Mystic. 1 River, c.10 mi (16 km) long, rising in SE Conn. and flowing S past Old Mystic and Mystic villages to the Long Island Sound. Mystic Seaport, a maritime museum, is at its mouth. 2 River, c.7 mi (11 km) long, rising in Mystic Lakes, E Mass., and flowing SE, past Medford, into Boston Harbor at Charlestown. Medford was one of the important early settlements on its banks.