Building the Reservoir required displacing some of the earliest settlements along the West Branch of the Croton River including Coles Mills.

Primarily, however, it receives water from the much larger Rondout Reservoir in the Catskill Mountains on the west bank of the Hudson River via the Delaware Aqueduct.

It serves as a supplementary settling basin for these waters before releasing its flow back into the aqueduct to be carried to the Kensico Reservoir in southern Westchester County.

[3] West Branch Reservoir has a 20 square mile (32 km2) drainage basin, and can hold up to 8 billion US gallons (30,000,000 m3) of water at full capacity.

There it enters the city's direct water supply distribution system, flowing via tunnels through the boroughs of The Bronx, Queens, and Brooklyn.