Desert lakes: playa lakes and sebkhas
393
The largest one is called La Playa, and it is more than 4 km in length. The dissolution of
the underlying Miocene formations composed of alternating beds of limestone and
gypsum is responsible for the genesis of many small depressions. The basins are elongated
with the main axis following a NW direction, which corresponds to the prevailing
winds ("cierzo") and the main tectonic lineaments and fractures (Quirantes, 1965; Pueyo,
1978 to 1979, Sancho and Guti6rrez, 1993).
Piping is another process leading to the appearance of closed depressions in semi-arid
regions (Figure 13.23). In Lupifien in the Huesca Province of Spain in the central sector of
the Ebro Depression, piping occurs along the contact between the loose, very-porous, thick
(up to 15 m) slope deposits and the Miocene Formations. The enlargement of the tunnels
and pipes decreases the mechanical resistance of the ceilings and the cavities finally
collapse. On the surface, the collapsed areas are indicated as pseudolines up to 35 m wide
and 20 m deep and with funnel or cylinder shapes (Figure 13.22 and Figure 13.23)
(Guti~rrez et al., 1988).
Animal activity provides some examples of erosive basins. Erosive processes may be
accelerated because of increased grazing by large herbivores and by soil compaction
caused by large herds. Elephants excavate with their legs and trunks closed depressions
up to 23 m in diameter and 1.5 m deep (Thomas, 1988).
Sedimentary depressions can be formed after a drainage system is abandoned or
changed by another fluvial network. Maizels (1987) described a complex series of
paleochannels in Oman that during the Plio-Pleistocene alternation of humid and dry
periods caused closed depressions. Also in floodplains some lacustrine depressions can be
formed. Dunes can impound creeks and rivers, as it happens south and east of Timbuktu in
Mali. Lava flows may produce the same effects. Littoral evolution causes the appearance
of beaches and bars, and the formation of littoral sebkhas (Glennie, 1970). Some of the
albuferas in the Spanish Mediterranean coast, such as the Pego and Elche Albuferas, have
the same origin (Sanjaume, 1985).
Finally, volcanic activity builds craters that could be partially filled up with water as
with Zuni Salt Lake in New Mexico. Meteoritic impacts in arid zones form impact craters;
Meteor Crater in Arizona is the most outstanding example of this type of closed
depression.
3. General features of hydrology of lakes from arid zones
Lake level fluctuations depend upon water input and output balance. Inputs are surface
run-off, groundwater, and secondarily, direct rainfall over the lake. Outputs are
evaporation, infiltration and, when the lake basin overflows, the flow of the outlet.
When input and output are not equal, the lake level fluctuates and shorelines retreat and
advance. In arid regions these fluctuations are very frequent, except in very deep lakes
such as the East Africa rift valley lakes. Desert climates are characterized by scarce
rainfall and intense evaporation, and consequently, lakes dry out very often.
Lake water volume depends on the relative altitude of the watershed boundaries in
relationship to the lake bottom and the lake bathymetry. With time, watershed highs are
eroded and bottom irregularities are covered with sediments, and, consequently, water
storage capacity of the basins decreases. Height and width can be modified by aeolian
deflation, which usually tends to widen and deepen the closed depression. The shape and
393
The largest one is called La Playa, and it is more than 4 km in length. The dissolution of
the underlying Miocene formations composed of alternating beds of limestone and
gypsum is responsible for the genesis of many small depressions. The basins are elongated
with the main axis following a NW direction, which corresponds to the prevailing
winds ("cierzo") and the main tectonic lineaments and fractures (Quirantes, 1965; Pueyo,
1978 to 1979, Sancho and Guti6rrez, 1993).
Piping is another process leading to the appearance of closed depressions in semi-arid
regions (Figure 13.23). In Lupifien in the Huesca Province of Spain in the central sector of
the Ebro Depression, piping occurs along the contact between the loose, very-porous, thick
(up to 15 m) slope deposits and the Miocene Formations. The enlargement of the tunnels
and pipes decreases the mechanical resistance of the ceilings and the cavities finally
collapse. On the surface, the collapsed areas are indicated as pseudolines up to 35 m wide
and 20 m deep and with funnel or cylinder shapes (Figure 13.22 and Figure 13.23)
(Guti~rrez et al., 1988).
Animal activity provides some examples of erosive basins. Erosive processes may be
accelerated because of increased grazing by large herbivores and by soil compaction
caused by large herds. Elephants excavate with their legs and trunks closed depressions
up to 23 m in diameter and 1.5 m deep (Thomas, 1988).
Sedimentary depressions can be formed after a drainage system is abandoned or
changed by another fluvial network. Maizels (1987) described a complex series of
paleochannels in Oman that during the Plio-Pleistocene alternation of humid and dry
periods caused closed depressions. Also in floodplains some lacustrine depressions can be
formed. Dunes can impound creeks and rivers, as it happens south and east of Timbuktu in
Mali. Lava flows may produce the same effects. Littoral evolution causes the appearance
of beaches and bars, and the formation of littoral sebkhas (Glennie, 1970). Some of the
albuferas in the Spanish Mediterranean coast, such as the Pego and Elche Albuferas, have
the same origin (Sanjaume, 1985).
Finally, volcanic activity builds craters that could be partially filled up with water as
with Zuni Salt Lake in New Mexico. Meteoritic impacts in arid zones form impact craters;
Meteor Crater in Arizona is the most outstanding example of this type of closed
depression.
3. General features of hydrology of lakes from arid zones
Lake level fluctuations depend upon water input and output balance. Inputs are surface
run-off, groundwater, and secondarily, direct rainfall over the lake. Outputs are
evaporation, infiltration and, when the lake basin overflows, the flow of the outlet.
When input and output are not equal, the lake level fluctuates and shorelines retreat and
advance. In arid regions these fluctuations are very frequent, except in very deep lakes
such as the East Africa rift valley lakes. Desert climates are characterized by scarce
rainfall and intense evaporation, and consequently, lakes dry out very often.
Lake water volume depends on the relative altitude of the watershed boundaries in
relationship to the lake bottom and the lake bathymetry. With time, watershed highs are
eroded and bottom irregularities are covered with sediments, and, consequently, water
storage capacity of the basins decreases. Height and width can be modified by aeolian
deflation, which usually tends to widen and deepen the closed depression. The shape and