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