AN ESTUARY IS NEITHER OCEAN NOR RIVER, but a blending of both. According to one classic definition, it's a semi-enclosed body of water, open to the sea, where salty water from the ocean mixes with freshwater from the land.

That definition dates from 1952 and came from Don Pritchard, the first professional oceanographer to study the physics of Chesapeake Bay. It's a definition that has weathered the passing decades and the alternate definitions proposed by later scientists, largely because it captures the essential element, the mixing of fresh and salt, from which so many other estuarine features flow.

Source: Office of Naval Research

Based on its geology, the Chesapeake Bay is classified as a coastal plain estuary or a drowned river estuary because it began life when rising sea levels flooded into its major tributary following the end of the last ice age. Estuaries like Pamlico Sound in North Carolina, on the other hand, began as bar-built, semi-enclosed bodies of water created by ocean currents piling up sand bars or barrier islands. Delta estuaries like the Mississippi were also built by sand and sediment, but the sand was carried there by a river not the ocean. San Francisco Bay is a tectonic estuary, created when a sudden movement of the earth's crust formed a basin that ocean and river water quickly filled in. Glacier Bay in Alaska and parts of Puget Sound are fjords, deep channels carved into the earth by glaciers.

Estuaries are common along the Atlantic and Gulf coasts, where they cover 80-90% of the coastline. They are uncommon along the Pacific coast, where they account for only 10-20% of the coastal waters.

Of all this country's estuaries, the Chesapeake Bay is the largest, and has been historically the richest in biological productivity. Its great rivers, streams, creeks, and coves intertwine with islands and peninsulas and necks to create a variety of habitats for wildlife, waterfowl, wetlands, and fish. The bottom of the Bay is home to seagrass beds, oyster reefs, and many bottom-dwelling shellfish like blue crabs, clams, and mussels. An estimated 350 fish species live part of their lives in the Bay. Despite declining water quality and shrinking habitats, the Chesapeake is still a rich estuary — in part because of restoration efforts launched in recent decades, in part because of the resilience of the animals and plants that adapted to the variable conditions in an ecosystem that mixes river and ocean.

— Michael W. Fincham