Dam on the Charles River. The Watertown Dam spans the Charles River 980 feet (300 m) upstream from the Watertown Bridge near Watertown Square in Watertown. The dam is located where the Charles River tidal estuary historically ended (the tides no longer reach this point because of the downstream Charles River Dam). Watertown Dam is of earthen construction, a gravity dam. The history of the dam traces back to 1632 when construction of a fish weir was authorized. The current dam, maintained by the Department of Conservation and Recreation, dates from 1900.