Transitions in WaterWhen 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 theoceans. Because fresh water is lighter, or less dense, than salt water, unless thetwo 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 countercurrentand produce a saltwater wedge below the freshwater surface waters (Bellamy

62).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, wavemotions, and river runoff can also be important locally (Braun 45). Salt water andfresh water mix to form brackish water. The three main estuarineones saltwater, brackish, and freshwater can shift seasonally and vary greatlyfrom one area to another because of changes in river flow. Also, an area of anestuary can change from stratified to well-mixed during the spring neap-tidecycles.The most highly stratified estuaries are the ones that receive a largeamount of fresh water but that have a relatively low tidal range. Partially mixedestuaries have moderate freshwater inflow and tidal range. The brackish zone ofsuch estuaries may have a salinity of 2 to 10 parts per thousand (ppt), comparedwith the salinity of salt water, which is about 35 ppt. Where there is a large tidalrange but little freshwater inflow mixing is more complete. In coastal lagoons,where there are large open waters, small tidal range, and low freshwater inputs,wind is usually a more important mixing agent than tides. It is truly evident theawesome role estauries play in the transition between salt and fresh water.