Physical Characteristics of Lakes
Lakes vary to a large degree in location, origin, type of water, and size.
Lakes occur throughout the world, but most are in regions of abundant rainfall. In dry areas lakes usually exist temporarily after rains as playa lakes, or they vary greatly in size with wet and dry seasons. Lake Chad, near the Sahara desert in Africa, is an example of one that greatly changes size. Lakes also occur at almost all elevations. The Dead Sea is 1,312 feet (400 m) below sea level; Lake Titicaca, in South America, lies about 12,510 feet (3,810 m) above sea level.
Lakes are formed in many ways. Glaciers, by scouring depressions and depositing debris that blocks water drainage, have created most of the world's lakes. The thousands of lakes dotting parts of northern Asia, Europe, and North America are of glacial origin. Among the largest of these are the Great Lakes and Canada's Great Bear and Great Slave lakes.
Some lakes are trapped remnants of prehistoric seas and oceans. Among the most notable of these is the Caspian Sea. Craters of extinct volcanoes occasionally are lake sites. Crater Lake in Oregon is of this type.
Lakes may form in depressions caused by faulting, or fracturing, of the earth's surface. Russia's Lake Baykal, the world's deepest lake, originated in that way. Of similar origin are lakes in the Great Rift Valley, which extends from the Jordan River valley to southeastern Africa. Lakes originating here include the Dead Sea and Lakes Rudolf, Tanganyika, and Nyasa. Reelfoot Lake, in Tennessee near the Mississippi River, was formed by earthquake activity in 1811-12. Lakes are often formed when rivers flood and change their courses. Among lakes of this type are the Salton Sea and the many oxbow (U-shaped) lakes along the lower course of the Mississippi River.
Lakes are often classed according to the kind of water they contain—fresh or saline (salt). In freshwater lakes, the amount of water entering by rivers, springs, and precipitation equals or exceeds the amount lost by evaporation and outflow. Such lake waters contain relatively small amounts of soluble salts.
Saline lakes have a high salt content. They usually occur in dry areas. Most saline lakes are gradually decreasing in size because of excessive evaporation. They may be isolated bodies of seawater, such as the Caspian Sea, or they may be remnants of former freshwater lakes. Great Salt Lake, for example, is one of the few remaining segments of glacial Lake Bonneville, a huge freshwater lake that existed thousands of years ago. Salt lakes that dry up leave crusts of hard mud and salt called alkali flats.
Over time, lakes change in size. Lakes typically become smaller, eventually turning into marshes, swamps, or even dry land. A lake decreases in size as sediment accumulates on its bottom and along its edges. Some of the sediment is carried by streams into the lake. Other sediment accumulates from the decayed remains of algae, plants, fish, and other organisms that die in the lake.