Estuaries are semi-enclosed embayments open to the sea at one end. The drowned mouths of rivers formed by rising sea level and Scandinavian fjords formed by ancient glaciers extending out to sea at times of lower sea level are classic examples. Estuaries are places where fresh water meets and mixes with ocean salt water. Estuaries are characteristic of low-gradient, broad coastal plains and broad shelves on a passive margin. Estuaries are strongly affected by tides and river discharge.
Estuaries are short-lived geologically and soon fill with sediment.
B. Estuarine Circulation 1. Mixing occurs at the base of the fresh water layer as it moves seaward over salt water. The upper layer thus becomes saltier as it moves seaward. Salt water continues to flow inward under the fresh water to replace what has been lost through mixing with the surface layer. Thus there is normally a net inflow at the bottom of an estuary and net outflow at the surface.