Fresh river water and salt seawater meet and mix in a semi-enclosed coastal embayment. An estuary is that transition zone. Tides push salt water upstream while river flow pushes fresh water seaward, creating a salinity gradient.

Estuaries rank among the most productive ecosystems on Earth. Nutrients from land fuel rich food webs that support fish, shellfish, and migratory birds. Many commercially important species use estuaries as nurseries. Mangroves, salt marshes, and seagrass beds stabilize the shoreline and filter water.

Human pressures are intense because major ports and cities often developed on estuaries. Pollution, dredging, and land reclamation have degraded many systems. Restoration efforts now focus on reconnecting tidal flows and reducing nutrient loads. The daily pulse of the tide remains the defining rhythm.

• Coastal zone where river meets sea
• Features mixing of fresh and salt water
• Highly productive nursery habitat
• Often heavily modified by human use