Surface water

The Housatonic River drains about 500 mi2 of western Massachusetts. The basin contains a few wetlands and, compared to other basins to the west of the Connecticut River, a relatively large number of lakes and ponds. There are 113 lakes and ponds, 70 of which are larger than 10 acres. Only one lake, Onota Lake in Pittsfield (617 acres), is larger than 500 acres.

The main stem of the Housatonic River is formed by the confluence of the East Branch and West Branch Housatonic Rivers at Pittsfield. The main river flows generally southward through Massachusetts and eventually empties into Long Island Sound. In Massachusetts, the Housatonic River has a moderate gradient, averaging 16.5 ft/mi from its headwaters to the USGS streamflow-gaging station near Great Barrington, a distance of almost 50 river mi.