Lake

Lakes are inland bodies of water—millions of which are scattered over the earth's surface. Lakes are classified on the basis of origin, age, salinity, fertility, and water circulation. Lakes can be formed by glaciers, tectonic plate movements, river and wind currents, and volcanic or meteorite activity. Lakes can also be a phase of evolution in the aging process of a bay or estuary. Some lakes are only seasonal, drying up during parts of the year. As a lake reaches old age, it can become a marsh, bog, or swamp. Young lakes have clear water with less organic matter, while older lakes have murkier water and higher levels of organic matter as well as nitrogen, phosphorous, and detritus or decaying matter. Salinity is a measure of the dissolved ionic components in lake water. High salinity lakes, salt lakes, have high levels of precipitates and less organic matter, whereas freshwater or low salinity lakes have fewer precipitates and more organic matter. Lake shape, climate, and salinity each effect water movement within a lake, contributing to an individual lake's annual circulation patterns. Most lakes exchange surface water with bottom water at least once during the year, but multiple factors influence this complex process. Life within any given lake is determined by multiple factors as well and is of considerable interest to fishermen and marine biologists. Lakes are used for several purposes other than for the food they contribute to the food chain: they are used for recreation and enhance scenic beauty. The study of fresh water, including lakes and ponds, is called limnology.