Estuaries are partially enclosed bodies of water formed where fresh river water meets and mixes with salty ocean water. They are formed through sea level rise flooding river valleys or filling glacial trenches. Estuaries act as sediment traps, collecting mud, clay and other materials transported by rivers and the sea over time. Key features include strong winds causing surface water circulation, rivers continuously supplying fresh water and sediment, and the fresh water gradually mixing with estuarine water before exiting into the coast.