Trapped In Time 82 The Lake Speaks Part 4

Alkali sink, also known as salt flats, are lakes on the other extreme of the scale from the large and deep saline lakes. They are shallow saline features that occupy low-lying areas of the arid regions and in groundwater discharge zones. These are typically cla.s.sifiable as either playas or playa lakes because they are periodically flooded by either rain or flood events and then dry up during drier intervals, leaving acc.u.mulations of brines and evaporitic minerals.

A salt pan (saltpan) is either an undrained small shallow natural depression in which water acc.u.mulates and evaporates leaving a salt deposit or the shallow lake of brackish water occupying a salt pan. This term is also used for a large pan for recovering salt by evaporation.

A saline pan is a name for an ephemeral saline lake which precipitates a bottom crust that is subsequently modified during subaerial exposure.

Lakes composed of other liquids

Lava lake is a large volume of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression.

Hydrocarbon lakes are bodies of liquid ethane and methane that occupy depressions on the surface of t.i.tan. They were detected by the Ca.s.sini–Huygens s.p.a.ce probe.
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Paleolakes

A paleolake also spelled palaeolake, is a lake that existed in the past when hydrological conditions were different. Quaternary paleolakes can often be identified on the basis of relict lacustrine landforms such as relict lake plains and coastal landforms that form recognizable relict sh.o.r.elines, which are called paleo sh.o.r.elines. Paleolakes can also be recognized by characteristic sedimentary deposits that acc.u.mulated in them and any fossils that these sediments might contain. The paleo sh.o.r.elines and sedimentary deposits of paleolakes provide evidence for prehistoric hydrological changes during the times that they existed.

Types of paleolakes include:

A former lake is a lake which is no longer in existence. Such lakes include prehistoric lakes and lakes which have permanently dried up often as the result of either evaporation or human intervention. Owens Lake in California, USA, is an example of a former lake. Former lakes are a common feature of the Basin and Range area of southwestern North America.[46]

A shrunken lake is a lake which has drastically decreased in size over geological time. Lake Aga.s.siz, which once covered much of central North America, is a good example of a shrunken lake. Two notable remnants of this lake are Lake Winnipeg and Lake Winnipegosis.[46]

Paleolakes are of scientific and economic importance. For example, Quaternary paleolakes in semidesert basins are important for two reasons. First, they played an extremely significant, if transient, role in shaping the floors and piedmonts of many basins. Finally, their sediments contain enormous quant.i.ties of geologic and paleontologic information concerning past environments.[47] In addition, the organic-rich deposits of pre-Quaternary paleolakes are important either for the thick deposits of oil shale and shale gas that they contain or as source rocks of petroleum and natural gas. Although of significantly less economic importance, strata deposited along the sh.o.r.e of paleolakes sometimes contain coal seams.[48][49]

Characteristics

Lakes have numerous features in addition to lake types, such as drainage basin (also known as catchment area), inflow and outflow, nutrient content, dissolved oxygen, pollutants, pH, and sedimentation.

Changes in the level of a lake are controlled by the difference between the input and output compared to the total volume of the lake. Significant input sources are precipitation onto the lake, runoff carried by streams and channels from the lake's catchment area, groundwater channels and aquifers, and artificial sources from outside the catchment area. Output sources are evaporation from the lake, surface and groundwater flows, and any extraction of lake water by humans. As climate conditions and human water requirements vary, these will create fluctuations in the lake level.

Lakes can be also categorized on the basis of their richness in nutrients, which typically affect plant growth. Nutrient-poor lakes are said to be oligotrophic and are generally clear, having a low concentration of plant life. Mesotrophic lakes have good clarity and an average level of nutrients. Eutrophic lakes are enriched with nutrients, resulting in good plant growth and possible algal blooms. Hypertrophic lakes are bodies of water that have been excessively enriched with nutrients. These lakes typically have poor clarity and are subject to devastating algal blooms. Lakes typically reach this condition due to human activities, such as heavy use of fertilizers in the lake catchment area. Such lakes are of little use to humans and have a poor ecosystem due to decreased dissolved oxygen.

Due to the unusual relations.h.i.+p between water's temperature and its density, lakes form layers called thermoclines, layers of drastically varying temperature relative to depth. Freshwater is most dense at about 4 degrees Celsius (39.2 °F) at sea level. When the temperature of the water at the surface of a lake reaches the same temperature as deeper water, as it does during the cooler months in temperate climates, the water in the lake can mix, bringing oxygen-starved water up from the depths and bringing oxygen down to decomposing sediments. Deep temperate lakes can maintain a reservoir of cold water year-round, which allows some cities to tap that reservoir for deep lake water cooling.

Lake Teletskoye, Siberia

Since the surface water of deep tropical lakes never reaches the temperature of maximum density, there is no process that makes the water mix. The deeper layer becomes oxygen-starved and can become saturated with carbon dioxide, or other gases such as sulfur dioxide if there is even a trace of volcanic activity. Exceptional events, such as earthquakes or landslides, can cause mixing which rapidly brings the deep layers up to the surface and releases a vast cloud of gas which lay trapped in solution in the colder water at the bottom of the lake. This is called a limnic eruption. An example is a disaster at Lake Nyos in Cameroon. The amount of gas that can be dissolved in water is directly related to pressure. As deepwater surfaces, the pressure drops and a vast amount of gases come out of solution. Under these circ.u.mstances, carbon dioxide is hazardous because it is heavier than air and displaces it, so it may flow down a river valley to human settlements and cause ma.s.s asphyxiation.