Explanation:

Most estuaries can be grouped into four geomorphic categories based on the physical processes responsible for their formation: (1) rising sea level; (2) movement of sand and sandbars; (3) glacial processes; and (4) tectonic processes.

1.  Drowned river valley or coastal plain estuaries were formed by rising sea level during the last interglacial period (about 15,000 years ago) which flooded river valleys that were cut into the landscape when sea level was lower. Chesapeake Bay (Virginia and Maryland) and Galveston Bay (Texas) are classical examples of this type of estuary.

2.  The movement of sand and formation of sandbars along the coastline can enclose bodies of water and form lagoon-type or bar-built estuaries such as Laguna Madre, Texas.

3. In colder climates, glaciers cut deep valleys in the landscape. When glaciers recede during warmer climate periods, coastal waters fill the valley to form fjord-type estuaries, which are common in New England and Alaska.

4.  Finally, earthquakes and faulting (the development of faults ) may cause the rapid sinking of coastal areas below sea level to form tectonically produced estuaries such as San Francisco Bay, California.