Winsor Dam

The Winsor Dam impounds the waters of the Swift River and the Ware River Diversion, forming the Quabbin Reservoir. Its 38.6 square miles (99.9 km²) make it the largest water body in Massachusetts. The reservoir is the primary water source for Boston, 65 miles (~100 km) to the east.

http://en.wikipedia.org/wiki/Quabbin_Reservoir