An inland body of water that occupies a sizable depression
in the Earth’s surface and is too deep to allow terrestrial
vegetation to grow across the entire body. Lakes have many
sizes and shapes, ranging from just bigger than a pond to virtual
inland seas thousands of square kilometers in area. Lakes
may form in depressions left by erosion, melting ice, in wide
parts in streams, behind natural and manmade dams, and in
fault-controlled depressions. Many lakes are located in tectonic
depressions, including extremely deep rift basins such as Lake
Baikal in Russia, Lakes Malawi, Tanganyika, Edward, Albert,
and Turkana in the East African rift. Lake waters have a large
variation in chemistry, including fresh and saline varieties, as
well as those with variable contributions from hot springs and
other deep crustal fluids. Other chemical variations depend in
part on the chemistry of inflowing streams, evaporation, and
biological activity.
Being water-filled depressions, lakes tend to become
filled with sediments over time. There is a huge variety in
the types and styles of sedimentation in lakes, depending on
the types of sediment available in the source area, the physiography
of the lake and surrounding area, the climate,
fauna and flora present, and the manner in which the sediments
settle in the basin. Many lakes exhibit a density stratification,
with warm, oxygenated, well-mixed waters near
the surface in the epilimnion layer, and cold, dense, anoxic
waters in the hypolimnion layer near the bottom. Oligotrophic
lakes are those where winter surface cooling leads
to overturning of the lake waters and mixing of the waters.
In low latitudes where there is no appreciable winter cooling,
the surface layer does not cool and the lake waters do
not overturn. These eutrophic lakes have bottom waters
that remain anoxic.
See also EUTROPHICATION; FACIES; SEDIMENTARY ROCKS.
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