II.

Geological Formation

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 (a warm period during which polar caps melt), when sea level rose about 120 m (about 394 ft). 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. See also Glacier; Ice Ages.

Tidal bores—sudden, extreme rises in tidal water—occur in some estuaries, such as the Bay of Fundy and the mouth of the Amazon River. When strong tides are forced upward through narrow channels, the water can rise swiftly and often to great heights.