The Monongahela River () — often referred to locally as the Mon — is a 130-mile-long (210 km)river on the Allegheny Plateau in north-central West Virginia and southwestern Pennsylvania. The Monongahela joins the Allegheny River to form the Ohio River at Pittsburgh.
The Native American word Monongahela means "falling banks", in reference to the geological instability of the river's banks. Moravian missionary David Zeisberger (1721–1808) gave this account of the naming: "In the Indian tongue the name of this river was Mechmenawungihilla (alternatively spelled Menawngihella), which signifies a high bank, which is ever washed out and therefore collapses."
The Lenape Language Project renders the word as Mënaonkihëla (pronounced [mənaoŋɡihəla]), translated "where banks cave in or erode", from the verbs mënaonkihële "the dirt caves off" (such as the bank of a river or creek, or in a landslide) and mënaonke (pronounced [mənaoŋɡe]), "it has a loose bank" (where one might fall in).