Tidal estuaryA tidal estuary is an arm of the sea where salty seawater meets fresh water running off the land.
The Hudson River in the New York Harbor is an example of a tidal estuary. The Hudson feels what is called the tidal pulse of the Atlantic Ocean from the city of Troy, New York 153 miles away, making it flow in different directions throughout each day, sometimes pushing salt water upstream. That makes it different from a river, which simply flows into a larger body of water.
A tidal estuary differs from a river in another respect: because of the constant changes in river flow, the estuary is deeper than the body of water into which it flows.