Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 95

large lakes having a major impact on total area (Fig. 6.3). A variety of ge-
ological processes lead to the formation of these lakes (Table 6.3). Hutchin-
son (1957) described these processes in detail; I give a brief version here.
Tectonic movements of the earth's crust (Fig. 6.4) form some of the largest
and oldest lakes. For example, warping of the earth's crust formed the Great
Rift Valley in Africa and has given rise to Lakes Edward, Albert, Tanganyika,
Victoria, Nyasa, and Rudolf. This group contains some of the oldest, deepest,
and most ecologically and evolutionarily interesting lakes on Earth. Although
small tectonic lakes are more numerous than large lakes, the large tectonic
lakes cover an area that is greater than that covered by the small ones on a
global scale (Fig. 6.5). Graben lakes are tectonic lakes formed where multiple
faults allow a block to slip down and form a depression. Lake Baikal of
Siberia, the deepest and oldest lake on Earth, is a graben lake. About 7 km of
sediment has accumulated on the bottom of Lake Baikal over 16 million years
(Fig. 6.6). Tectonic movements also form horst lakes. In this case, the blocks
tilt and leave a depression that can be filled by water (Fig. 6.4).
Damming by natural processes can form lakes. Examples of these
processes include landslides, lava flows, drifting sand dunes, and glacial
moraines. In addition, beaver ponds, damming by excessive plant growth,
flows of rivers at deltas, glacial ice dams, and pools formed at the edges of
large lakes by shore movement are classified into this general category.
These lakes usually are not large, but some exceptions exist (e.g., Lake
Sarez, a large landslide lake in Russia, is 500 m deep).
7
6
5
4
3
2
1
0
10 6
10 5
10 4
10 3
10 2
10 1
Lake size range (km 2 )
FIGURE 6.3 Global numbers and total areas of lakes by surface area size class (data from
Meybeck, 1995).