Lakes

Lakes

Lakes and ponds are bodies of standing fresh water impounded in basins and depressions in the earth's continental crust . Lakes are temporary catchment basins for flowing surface and groundwater . Freshwater reservoirs form behind natural and man-made dams, surface water collects in topographic lows, and groundwater discharges into ephemeral lakes, but eventually all continental runoff drains to the ocean. Lakes provide humans with fresh drinking water, recreation areas and, in the case of the world's largest lakes, navigable waterways for ship traffic. Regional climate strongly affects the chemical and hydrological properties of lakes, and lake sediments often provide high-resolution records of climatic fluctuations. Lake basins typically fill with interlayered coarse and fine sediments, and organic material. Many ancient lacustrine deposits contain petroleum reservoirs. Because ponds, lakes, and inland seas are smaller and less well-mixed than the oceans , they are particularly susceptible to pollution.

Tectonic motion created the crustal basins and sags that contain the world's largest lakes. Elongate, deep lakes fill the axes of incipient divergent plate tectonic boundaries, or rift zones. The lakes of the East African Rift system—Lakes Turkana, Kiva, Tanganyika, and Malawi—fill the central grabens of the rift zone between the African and Somali Lithospheric Plates . Lake Baikal, the worlds deepest (5,370 ft,

or 1,637 m) and most voluminous (Lake Baikal contains about 20% of the earth's fresh surface water) lake, occupies a rift valley in southern Siberia. Lakes also fill broad, shallow intercra-tonic basins that form during the earliest stages of continental rifting . Lake Eyre in central Australia , and Lakes Victoria and Chad in Africa are examples of lakes in shallow extensional basins.

Many modern lakes, including the Great Lakes of North America , occupy basins created by Northern Hemisphere ice sheets of the Pleistocene Epoch . The weight of the Laurentide and Eurasian ice sheets depressed large regions of the continental crust into the mantle, a phenomenon called glacial isostasy . Since the ice sheets retreated about 20,000 years ago, meltwater and stream runoff have collected in these broad depressions. Large regions of the northern continents—the Great Lakes region and the Scandinavian Peninsula for example—are presently undergoing rapid uplift, known as isotatic rebound, as these glacially depressed regions continue to readjust. Small ponds and lakes are also common in glacial environments. Erosion by moving ice carves bedrock depressions where lakes form, and leaves sills that impound glacial streams. Glacial sedimentary landforms , including moraines , kame terraces, and eskers serve as natural dams for glacial lakes. Glacial terrains are dotted with small ponds that fill circular depressions called kettles that form when ice blocks buried in glacial till deposits melt.

Lake basins also form in a number of other geologic environments. Small lakes and ponds are common in continental fold belts where outcrops of resistant bedrock divert and dam perennial streams. Abandoned meanders along low-gradient streams form circular lakes called oxbow lakes. Groundwater discharge zone lakes form where the top of the saturated zone , the water table , intersects the land surface. In humid and temperate climates, where the water table is close to the land surface, discharge lakes typically have an outlet stream. In arid regions, ephemeral groundwater discharges into closed, saline playa lakes that fill and dry seasonally.

Man-made lakes are a significant component of the earth's present-day hydrologic cycle . Most of the world's rivers have been dammed, creating reservoirs for human water supplies, recreation, and generation of electrical power. While reservoirs provide many benefits to human populations, they also force numerous readjustments to natural and artificial systems. Ecosystems must compensate for the loss of drowned habitats, human populations are displaced, and water quality is often compromised. Streams that have been segmented by dams regrade their equilibrium profiles, creating new patterns of erosion and deposition throughout the stream system. In fact, natural stream processes act to remove obstacles like dams by eroding the streambed below them, and depositing sediment in the reservoir above them. Poorly constructed and maintained dams are thus a safety hazard for downstream inhabitants.

Climatic factors control the chemical and hydrological properties of lakes. Regional variations of temperature , precipitation , and winds determine water levels, circulation patterns, vertical stratification, and the concentration of dissolved materials in lake water. The quantity and seasonality of rainfall in a drainage basin controls the balance between recharge and discharge that maintains lake level. Lake salinity is a function of the relative concentrations of dissolved ions and diluting water. During a drought , lake levels fall, salinity increases, and a lake can change from a permanent freshwater reservoir to an ephemeral saline lake. The Great Salt Lake in Utah is all that remains of Lake Bonneville, a much larger freshwater lake that existed during the wet period at the end of the Pleistocene Epoch. A 25% decrease in freshwater flow to the Aral Sea in central Asia has led to a 50% decrease in surface area and a four-fold increase in salinity since the 1960s.

Seasonal temperature variations, changes in the balance between precipitation and evaporation , and wind patterns affect lake circulation and stratification. High-latitude lakes that are subject to large diurnal temperature variations and strong winds are typically so well mixed that the water column is unstratified. Warm, stagnant low-latitude lakes are often permanently stratified. Without mixing, the lower water column of these stagnant, oligomictic lakes becomes depleted in oxygen , and aquatic plants choke the ecosystem in a process called eutrophication. Human water pollutants that contain phosphates—detergents for example—also encourage eutrophication. Temperate lakes that experience large seasonal temperature fluctuations undergo seasonal overturns in which a layer of cold surface water circulates to the bottom of the lake or the pond. This process of periodic restratification oxygenates the base of the water column and infuses lake-bottom ecosystems with nutrients. Lakebed sediments record these seasonal patterns, and can be used to deduce and date the regional climate history. Lake stratigraphers, or limnologists, use features like preserved pollens and winter-summer couplets of thin sedimentary laminae, called varves, to recreate the geochronology of a lake basin.

See also Hydrogeology