The Hudson River flows through the Hudson Valley into New York Bay. Between New York City and Troy, New York, the river is an estuary. The Hudson River separates the city from the U.S. state of New Jersey. The East River—a tidal strait—flows from Long Island Sound and separates the Bronx and Manhattan from Long Island. The Harlem River, another tidal strait between the East and Hudson Rivers, separates most of Manhattan from the Bronx. The Bronx River, which flows through the Bronx and Westchester County, is the only entirely fresh water river in the city.

The Hudson River serves as a dividing line between New York and what state?