The Ramapo river rises in Round Lake, a small freshwater lake in the Village of Monroe, New York in a mountainous area of central Orange County, New York, ] and flows southeast through the village where the river was dammed in 1741 for a sawmill and grist mill, to Harriman, Here a chemical plant, Nepera Chemical, was built, and while the plant has been dismantled, a superfund site has been designated at the location where barrels of toxic chemicals were buried. At Harriman, the river then turns south into western Rockland County, where it flows through the hamlet and town of Ramapo, New York, then into northern Bergen County, New Jersey. In New Jersey, it flows SSW, along the east side of the ridge of the Ramapo Mountains. The river flows into Potash Lake in Oakland and from there, into Pompton Lake in Pompton Lakes. The river that flows out of Pompton Lake, forming the border between Pompton Lakes and Wayne. The Ramapo forms a confluence with the Pequannock River along the border between Pequannock and Wayne, forming the Pompton River. A portion of the river's water is diverted to the nearby Wanaque Reservoir.