Earth Science: Earth – Lakes – Circulation
Geologically speaking, most lakes are both young and short-lived. Human activity also contributes to the fact that the sensitive ecological balance of many lakes is severely threatened.
A lake is a water-filled hollow, which is enclosed by land and does not have a direct connection to the sea. The water may originate from precipitation, or aboveground or subterranean inflow; it is lost again through evaporation.
Spring lakes are those with no surface inflow; flow-through lakes have both in-and outflow; in blind lakes, above-surface in- and out- flow is absent; and terminal lakes lack an outflow. A distinction can also be made between permanent or perennial lakes, which have sufficient inflow throughout the entire year; periodic lakes, which carry water only during the rainy season; and episodic lakes, which retain water only after periods of heavy precipitation.
Large lakes can influence the local climate, as their ability to store heat tends to buffer climatic extremes. They store water during periods of precipitation, and humidify their surroundings in the dry season.
Transformation characteristics
In geological terms, lakes have only a short life expectancy, because they become terrestrialized due to a buildup of river sediments and plant matter. The biological aging process is influenced decisively by eutrophication. The lake is gradually enriched with nutrients from the atmosphere and inflowing water. This results in increased growth of phytoplankton and in an elevated oxygen production in the upper layers (epilimnion).
Because of the breakdown of dead organic material in the lower region (hypolimnion) of the lake, oxygen is quickly used up again. Rapid exchange of water slows down eutrophication. In some Canadian lakes the rates of in- and outflow are so high that the entire water volume is exchanged within a few weeks. At the other end of the scale, it takes 700 years for the water in Lake Tahoe (in California) to be exchanged.
SALT LAKES
Most salt lakes were originally freshwater lakes. If inflow does not equal evaporation over an extended period, the concentration of salts and minerals in a lake increases steadily. In an extreme case, the salt content can exceed that of natural seawater and then salt precipitates in a crystalline crust.
These types of lakes are found principally in dry regions. Some of the best known are the Salar de Uyuni in Bolivia, the Great Salt Lake in Utah, Lake Eyre in Australia, and the Dead Sea on the Israeli-Jordan border.