What Is an Estuary? Where Rivers Meet the Sea, per NOAA
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How we wrote this: A plain-English summary of NOAA's National Ocean Service page "What is an estuary?", checked October 29, 2026.
Where fresh water meets salt
Estuaries and their surrounding wetlands usually form where rivers meet the sea. Their plants and animals are adapted to brackish water, a mix of fresh water from the land and salty seawater.
There are also freshwater estuaries. Along the Great Lakes, river water mixes with lake water in coastal wetlands that tides and storms affect, much like ocean estuaries.
Why they matter
- They're among the most productive ecosystems in the world. Many animals rely on them for food, breeding and migration stopovers.
- NOAA's video calls them the nursery grounds for 75% of the fish we catch.
- Their marshes and sea grasses filter water flowing to the ocean and buffer the coast from storms.
- People rely on them for food, recreation, jobs and protection. 22 of the world's 32 largest cities sit on estuaries.
Visiting one
Congress created the National Estuarine Research Reserve System, which protects more than one million acres. NOAA manages it with coastal states as 28 reserves, including the Chesapeake Bay, South Carolina's ACE Basin and San Francisco Bay. The reserves are popular for fishing, boating, birding and hiking, and also serve as living laboratories and classrooms.
Related: barrier islands, horseshoe crabs, harmful algal blooms, why the ocean is salty.