394
Climatic Geomorphology
the storage capacity of the lake basin can be modified by tectonics in active regions.
Volcanism plays a similar role. This is the case of the numerous volcanoes located in the
East Africa rift valley, which in more active periods altered the morphology of the lakes
located in these large tectonic structures.
Some complex lake basins are composed of connected sub-basins, and water, solutes
and sediments are transferred and mixed from one sub-basin to another. Sediment and
solute transport occurs along the surface channels or during flooding episodes when one
sub-basin overflows (Currey, 1994).
Lake level decrease in desert lakes is mostly controlled by evaporation which can easily
account for 2 to 3 m loss per year. Near Timbuktu in Mali, 2.5 to 3 m of water evaporated
from the diffluent Niger lakes was recorded during an 8-month period (Tricart, 1969). As
precipitation in arid zones is very irregular, after a long drying-out period, intense rains
may flood the lakes for several years. These large flooding episodes are characteristic of
large basins, because small basins commonly dry out in less than a year.
Lake Eyre in the Simpson Desert of Australia has a surface area of 8000 km 2, a
watershed about 100 times larger, and an annual rainfall between 100 and 500 mm
(Bonython and Mason, 1953). During the mid-20th century, the lake was flooded after a
200 to 400 mm rainfall in the southern area of the lake during February and March of 1949
(Figure 16.12a). After more intense rainfall during the next year, particularly about
500 mm during the January to April period, the lake filled up in September 1950. The lake
level desiccation phase started the following month, with a salinity increase and lake level,
water volume, and lake surface area decreasing (Figure 16.12b). In 1953 the lake had
completely dried out. The average salt composition is NaC1 (96.14%) and CaSO4 (2.38%).
Lake Eyre fills up completely about 12 times per century.
From a geomorphological point of view, lacustrine processes are more active in the
shallow, littoral environments, where the water column is mixed by turbulent flow, and
low-energy waves are active transport agents towards the coast and along the littoral zone.
Beaches, bars, and other morphologies are common littoral features.
Detailed studies of littoral morphology and evolution, and analyses of stratigraphic
records have allowed reconstructions of raising and lowering lake levels of many
lacustrine systems during the Pleistocene and Holocene ages. These reconstructions are
the hydrograph of the lake during those periods (Figure 16.13).
4. Sedimentation in desert lakes
Sediment deposition in closed depressions located in arid zones is mostly controlled by the
hydrological regime. In dry playa lakes only affected by flooding, sediments are mostly
silts and clays transported by the water inflows. During higher energy floods, sand can be
transported into the playa and deposited in small channels. This kind of clastic
sedimentation occurs mostly along the margins of the basin; alluvial fans may also develop
in these areas. Waters reaching the central areas of the basins are subjected to strong
evaporation, and consequently, salinity increases and salts precipitate after chemical
saturation is reached. In wet playa lakes, saline ground waters are dominant and sediment
deposits comprise salt-imbedded clastics and evaporites (Mabbutt, 1977).
Climatic Geomorphology
the storage capacity of the lake basin can be modified by tectonics in active regions.
Volcanism plays a similar role. This is the case of the numerous volcanoes located in the
East Africa rift valley, which in more active periods altered the morphology of the lakes
located in these large tectonic structures.
Some complex lake basins are composed of connected sub-basins, and water, solutes
and sediments are transferred and mixed from one sub-basin to another. Sediment and
solute transport occurs along the surface channels or during flooding episodes when one
sub-basin overflows (Currey, 1994).
Lake level decrease in desert lakes is mostly controlled by evaporation which can easily
account for 2 to 3 m loss per year. Near Timbuktu in Mali, 2.5 to 3 m of water evaporated
from the diffluent Niger lakes was recorded during an 8-month period (Tricart, 1969). As
precipitation in arid zones is very irregular, after a long drying-out period, intense rains
may flood the lakes for several years. These large flooding episodes are characteristic of
large basins, because small basins commonly dry out in less than a year.
Lake Eyre in the Simpson Desert of Australia has a surface area of 8000 km 2, a
watershed about 100 times larger, and an annual rainfall between 100 and 500 mm
(Bonython and Mason, 1953). During the mid-20th century, the lake was flooded after a
200 to 400 mm rainfall in the southern area of the lake during February and March of 1949
(Figure 16.12a). After more intense rainfall during the next year, particularly about
500 mm during the January to April period, the lake filled up in September 1950. The lake
level desiccation phase started the following month, with a salinity increase and lake level,
water volume, and lake surface area decreasing (Figure 16.12b). In 1953 the lake had
completely dried out. The average salt composition is NaC1 (96.14%) and CaSO4 (2.38%).
Lake Eyre fills up completely about 12 times per century.
From a geomorphological point of view, lacustrine processes are more active in the
shallow, littoral environments, where the water column is mixed by turbulent flow, and
low-energy waves are active transport agents towards the coast and along the littoral zone.
Beaches, bars, and other morphologies are common littoral features.
Detailed studies of littoral morphology and evolution, and analyses of stratigraphic
records have allowed reconstructions of raising and lowering lake levels of many
lacustrine systems during the Pleistocene and Holocene ages. These reconstructions are
the hydrograph of the lake during those periods (Figure 16.13).
4. Sedimentation in desert lakes
Sediment deposition in closed depressions located in arid zones is mostly controlled by the
hydrological regime. In dry playa lakes only affected by flooding, sediments are mostly
silts and clays transported by the water inflows. During higher energy floods, sand can be
transported into the playa and deposited in small channels. This kind of clastic
sedimentation occurs mostly along the margins of the basin; alluvial fans may also develop
in these areas. Waters reaching the central areas of the basins are subjected to strong
evaporation, and consequently, salinity increases and salts precipitate after chemical
saturation is reached. In wet playa lakes, saline ground waters are dominant and sediment
deposits comprise salt-imbedded clastics and evaporites (Mabbutt, 1977).