Long Island Sound is an estuary of the Atlantic Ocean and various rivers in the United States that lies between the coast of Connecticut to the north and Long Island, New York to the south. The mouth of the Connecticut River at Old Saybrook, Connecticut empties into the sound. On the extreme western end, the sound is bounded on the north side by Westchester County, New York and the Bronx, and connects to the East River. On its eastern end it opens to Block Island Sound. The sound serves as a geographic border between New England and the Mid-Atlantic states.