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is a city in Washington County, Pennsylvania, United States (zip Code 15063) and is part of the Pittsburgh Metro Area. The population was 4,761 at the 2000 census. In 1940, the population of Monongahela was 9,823, in 1950, 10,387, in 1960, 12,948, in 1970, 11,726, in 1980, 8,590, and in 1990.
One of only two cities in Washington County, Monongahela sits at the intersection of Pennsylvania state routes 136, 88, and 837, all of which constitute the city's Main Street.
According to the United States Census Bureau, the city has a total area of 2.1 square miles (5.5 km²), of which, 1.9 square miles (5.0 km²) of it is land and 0.2 square miles (0.5 km²) of it (9.86%) is water.