If you happen to visiting Yorktown Heights, it's worth the 10 minute drive towards Croton to see the Croton Gorge Park. The web page shows a picture of the Croton Dam from below, and I understand it to be one of the largest stone dams in the country. The reservoir serves NYC but stands today as one of many reservoirs for the city. The water that spills over the dam forms the Croton River that flows toward the Hudson. There is an area in Croton where the water is held back a bit to form Silver Lake.

When the reservoir is full, the water spilling over the top is relatively warm. But when it's not full, water empties from the bottom and it's very cold. Feels great on a hot summer day.