The Hudson River is a 315-mile (507 km) river that flows from north to south largely through eastern New York state. It originates in the Adirondack Mountains at Henderson Lake in the town of Newcomb, and flows south to New York Bay, a tidal estuary between New York City and Jersey City, before draining into the Atlantic Ocean. The river marks out the eastern border between the U.S. states of New York and New Jersey and many upstate New York county borders. The lower half of the river is a tidal estuary, deeper than the body of water into which it flows, occupying the Hudson Fjord, an inlet that was carved out by North American glaciers. Even as far north as the city of Troy, the flow of the river changes direction with the tides.