The East River is a salt water tidal estuary in New York City. The waterway, which is actually not a river despite its name, connects Upper New York Bay on its south end to Long Island Sound on its north end. It separates the borough of Queens on Long Island from the Bronx on the North American mainland, and also divides Manhattan from Queens and Brooklyn, which is also on Long Island. Because of its connection to Long Island Sound, it was once also known as the Sound River. The tidal strait changes its direction of flow frequently, and is subject to strong fluctuations in its current, which are accentuated by its narrowness and variety of depths. The waterway is navigable for its entire length of 16 miles (26 km), and was historically the center of maritime activities in the city.