Saltwater estuaries exist across the globe where rivers, streams and creeks end and oceans begin, where freshwater and saltwater meet.
An estuary's salinity constantly fluctuates. Changing tides deliver saltwater into the system, while rainy and dry seasons determine the flow of freshwater. The resulting not-quite-fresh but not-so-salty water is called brackish.
Brackish water could spell death for many terrestrial plants, but others are well adapted to survive in the slightly salty water. Those marsh plants and sea grasses filter water and offer a natural buffer against coastal storms.
Animals also inhabit estuaries, many of which are seasonal visitors. For some, estuaries provide a suitable stopover for food and shelter during long migrations. Others arrive seeking sanctuary for their young. In fact, according to the National Oceanic and Atmospheric Administration, estuaries provide nursing grounds for approximately 75 percent of the species we fish.
Estuaries fan the coasts of the United States, from Washington's Puget Sound to Florida's Tampa Bay and the nation's largest estuary—the Chesapeake Bay.
As the last ice age came to a close, mile-wide glaciers retreated, carving out a landscape of streams and rivers along the Atlantic coast. Sea levels rose over time, causing ocean waters to flood the coastal plain, drowning the mouth of the ancient Susquehanna River and forming the Chesapeake Bay.
Other estuaries are the result of shifting tectonic plates, receding glaciers that formed fjords, or lagoons created between coasts and adjacent sandbars or barrier islands.
Estuaries are not just popular and productive homes for wildlife. Many people also occupy the surrounding land and rely on the ecosystem services estuaries offer. According to NOAA, 22 of the 32 largest cities in the world are situated on estuaries.
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