Estuaries

1) Fresh water from land enters the ocean through rivers, streams, and groundwater flowing through valleys. These valleys that channel fresh water from land to the salty ocean, which range from extremely narrow stream-cut channels to remarkably broad lagoons behind long barrier islands, are called estuaries.

2) A number of types of estuaries are commercially vital. Many commercially important estuaries are the mouths of major rivers. The powerful flow of water in major rivers maintains channels that are deep enough for navigation by ocean-bound vessels, and the rivers themselves provide transportation of goods to points farther inland. In addition, estuaries formed as a result of tectonic or glacial activity are sometimes sufficiently deep to provide ports for oceangoing vessels. The types of estuaries that are not viable as ports of call for ocean commerce are those that are not wide enough, not deep enough, and not powerful enough to prevent the buildup of sediment.

3) Estuary systems, which vary to reflect the geology of the coasts where they are found, can be broadly categorized as one of two different types. One type of estuary system is the type that is found in flooded coastal plains, the broad land areas that extend out to the continental shelves, on the Atlantic coasts of North and South America, Europe, and Africa, for example. The other category of estuary system encompasses the mountainous coasts, with their rugged topography, such as those found along the Pacific coasts of North and South America.

4) Today, much of the eastern coast of the United States is a flooded coastal plain. During the last Ice Age, much of what is today the submerged continental shelf was exposed as an extended part of the continent. Intricate river systems composed of main rivers and their tributaries cut valleys across the plains to the edge of the shelf, where they released the fresh water that they carried into the ocean. Then, as the ice melted at the end of the Ice Age, rising waters extended inland over the lower areas, creating today’s broad drowned river valleys. On today’s flooded coastal plains, the water is comparatively shallow and huge amounts of sand and sediment are deposited. 6A These conditions foster the growth of extensive long and narrow offshore deposits, many of which are exposed above the water as sandspits or barrier islands. 6B These deposits are constantly being reshaped, sometimes extremely slowly and sometimes quite rapidly, by the forces of water and wind. 6C It is common along flooded coastal plains for drowned river valleys to empty into lagoons that have been created behind the sandspits and barrier islands rather than emptying directly into the ocean. 6D These lagoons support vigorous biological activity inasmuch as they are shallow, which causes them to heat up quickly, and they are fed by a constant inflow of nutrient-rich sediments.

5) Unlike the flooded coastal plains, the mountainous coasts have a more rugged and irregular topography with deeper coastal waters. There is less sand and sediment, and external systems of barrier islands are not as pervasive as they are on flooded coastal plains because the mountainous topography blocks the flow of sediments to the coast and because the deeper ocean water inhibits the growth of barrier islands, and without the protection of barrier beaches, mountainous coasts are more exposed to direct attack by the erosive forces of waves. Different geological processes contribute to the rugged topography along mountain coasts. The tectonic activity that creates the mountains along a mountainous coast can cause large blocks of the Earth’s crust to fall below sea level; San Francisco Bay in California and the Strait of Juan de Fuca in Washington state in the north formed in this way. In the northern latitudes, coastal fjords were created as glaciers cut impressive u-shaped valleys through mountains and now carry fresh water from the land to the ocean.