Where a river meets the sea, freshwater and saltwater mix. That zone is an estuary. The mixing creates conditions that shift with the tide, the season, and the river's flow. Salinity rises and falls. Sediment settles. Nutrients concentrate. Estuaries are among the most productive ecosystems on Earth. They support shellfish beds, nursery grounds for fish, and vast flocks of migratory birds. Chesapeake Bay, the Thames, and the Ganges delta are all estuaries.

Estuaries absorb shocks. They buffer coastlines against storms, filter pollutants, and store carbon in their sediments. They also face intense pressure. Half the world's population lives within 60 miles of a coast, and many major cities sit on estuaries. Development destroys marshes and mudflats. Pollution from farms and cities flows in. Dams upstream reduce the sediment that builds and maintains the estuary. Sea-level rise pushes saltwater farther inland, drowning freshwater marshes.

Why estuaries matter

• Nursery habitat for fish and shellfish.
• Stopover for migratory birds.
• Natural buffer against storms and floods.
• Filter for pollutants and sediment.

Estuaries are not just transition zones. They are working ecosystems that support fisheries, wildlife, and coastal economies.