The Monongahela River (/məˌnɒŋɡəˈhiːlə/ mə-NONG-gə-HEE-lə, /-ˈheɪ-/ HAY-)[1] — often referred to locally as the Mon (/mɒn/) — is a 130-mile-long (210 km)[2] river on the Allegheny Plateau in north-central West Virginia and southwestern Pennsylvania, which flows from south to north. The Monongahela joins the Allegheny River to form the Ohio River at Pittsburgh.