Environmental Engineering Reference
In-Depth Information
small lakes or sag ponds. Block faulting creates grabens, resulting in some of the largest and
deepest lakes such as Tanganyika and Nyasa in Africa, and Lake Baikal in Central Asia.
Faulting and tilting together created lakes in the Great Basin and Sierra Nevada portions of
North America, including Lake Bonneville and its relict, the Great Salt Lake.
Glacial Lakes
Glaciers form lakes by scouring the surface (the Great Lakes of North America, Lake
Agassiz in northwestern United States), by deposition of damming rivers, by melting
blocks of ice leaving depressions (kettles), and by the very irregular surface of ground
moraine (
Figure 7.81)
.
Volcanic Activity
Lakes form in extinct volcanic craters and calderas, and in basins formed by the combina-
tion of tectonic and volcanic activity, such as the Mexico City basin.
Landslides
Natural dams are formed by landslides in valleys to create lakes such as the Gros Ventre
slide
(Section 9.2.3).
Solution Lakes
Depressions resulting from the collapse of caverns in limestone form solution lakes.
Floodplain Lakes
Floodplain lakes form in cutoff meanders (oxbows) or in floodplain depressions created
by natural levees (the backswamp zone).
Deflation Basins
Deflation basins, formed by wind erosion, are found in arid or formerly arid regions com-
mon to the Great Plains of the United States, northern Texas, Australia, and South Africa.
Depositional Characteristics
Sedimentation
The life cycle of a typical lake is illustrated in
Figure 7.58.
Deposition includes a wide
range of materials, the nature of which depends on the source materials, the velocity of the
streams entering the lake, and the movement of the lake water. Streams enter the lake to
form deltas; in large lakes currents carry materials away to form beaches and other shore-
line deposits.
The finer sediments are carried out into the lake where they settle out in quiet deep
water, accumulating as thick deposits of silts and clays. Since freshwater clays settle out
slowly they tend to be laminated and well stratified, and can be extremely weak. In rela-
tively small lakes with an active outlet, the fines may be carried from the lake by the exit-
ing stream.
As the lake reaches maturity and filling creates shallow areas with weak current and
wave action, plants grow and the accumulation of organic material begins.
Modern Lakes in Dry Climates
In arid climates water enters the basin, but its flow is inadequate to replenish the loss to
evaporation, and the lake quickly dries, forming a playa. As the water evaporates, salts are
