The East River is a salt water tidal strait in New York City where part of the Manhattan Island Marathon Swim is conducted. 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 Long Island – including the boroughs of Queens and Brooklyn – from the Bronx on the North American mainland, and the island of Manhattan. Because of its connection to Long Island Sound, it was once also known as the Sound River. The tidal strait changes its flow direction frequently.