The Housatonic River begins its 149 mile journey in southwestern Massachusetts. The main stem of the river is formed by the joining together of the West and Southwest branches of the Housatonic River in Pittsfield. The East Branch begins at Muddy Pond in Hinsdale and Washington and flows a total distance of approximately 17 miles, dropping 480 feet before merging with the main stem. The West Branch starts from Pontoosuc Lake and joins the Southwest Branch at Clapp Park in Pittsfield. From there it flows a short distance until it is joined by the East Branch near Pittsfield's Fred Garner Park.
The Housatonic River main stem, which flows in a southerly direction 132 miles to its outfall at Long Island Sound at Milford Point in Connecticut. The main stem of the river has an overall drop of 959 feet. The Housatonic River and its tributaries drain an area of 1,948 square miles. This area is referred to as the watershed. From its headwaters flowing south toward Great Barrington, the valley is narrow and the river flows quickly, characterized by several swift drops in elevation, before it emerges from the Berkshire Hills. In this section there is a good deal of commercial and industrial development. Below Great Barrington, the valley flattens and broadens out. This region is rich in farmland, and through this section the river flows more slowly, meandering its way through the valley to Falls Village in Connecticut.
As the Housatonic River moves into Connecticut, the valley changes dramatically. The valley walls narrow and are flanked by hills on either side. The river now flows through a much harder substrate consisting of limestone, quartz and granite, and the river bottom becomes much rockier. There are still some agricultural activities in this northwestern part of Connecticut due to the presence of the river's nutrient rich floodplains.
Just south of Bulls Bridge power station, water is diverted from the river and pumped uphill, through a penstock, to Candlewood Lake, the first pump storage reservoir built in the country. Constructed in 1926, it is the largest (5,400 acres) lake in Connecticut. When river levels are too low to support the power generation at the Rocky River Power Station in New Milford, lake water is sent rushing down the penstock and through the generators. Upon leaving New Milford, the river again changes dramatically, becoming a series of 3 in-stream lakes. Each lake is formed by a hydroelectric power dam. The Shepaug Dam forms Lake Lillinonah (1,900 acres) in Bridgewater. Farther south in Monroe, the Stevenson Dam, which is the largest, creates Lake Zoar (975 acres). The third lake is Lake Housatonic (328 acres), formed by the Derby Dam between Derby and Shelton.
The flow of the Housatonic River may vary in this area. River flows are periodically "ponded" behind the dams when normal river flows are inadequate. The water is then released to turn the turbines which produce electric power. Below the Derby dam, the river begins its final change, becoming an estuary, where salt and fresh water mix. The Housatonic River estuary produces one-third of all the seed oysters which are a vital part of Connecticut's commercial shellfish industry. In this lower 12 mile section of the river are tidal wetlands and salt marshes which provide important habitat for plants, birds, shellfish, finfish and other aquatic life. The Housatonic River enters Long Island Sound at Milford Point.