Estuary is a coastal river valley flooded by an ocean. Most estuaries are funnel-shaped, with the wide end toward the sea. Chesapeake Bay on the Atlantic Coast is an estuary in the United States. The Rio de la Plata is a major estuary on the Atlantic in South America. Estuaries in Europe include the Gironde in France and the Humber, Severn, and Thames estuaries in the United Kingdom.
Most estuaries formed as the seas rose during the past 11,500 years. The water level rose because of the melting of vast ice sheets that had accumulated during the most recent ice age.
Within an estuary, currents created by the tides mix the salty seawater with the fresh water of the river. Where those currents are weak, little mixing occurs. Because the seawater is salty, it is denser than the river water. Therefore, the river water overlies the seawater, and there are large differences in salinity (saltiness). But where the currents are strong, much mixing occurs, and so the salinity is the same throughout the estuary.
The river erodes soil from the land and deposits much of it in the estuary as sediment. Waves and tidal currents can redistribute the sediment and deposit it around the edges of the estuary. The sediment can accumulate as mudflats, stretches of muddy land that are uncovered at low tide.
See also River (River systems) ; Tide .
