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 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, 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 river flow. Also, an area of an estuary can change from stratified to well-mixed during the spring neap-tide cycles. The most highly stratified estuaries are the ones that receive a large amount of fresh water but that have a relatively low tidal range. Partially mixed estuaries have moderate freshwater inflow and tidal range. The brackish zone of such estuaries may have a salinity of 2 to 10 parts per thousand (ppt), compared with the salinity of salt water, which is about 35 ppt. Where there is a large tidal range 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 the awesome role estauries play in the transition between salt and fresh water.