The Earth's crust in parts of Alaska and the Great Lakes area, which was pressed down by the weight of the massive ice sheets that blanketed the north during the last great ice age, is now rising due to the retreat of the glaciers. In recently abandoned deltas, such as near the mouth of the Mississippi River, compaction of newly deposited sediment results in extensive land subsidence (as much as 1 meter per century). Along tectonically active coasts, such as the coast of earthquake-prone southern California, land may rise as much as 4 centimeters per century. Other local changes in water level occur when the land either rises or falls relative to the water. A prolonged period of wet weather in Utah in the early 1980's, for example, raised the Great Salt Lake's water level to record highs, flooding parts of Salt Lake City. Predominantly closed bodies of water such as lakes experience dramatic water-level changes in response to precipitation, spring snowmelt, and evaporation. Water-Level Changes In addition to the daily cycles of tides, many other forces lead to significant changes in water level. These seasonal trends result in a general difference between the winter "eroding" beach and the summer "building" beach that is most common along parts of the west coast. Storms often are concentrated in specific seasons along the eastern seaboard, for example, hurricanes occur in the late summer and early fall, and storms are especially frequent during the winter months. Some areas are more storm prone than others. Storms carry sand seaward, forming offshore bars much of this sand migrates landward during calm weather.
Although storms are sporadic, they are the primary cause of beach erosion along many coasts. Storm systems along coasts contain high winds, create large waves, and cause storm surges that raise water levels as much as 7 meters above normal. Rip tides, or undertow, occur along most beaches and can move significant amounts of sand offshore. Tides help determine where the waves break - low on the beach at low tide, high on the beach at high tide - and, therefore, where sand is deposited and removed. Tides ebb and flood in response to the gravitational attraction of the moon and sun exceptional high and low tides occur each month when the sun and moon are aligned. These longshore currents are a primary agent of coastal movement they are a major cause of sand migration along barrier and mainland beaches. Even the slightest angle between the land and the waves will create currents that transport sediment along the shore. In many areas, prevailing winds produce waves that consistently approach the coast at oblique angles. Winds create waves that ripple across the surface of lakes and seas until they break on the shallowing bottom and crash into the shore. Taken individually, each natural process of coastal transport is complex taken collectively, they create an extraordinarily intricate system that attempts to achieve a dynamic balance. Natural processes that change the water level also affect coastal dynamics.
Plants retain sediment in wetlands and impede movement of coastal dunes. Storms cause deep erosion in one area and leave thick overwash deposits in another. Rivers carry sediment to the coast and build deltas into the open water. Tidal cycles bring sand onto the beach and carry it back into the surf. Breaking waves move sand along the coast, eroding sand in one area and depositing it on an adjacent beach. Earth-science research on coastal dynamics can quantify these changes and improve our ability to predict coastal responses to human actions.Ĭoastal Focus: Montauk Lighthouse, New York Natural Processess Coastal lands and sediments are constantly in motion. Human activity adds yet another dimension to coastal change by modifying and disturbing, both directly and indirectly, the coastal environments and the natural processes of change. Regional and local characteristics of coasts control the differing interactions and relative importance of these natural processes. Diverse and complex natural processes continually change coasts physically, chemically, and biologically, at scales that range from microscopic (grains of sand) to global (changes in sea level).
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