The Mamaroneck River is a freshwater stream located in Southern Westchester County, New York.

[3] Originating 430 ft (130 m) above sea level in a small wetlands area in Harrison, the river flows south along the eastern boundary of White Plains.

The East Branch of the Mamaroneck River is the furthest upstream tributary and originates in north Harrison near the intersection of Westchester Avenue and Anderson Hill Road.

The east branch originates from a small pond in north Harrison at about 400 ft (120 m) above sea level and then flowing southeast 0.7 mi (1.1 km) into Forest Lake.

The west branch originates from a wetland and pond adjacent to Archbishop Stepinac High School on Mamaroneck Avenue and flows about 2.75 mi (4.43 km) south and southwest through several stormwater management basins to its confluence with the main stem near the intersection of Mamaroneck Avenue and Hutchinson River Parkway.