Middle Quaddick Reservoir is a 408-acre impoundment of the Five Mile River in Thompson, CT. Quaddick State Park lies along most of the eastern shore which allows public access in the form of a boat launch and a state beach. On average the lake is about 2.5 meters (7.6 feet) deep but reaches depths of 20 feet. The lake is stocked annually with pike fingerlings, and bass fishing tournaments are held throughout the season.

Our August 2007 aquatic vegetation survey found twenty-two plant species, including the invasives variable-leaf watermilfoil (Myriophyllum heterophyllum) and fanwort (Cabomba caroliniana). Both were found growing together throughout nearly the entire lake except for the deepest basins. Fanwort occurred in greater abundance. Native common bladderwort (Utricularia macrorhiza) was the most abundant species, occurring in densest stands throughout the lake. Also common were watershield (Brasenia schreberi) and white water lily (Nymphaea odorata) that occurred together as well as separately along most of the shoreline, especially towards the north. Purple bladderwort (Utricularia purpurea) was also common and in flower at the time of this survey. The densest vegetation occurred in the northernmost part of the lake.