Forming where rivers meet the sea, estuaries are highly dynamic ecosystems that are in constant motion and bursting with life. Their waters rise and fall with the ocean tides, and this ebb and flow merges incoming seawater with outgoing river water to create a mixture that varies in saltiness. The continuous blending also gathers and distributes nutrients, making estuaries some of the most productive regions on Earth. They teem with vast blooms of plankton and algae, huge schools of fish, and countless numbers of invertebrates.

The northern half of the Hudson estuary is fresh water, which gradually becomes saltier as it nears New York Harbor. © Steve Stanne / Hudson River Sloop Clearwater

IN THIS EXHIBIT: The Hudson River Estuary
Winding some 245 kilometers (150 miles) from New York City to Troy's Federal Dam, the Hudson River estuary flows both ways, following the tides of the Atlantic Ocean. The Hudson reaches depths of more than 61 meters (200 feet) and spans over 5 kilometers (three miles) at its widest point. Like many other estuaries, the Hudson has many tidal marshes that form along its edges.