Oh! It's Barkhampstead resevoir, which is in Hartland, one town west of Granby. I misremembered and thought you'd said north of Granby, not west (I hadn't looked at the map link you sent before), so I was thinking of the lakes in the "notch".

The resevoir was a popular place to go hang out and drink. It was pretty damn rural. I didn't spend much time exploring around there, though.