The Hudson River is a special kind of river called an estuary. There is some salt in the water from New York City north until Poughkeepsie. It is then fresh water from Poughkeepsie north until Lake Tear of the Clouds. The border for the salt in the water moves when the tide rises and falls. The Hudson is part of the Erie Canal from New York City to Albany. The Hudson River used to have a lot of pollution from industry, but it is becoming cleaner now.