The reservoir is 5.5 miles (8.9 km) long, and holds 12.3 billion US gallons (47,000,000 m3) when full.

[3] The reservoir supplies water to about 50,000 residents in Troy, as well as to about 85,000 residents in the nearby municipalities of Brunswick, East Greenbush, Halfmoon, Menands, North Greenbush, Poestenkill, Rensselaer, Schaghticoke and Waterford.

[4] After eight years of repairs to the original 33 inches (84 cm) main, a supplemental 30 inches (76 cm) cast iron[4] main was installed to stabilize the city's water supply.

In 1960, a study determined that the Tomhannock Reservoir could serve the city of Troy and the region immediately adjacent to the city (East Greenbush, North Greenbush, Brunswick, a portion of Schaghticoke, and West Sand Lake) in southern Rensselaer County.

During the early 1960s a treatment plant was constructed, which provided filtration and additional supplementation with alum, carbon, potassium permanganate and fluoride.