Introduction to Lake
Lake, a sizable inland body of standing water. Not having a precise meaning, the word lake has been applied to many different bodies of water. It is usually applied to natural inland waters such as Lake Michigan, but it is also used for many man-made reservoirs, such as Lake Mead behind Hoover Dam. The word is sometimes loosely applied to coastal bodies of water, examples being Lake Maracaibo in Venezuela and Lake Pontchartrain in Louisiana. Widened parts of rivers are occasionally called lakes. An example is Lake Pepin on the Mississippi between Minnesota and Wisconsin.
Some large bodies of water that are really lakes are commonly called seas. These include the Aral, Caspian, and Dead seas in southwestern Asia and the Salton Sea in California.
Lakes are major natural resources. Large lakes, such as the Great Lakes of North America, have a tempering influence on the local weather. They provide low-cost transportation, supply water for homes and industries, and offer a wide variety of recreational facilities. Lakes provide water for irrigation systems and water power projects. Some lakes support important commercial fisheries.
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.
Eutrophication
Nitrates, phosphates, and certain other chemical compounds promote the growth of algae and plants in a lake. These chemical compounds increase in a lake over time—in a process called eutrophication. Under natural conditions, the increase is slow and gradual. However, pollution (as from city sewage or runoff from fields that have been fertilized) greatly speeds up the process. As the quantity of the chemical compounds in the lake increases, the population of algae and plants also increases.
One consequence of eutrophication is an increase in the accumulation of decaying algal and plant matter in a lake. In a lake in the advanced stages of eutrophication (such as a heavily polluted lake), the bacteria that feed on this matter use up much of the oxygen dissolved in the water. The reduction in the amount of dissolved oxygen leads to changes in the types of species in the lake. For example, commercially and recreationally desirable fish such as trout are replaced by undesirable fish such as carp.
