Long Island Sound, arm of the Atlantic Ocean, c.90 mi (145 km) long and 3–20 mi (5–32 km) wide, separating Long Island, N.Y., from the SE New York mainland and Connecticut. On the W the East River joins it with New York Bay. The sound is fed from the north by the Housatonic, Connecticut, and Thames rivers. The Long Island Sound is a popular leisure-boating center. New Haven, New London, and Bridgeport, Conn., are the largest port cities; many residential communities line the sound. Federal and state environmental studies in the mid-1990s identified serious problems in the sound and recommended a massive cleanup.