- Open and closed lakes
Open and closed lakes refer to the major subdivisions of lakes - bodies of water surrounded by land.
An open lake is a lake where water constantly flows out under almost all climatic circumstances. Because water does not remain in an open lake for any length of time, open lakes are always fresh water: dissolved solids do not accumulate. Open lakes form in areas where precipitation is greater than evaporation. Because most of the world's water is found in areas of highly effective rainfall, most lakes are open lakes whose water eventually reaches the sea. For instance, the Great Lakes' water flows into the St. Lawrence River and eventually the Atlantic Ocean.
Open lakes typically have stable levels which do not fluctuate because input is always matched by outflow to rivers downstream. If more water enters an open lake than was previously leaving it, then more water will leave the lake. The drainage from an open lake, like that from ordinary rivers, is referred to as exorheic (from the Greek exos, outside and rhein, to flow).
A closed lake (see endorheic drainage) is exactly the opposite. In a closed lake, no water flows out, and water which is not evaporated will remain in a closed lake indefinitely. This means that closed lakes are always saline, though this salinity varies greatly from around 3 parts per thousand for most of the Caspian Sea to as much as 400 parts per thousand for the Dead Sea. Only the less salty closed lakes are able to sustain life, and it is completely different from that in rivers or freshwater open lakes. Closed lakes typically form in areas where evaporation is greater than rainfall, although most closed lakes actually obtain their water from a region with much higher precipitation than the area around the lake itself, which is often a depression of some sort.
The level of most closed lakes is unstable because if runoff into the lake is lessened, the water balance of a closed lake is altered, and the amount of water in the lake falls. This is what has caused the shrinkage of the Aral Sea, formerly the world's second largest closed lake. Similarly, if runoff into a closed lake is increased, then the level will increase because evaporation is not likely to increase at all - let alone enough to stabilise the level of the lake.
Fluctuation in the level of closed lakes are therefore much more useful in paleoclimatology than are studies of open lakes which can reduce the level of outflow if inflow decreases.
Conversion between an open and a closed lake
If the amount of water entering a closed lake is increased beyond a certain level - for most closed lakes far more than would normally enter the lake - it is possible that the water level in the lake would rise so much that the lake would no longer be capable of absorbing it. This is rare but known conclusively in the case of Lake Corangamite in southern Victoria, when a succession of wet years during the 1950s caused the level of the lake to rise so much that it nearly overflowed. There is a great deal of evidence that in fact Lake Corangamite was an open lake prior to the end of the Little Ice Age - some evidence says it was an open lake for much of the Holocene due to a wetter climate.
It is, of course, also possible that the inflow into an open lake could decrease so much that any outflow would dry the lake up completely — that is, the open lake would become a closed lake. The only lake that is currently an open lake for which there is any evidence of a closed lake existing in the same location in the past is Lake Malawi in Africa. It is known that in very dry periods, outflow from the Shire River — Lake Malawi's only outlet — can drop precipitously, and it is strongly believed that during past dry periods outflow from Lake Malawi has ceased entirely and salt has accumulated in the lake, especially during the Last Glacial Maximum and other similarly dry periods.
Apart from Australia and Southern Africa, it is rare for runoff variability to be high enough for these changes to take place on a useful time scale, and lakes forming in areas where conversions from closed to open or open to closed are likely are very rare. However, Lake Titicaca, the largest lake in South America, has historically been a semi-open lake with very limited outflow, but recent glacial retreat has reduced inflows so much that some believe it could become a completely closed lake in the future. In North Dakota Devils Lake, historically a closed lake, is known to have been an open lake during the Medieval Warm Period. Since 1999, probably due to global warming, Devils Lake has overflowed into another terminus called Stump Lake, which could overflow into the Red River of the North if present wet conditions in the region continue.
- ^ Water Research Foundation of Australia; 1975 symposium: the 1973-4 floods in rural and urban communities; published 1975
- ^ See McMahon T.A. and Finlayson, B.; Global Runoff: Continental Comparisons of Annual Flows and Peak Discharges
- ^ Comments on Statements by North Dakotas Senator Kent Conrad in His Letters to the Great Lakes Commission Regarding the Commission’s January 23, 2003, Letter to Senators Urging Support of Senator McCain’s Amendment on the Devil’s Lake Outlet in the Omnibus Appropriations Bill for Fiscal Year 2003
- ^ North Dakota Precipitation, 1895-2010
- ^ Minnesota Precipitation 1895-2010
Wikimedia Foundation. 2010.
Look at other dictionaries:
Closed city — Central entry checkpoint to the closed city of Seversk A closed city or closed town is a settlement with travel and residency restrictions in the Soviet Union and some of its successor countries. In modern Russia, such places are officially known … Wikipedia
Romney, Hythe and Dymchurch Railway — Romney, Hythe Dymchurch Railway Northern Chief at New Romney Overview Type Light railway Locale Ken … Wikipedia
HISTORICAL SURVEY: THE STATE AND ITS ANTECEDENTS (1880–2006) — Introduction It took the new Jewish nation about 70 years to emerge as the State of Israel. The immediate stimulus that initiated the modern return to Zion was the disappointment, in the last quarter of the 19th century, of the expectation that… … Encyclopedia of Judaism
Great Lakes — a series of five lakes between the U.S. and Canada, comprising Lakes Erie, Huron, Michigan, Ontario, and Superior; connected with the Atlantic by the St. Lawrence River. * * * I Chain of lakes, central North America. Comprising Lakes Superior,… … Universalium
East African lakes — ▪ lake system, East Africa Introduction group of lakes in East Africa (eastern Africa). The majority of the East African lakes lie within the East African Rift System, which forms a part of a series of gigantic fissures in the Earth s crust… … Universalium
Outline of canoeing and kayaking — Kayakers view of Lake Coniston, Cumbria The following outline is provided as an overview of canoeing and kayaking: Canoeing is an outdoor activity that involves a special kind of canoe. Open canoes may be poled (punted), sailed, lined and tracked … Wikipedia
ALIYAH AND ABSORPTION — GENERAL SURVEY Introduction Aliyah, ascension or going up, is the coming of Jews as individuals or in groups, from exile or diaspora to live in the Land of Israel. Those who go up for this purpose are known as olim – a term used in the Bible for… … Encyclopedia of Judaism
Geography and the Church — • Explains the nature of this science and the course of its evolution Catholic Encyclopedia. Kevin Knight. 2006. Geography and the Church Geography and the Church … Catholic encyclopedia
Late-May 1998 tornado outbreak and derecho — The Late May 1998 Tornado Outbreak and Derecho was a historic tornado outbreak and derecho that began on the afternoon of May 30 extending throughout May 31, 1998, across a large portion of the northern half of the United States and southern… … Wikipedia
Great Lakes Council (Boy Scouts of America) — For the Australian local government area, see Great Lakes Council. Great Lakes Council Owner Boy Scouts of America … Wikipedia