peters Creek, Streets Run, Turtle Creek, Tygart Valley River, West Fork River, Youghiogheny River. See also: List of dams and reservoirs in Pennsylvania The river is navigable its entire length with a series of locks and dams that maintain a minimum depth of 9 feet to accommodate coal-laden barges. All dams are operated by the Pittsburgh District Army Corps of Engineers.