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Many lakes were formed as a result of glacial action during the Pleistocene ice sheets. In some areas, as exemplified by the Great Lakes, basins were carved into bedrock by the erosive action of the advancing ice mass. Lake basins are also formed by glacial moraine deposits that dam preexisting stream valleys. Lakes also form in calderas, created by the collapse of volcanic craters. Where extensive limestone deposits underlie a region, groundwater can dissolve great volumes of the limestone, forming caves that often contain underground lakes and eventually, if the roofs collapse, leave deep lake basins. Tectonic activity in the earth's crust forms lake basins in many ways, such as fault-generating rift valleys as those found in E Africa, that often fill with water. Oxbow lakes form in abandoned stream channels in floodplains of meandering rivers. Deposition of sediment along a shoreline can cut off bays, forming coastal lagoons. Humans often form lakes by building dams across river valleys for flood control, hydroelectric generation, or recreational purposes.
Lakes are transient features on the earth's surface and generally disappear in a relatively short period of geologic time by a combination of processes (e.g., erosion of an outlet or climatic changes that bring drier conditions). In a process called eutrophication, a lake gradually fills with organic and inorganic sediment, becoming a swamp or bog, and eventually a meadow. Human activity has greatly increased the rates of eutrophication; urban and suburban land construction activities result in increased discharge of soil debris into streams draining into lakes, filling them.
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