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).

In a closed lake (see endorheic drainage), 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.

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