Causes of Relative Sea Level Change

Relative sea level may change in response to vertical land movement or changes in the level of the ocean surface as shown in Figure 1.

Vertical movements of the land may give the appearance of a rising sea level. For instance, sediment carried by a major river such as the Mississippi River is deposited in its delta area where it empties into the ocean. The sediment and nutrients discharged by slowly moving waters as the river meanders over the low-lying flood plain have built up marshes at a rate that has matched or exceeded the natural subsidence of the land due to settling and horizontal expansion of the saturated silt that comprises the delta subsurface. Flood control structures and drainage practices of the last 50 years have prevented the marshes from receiving replacement soil, leaving them in a state of slow and irreversible subsidence. Coastal land in Louisiana presently is being lost at a rate of 50 square miles per year, which will mean loss of land equivalent to the size of the state of Rhode Island in 21 years.

Extraction of groundwater or oil in coastal areas also can lead to settling or outward movement of the land surface. Natural vertical movements in coastal regions may be due to interaction of continental plates on geological time scales. Another process operating on time scales of 10,000 years is the process of isostatic rebound. During the last ice age, the whole northern North American Continent was covered with ice several kilometers thick. This represents an enormous amount of weight on the continent. Melting of this ice over a few thousand years (which may be considered abrupt on geological time scales) leads to an upward rebound of the continental plate due to its elastic characteristics. This rebound is similar to the rebound of a bed mattress when you jump out of bed. Vertical motion due to elastic rebound can be calculated and eliminated from sea-level measurements on the basis of known elastic properties of earth materials and knowledge of the mass of ice previously located over the continent.

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