I took Columbus Day off to go to some local dams that I found by browsing Google Maps.
These are all on the Charles River in Waltham and Watertown, MA.
At one point there were 32 dams on the Charles, with diversions to power the mills that lined the river. This was the birthplace of the automated textile industry, and the proving grounds for mills that were setup in Lowell, Lawrence, and other New England towns.
They don't power anything anymore, and are part of the DCR Riverwalk park system.