Low tide in the estuary of the Tyne
Definition: the wide and often shallow lower course of a river where fresh and salt water meet under the influence of tides
An estuary is a semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea (Cameron and Pritchard, 1963). Estuaries are often associated with high rates of biological productivity.
An estuary is typically the tidal mouth of a river (aestus is the Latin word for tide). Estuaries are characterized by sedimentation of silt carried in from terrestrial runoff and, frequently, from offshore. Estuaries are more likely to occur on submerged coasts, where the sea level has risen in relation to the land.
Estuarine circulation is common. Fresh or brackish water flows out near the surface, while denser saline water flows inward near the bottom. A compensating flow occurs when dense water flows out near the bottom and less dense water circulates inward at the surface. The intensity of salinity gradients in estuaries is highly variable, reflecting the balance of freshwater and salt water inputs, wave action, winds and tides.
The contact between suspended clays and salt water leads to flocculation - the clumping together of fines into mud-sized particles which then settle in areas of slack water. Flocculation is very important for the flux of metal ions in estuarine waters and sediments.
Raised estuarine mudflats are known as carselands in Scotland.