lake, body of water

Introduction

lake, inland body of standing water occupying a hollow in the earth's surface. The study of lakes and other freshwater basins is known as limnology. Lakes are of particular importance since they act as catchment basins for close to 40% of the landscape, supply drinking water, generate electricity, are used to irrigate fields, and serve as recreational areas.

Formation and Fate of Lakes

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.

Global Distribution of Lakes

Lakes are not evenly distributed on the earth's surface; most are located in high latitudes and mountainous regions. Canada alone contains nearly 50% of the world's lakes. Although lakes are usually thought to be freshwater bodies, many lakes, especially in arid regions, become quite salty because a high rate of evaporation concentrates inflowing salts. The Caspian Sea, Dead Sea, and Great Salt Lake are among the greatest of the world's salt lakes. The Great Lakes of the United States and Canada is the world's largest system of freshwater lakes. Lake Superior alone is the world's largest freshwater lake with an area of 31,820 sq mi (82,414 sq km), although there is a larger volume of freshwater in Lake Baykal. The Caspian Sea is the largest lake in the world, with an area of c.144,000 sq mi (372,960 sq km). Lake Titicaca in the Andes Mts. of South America is the world's highest large lake at 12,500 ft (3,800 m) above sea level; the Dead Sea is the lowest at c.1,400 ft (425 m) below sea level.

The Environment of Lakes

The primary source of lake water is precipitation that may enter the depression directly, as runoff from surrounding higher ground, or through underground springs. Unique flora and fauna live around a lake and vary depending on the size and shape of the lake and the surrounding rocks and soil. Flora and fauna in the lake are usually found in three zones: the littoral zone closest to the shallow water shore; the limnetic, in the open, well-lit water away from most vegetation; and the lower profundal zones areas of low oxygen and light.

Ponds are generally small, shallow lakes; the criterion for differentiating between ponds and lakes is usually temperature. Ponds have a more consistent temperature throughout; while lakes, because they are deeper, have a stratified temperature structure that depends on the season.

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