Where Does Our Drinking Water Come From?

Cambridge drinking water supply system map showing watershed boundaries, reservoir locations, and aqueduct location.

Water Supply

The drinking water supply system consists of three in-series reservoirs, the Hobbs Brook, Stony Brook, and Fresh Pond Reservoirs. Hobbs and Stony Brook Reservoirs drain a 24 square mile basin in Lexington, Lincoln, Waltham and Weston, MA. Fresh Pond Reservoir is a glacial kettle-hole lake located in Cambridge with no natural inlets or outlets. Hobbs Brook Reservoir is fed by Hobbs Brook and other unnamed tributaries that discharge directly into the reservoir. Water is released from the Hobbs Brook Reservoir dam at Winter Street in Waltham, which joins Stony Brook about 1.5 miles downstream. Stony Brook Reservoir is fed by Stony Brook and a small tributary from Weston. Both reservoirs receive additional inflows from surface overland flow, engineered drainage systems, groundwater, and direct precipitation. From the Stony Brook Reservoir, water is piped through an underground aqueduct to Fresh Pond Reservoir where it is stored prior to treatment. Fresh Pond raw water is purified at the Walter J. Sullivan Water Treatment Plant (WTP) and pumped to Payson Park Reservoir, a covered storage facility located in Belmont, MA. From there, water flows by gravity to the City of Cambridge distribution system.

The largest of the reservoirs, Hobbs Brook Reservoir, reaches its maximum elevation at ~169.6 feet (NAVD88), its maximum depth at approximately 25 feet, and at full capacity, holds approximately 2.5 billion gallons of water. Stony Brook Reservoir reaches its maximum elevation at ~68.9 feet, its deepest point is approximately 30 feet, and at full capacity contains roughly 418 million gallons of water. Fresh Pond Reservoir reaches its maximum elevation at ~5.3 feet, its maximum depth is 50 feet, and at full capacity, holds roughly 1.5 billion gallons.   

While the Watershed's primary storage reservoir is Hobbs Brook Reservoir, in the winter and spring months it is largely unused. Due to its vast size and relatively small watershed, Hobbs Brook Reservoir is slow to fill up. This winter hiatus is necessary for Hobbs Brook Reservoir to regain the water transferred to the downstream reservoirs in the summer months. Conversely, Stony Brook Reservoir is relatively small compared to its large watershed and fills much faster than Hobbs Brook Reservoir. Water from Stony Brook Reservoir is enough to supply our demand during the winter and spring months and is piped to Fresh Pond Reservoir. Excess water is released to the Charles River to maintain safe dam operating levels. The average annual daily water demand in the City of Cambridge is typically 12 to 13 million gallons per day (MGD).

Any questions or comments call 617-349-4799 or email us.