Boston Harbor is in the western edge of Massachusetts Bay, formed by the estuary of three rivers: the Charles River (which flows through Boston itself), the Mystic River, and the Chelsea River. It is one of the best natural harbors on the East Coast of the US, and it was one of the three busiest seaports, besides New York and Philadelphia, in the 18th and early 19th centuries, until New York Harbor began to dominate sea trade with Europe.