The Effects of Freshwater on Sea Water

Estuaries

• An estuary is a site where a river or stream empties into the ocean, discharging freshwater directly into the sea. The mixing process is seldom perfect and in some estuaries stratification patterns develop, where the salt water and fresh water remain largely segregated. In these kinds of estuaries, salt and fresh water form distinct layers. The fresh water flows out into the sea, mixing gradually with the salt water as it goes.

Types of Estuaries

• There are four main types of estuaries: drowned river mouth, fjord, bar-built and tectonic. Drowned river mouths are places where the sea level has risen and flooded the old river mouth. Fjords are deep u-shaped channels formed by glaciers. A barrier island built offshore through sediment deposition forms a bar-built estuary. Tectonic estuaries formed through the movement of plates along faults in the Earth's crust, which sometimes creates a basin that later becomes filled by seawater.

Stratification

• The type of estuary, the water density and the flow all help determine how fresh and seawater mix. If river flow dominates the tidal motion, the estuarine waters may become highly stratified; the fresh water has lower density than salt water, so it will flow over the sea water beneath. Fjords often exhibit this kind of stratification. Fjords with steep sills can actually develop a layer of cold, stagnant, nutrient-poor water just above the floor of the estuary.

  Other estuaries with large, rapid river flow and low or moderate tidal range can create a salt wedge, where a wedge of seawater is held back by the outrushing fresh water. Mixture between salt and fresh water occurs along the boundary between them, but the waters in these estuaries are poorly mixed. The seawater lost from the top of the wedge is replaced by more seawater from beneath, which enables sediments and nutrients to enter the estuary.

Well-Mixed Estuaries

• If the river flow is slow and the tidal range moderate to high, a well-mixed estuary may form. Here, tidal turbulence mixes the salt and fresh water together throughout most of the estuary, and there is a steady increasing gradient of salt concentration from the river out into the open ocean. Partially-mixed estuaries are midway between salt-wedge and well-mixed estuaries, with seawater flowing inwards beneath the surface freshwater but with much better mixing than occurs in a salt-wedge estuary. The Chesapeake Bay, a drowned river mouth estuary, is one especially prominent example.