Transitions in Water

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The Keys To Unlocking Transitions in Water

When examining waters transition from fresh to salt as well as from salt to

fresh one quickly finds the importance of estuaries. In terms of geology,

present-day estuaries are young and ephemeral coastal features. Today's estuaries

began to take their current form during the last interglacial period, when sea level

rose about 120 m (Braun 36). However, the relatively high sea levels and extensive

estuaries found today have been characteristic of only about 10 to 20 percent of the

last million years. When sea level was lower, during glaciation periods, estuaries

were much smaller than they are at present and were located on what is now the

continental slope. Unless sea level rises, estuaries tend to fill with sediments and

become much smaller. The sediments come from riverborne terrestrial materials

from the eroding continents and from sand transported upstream by the tides from

the continental shelf (Braun 55).

It is in estuaries that most of the world's freshwater runoff encounters the

oceans. Because fresh water is lighter, or less dense, than salt water, unless the

two are mixed by the tides or winds, the fresh water remains at the surface,

resulting in a salinity gradient. Tides force seawater inland as a countercurrent

and produce a saltwater wedge below the freshwater surface waters (Bellamy


Estuaries are always in a state of change and hardly ever in a steady state.

The principal energy source are tides, causing estuarine mixing, but wind, wave

motions, and river runoff can also be important locally (Braun 45). Salt water and

fresh water mix to form brackish water. The three main estuarine

ones--saltwater, brackish, and freshwater--can shift seasonally and vary greatly

from one area to another because of changes in...