Sea Level Factors and the Estuary

Sea Level Factors and the Estuary
Developed by Susan Soberhad
Fall 97

Summary:

Estuaries are greatly affected by changes in sea level--both on a long-term and short-term basis. Tidal changes that affect estuaries occur daily, fortnightly, and annually. These regular, observable changes affect the rate of the growth of intertidal landforms likes salt marshes, coral reefs, and barrier islands. Tides also set parameters for ecological zones in the estuary.

Secular changes in sea level relate to periods of geologic time--from 100 to 100,000 years. These changes affect the shape of the estuary, and associated changes in climate may also contribute to the rate at which an estuary fills.

The Tidal Factor:

Tides worldwide may fluctuate anywhere from 0.1 meters to nearly 20 meters. In addition, storm waves and tsunamis may also cause periodic increases in the height of sea level by up to two meters. Atmospheric low pressure systems also can cause sea level changes by about 10 mm per 1 mb of atmospheric pressure. A 700 mb hurricane could raise local sea levels by as much as 3 meters.

Estuaries in areas of low tidal action tend to accumulate sediments more rapidly. This is due to minimal "flushing out" of sediments by the tides. Low tidal action also creates areas of high ecological stress, making it difficult for plants and animals to survive.

In areas of large tides, sediments are easily removed from the main channels by the flow of water, and plants and animals flourish. The boundaries of this type of estuary tend to remain constant for centuries. This type of estuary favors the growth of coral reefs, especially in tropical estuaries with low runoff.

The Historical Component:

Slow change in estuaries over a much longer period of time (one or more years), is affected by two variables.

1. First there is the eustatic variable that relates to the volume of sea water in the entire ocean. In the four successive ice ages which have occurred over the last 65 million years of geologic history, sea level has risen as glaciers melted, and decreased as glaciers formed. Over the last two million years (this time frame corresponds to the Quaternary Ice Age), a cycle of rising and sinking sea levels has been noted. Every 100,000 years or so, sea level has risen and sunk from 100 - 150 meters. The last glaciers to advance did so about 17,000 years ago, and sea level was down from present-day levels by about 135 meters. This means that, at that time, the shoreline was close to where the continental shelf is today. Present-day estuaries were not in existence.

As the advancing glaciers began to melt, sea level rose rapidly and continental shorelines were inundated. Sea level was rising at the rate of about 10 millimeters a year, and today's current estuaries began to take shape. Six thousands years ago, the mean sea level reached ins present average level, and the clock began ticking for the infilling cycle of today's estuaries around the world. Presently, most estuaries have reached their dynamic equilibrium. Over long periods of geologic time, they will become progressively filled, partially or completely, by prograding deltas. Deltaic sediments are deposited in the estuary until is completely full, and the delta continues to grow until is near the edge of the continental shelf.

Since 6,000 years ago, sea level has fallen about three meters. However, beginning in about 1890, sea level has been rising steadily at the rate of about 1 millimeter per year.

2. The second variable that affects long-term sea level changes and estuarine development is any vertical change in the height of the earth's crust.

Glaciers placed a great weight on the earth's lithosphere, and large areas of the earth's crust were depressed. As the glaciers melted and the land was relieved of its heavy burden, the lithosphere itself began to rise in a process called isostatic uplift. Although the initial "bounce back" was fairly rapid, in some areas, the land is still uplifting at a rate of about 10 millimeters per year. In areas of dramatic uplift, the land is rising faster than sea level is rising, and the result is that exposed land surfaces are growing toward the sea. For estuaries in these areas, this continued uplifting means that the estuaries are getting shallower and are drying out--much of Stockholm and Montreal are built on emerged estuaries.

As areas under glaciers were depressed, land areas surrounding the glaciers bulged upward. With the melting of the glaciers, these bulging areas began to sink back down to former levels. The estuaries found in these areas (e.g., the eastern coast of the United States), are deepening, although the deepening tends to be offset by heavy sedimentation in local areas.

The shape of estuaries today was determined in part by these ancient geological processes.

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