484
Climatic Geomorphology
5. Lake systems
Closed depressions, or undrained lakes (Langbein, 1961), are relatively common in arid
areas, and are often found grouped together. They are not usually very deep and are
ephemeral in nature. Because evaporation commonly exceeds water input, they are often
dry and saline, and the water level fluctuates because of the high variability in available
water.
These lakes can, however, overflow as a consequence of excess inflow. In September
and October 1968, it rained for 38 days over the greater part of Tunisia. Rivers burst their
banks and lakes overflowed, and the excess water ran down overflow channels giving rise
to flooding. Some 542 people were drowned and hundreds of thousands made homeless.
Damage to construction and agriculture was severe (Davis, 1992).
The salinity of these desert lakes can be related to the groundwater flow toward the
lake, as occurs in the closed Bujalaroz basin in the Province of Zaragoza, Spain. The
continuous supply of water provides salts to the system - and the only way to get rid of
them is the wind. This makes the use of such lands for irrigated agriculture inadvisable; the
water supplied to the irrigation system increases the level of these lakes, perhaps making
them permanent. This can affect the flora and fauna of the region. Additional irrigation
water can cause changes that may lead to serious environmental problems (S~inchez et al.,
1993).
Human activity associated with the lakes of arid regions can be very varied, but often
they lead to their slow eutrophication or degradation (Goudie, 1986). Sometimes, in semiarid environments, lakes are partially dried to make way for crops. This is quite a normal
fate for small lakes. In the Caspian Sea, the biggest lake in the world, man-made activities
have caused a fall in the water level of over 3 m since 1929 (Goudie, 1992). In Israel, the
largest hydrological project ever undertaken was the National Water Carrier, which
transports water from the Sea of Galilee to the whole of the country' s Mediterranean coast
where it is required for irrigation (Beaumont, 1993). Consequently, the salinity of the
Dead Sea has increased considerably as the quantity of fresh water flowing into it from the
River Jordan, rising just north of the Sea of Galilee, has decreased. This increase in salinity
has killed off the Dead Sea's animal and plant life, although it has favoured the salt
extraction industry at the lake's southern tip.
Over the last few decades, the Aral Sea, the fourth largest lake in the world, has been
the focus of one of the greatest ecological disasters ever recorded. This lake lies in a classic
arid region and is fed mainly by the Rivers Amu Daria and Syr Daria, which arise in the
mountains of Pamir and Tian Shan, respectively. Since 1960 this lake has suffered a
serious reduction of its perimeter because of huge irrigation projects (L6tolle and
Mainguet, 1993) (Fig. 19.20). It once had a mean depth of 16 m and a maximum of 68 m
close to the eastern shore. Table 19.2 shows the main hydrological data for the period
1960-1990.
The removal of water has left ports high and dry, fishing has fallen from
48,000 metric t/yr in 1957 to zero, fishing villages have been reclaimed by the desert,
the water is polluted by pesticides and its salinity has increased considerably, the water
level in the area's wells has dropped by 10 m and is polluted, and the fauna and flora are
degraded, etc. (L6tolle and Mainguet, 1993). Moreover, many of the irrigated soils have
become saline and have been abandoned (Beaumont, 1993).
Climatic Geomorphology
5. Lake systems
Closed depressions, or undrained lakes (Langbein, 1961), are relatively common in arid
areas, and are often found grouped together. They are not usually very deep and are
ephemeral in nature. Because evaporation commonly exceeds water input, they are often
dry and saline, and the water level fluctuates because of the high variability in available
water.
These lakes can, however, overflow as a consequence of excess inflow. In September
and October 1968, it rained for 38 days over the greater part of Tunisia. Rivers burst their
banks and lakes overflowed, and the excess water ran down overflow channels giving rise
to flooding. Some 542 people were drowned and hundreds of thousands made homeless.
Damage to construction and agriculture was severe (Davis, 1992).
The salinity of these desert lakes can be related to the groundwater flow toward the
lake, as occurs in the closed Bujalaroz basin in the Province of Zaragoza, Spain. The
continuous supply of water provides salts to the system - and the only way to get rid of
them is the wind. This makes the use of such lands for irrigated agriculture inadvisable; the
water supplied to the irrigation system increases the level of these lakes, perhaps making
them permanent. This can affect the flora and fauna of the region. Additional irrigation
water can cause changes that may lead to serious environmental problems (S~inchez et al.,
1993).
Human activity associated with the lakes of arid regions can be very varied, but often
they lead to their slow eutrophication or degradation (Goudie, 1986). Sometimes, in semiarid environments, lakes are partially dried to make way for crops. This is quite a normal
fate for small lakes. In the Caspian Sea, the biggest lake in the world, man-made activities
have caused a fall in the water level of over 3 m since 1929 (Goudie, 1992). In Israel, the
largest hydrological project ever undertaken was the National Water Carrier, which
transports water from the Sea of Galilee to the whole of the country' s Mediterranean coast
where it is required for irrigation (Beaumont, 1993). Consequently, the salinity of the
Dead Sea has increased considerably as the quantity of fresh water flowing into it from the
River Jordan, rising just north of the Sea of Galilee, has decreased. This increase in salinity
has killed off the Dead Sea's animal and plant life, although it has favoured the salt
extraction industry at the lake's southern tip.
Over the last few decades, the Aral Sea, the fourth largest lake in the world, has been
the focus of one of the greatest ecological disasters ever recorded. This lake lies in a classic
arid region and is fed mainly by the Rivers Amu Daria and Syr Daria, which arise in the
mountains of Pamir and Tian Shan, respectively. Since 1960 this lake has suffered a
serious reduction of its perimeter because of huge irrigation projects (L6tolle and
Mainguet, 1993) (Fig. 19.20). It once had a mean depth of 16 m and a maximum of 68 m
close to the eastern shore. Table 19.2 shows the main hydrological data for the period
1960-1990.
The removal of water has left ports high and dry, fishing has fallen from
48,000 metric t/yr in 1957 to zero, fishing villages have been reclaimed by the desert,
the water is polluted by pesticides and its salinity has increased considerably, the water
level in the area's wells has dropped by 10 m and is polluted, and the fauna and flora are
degraded, etc. (L6tolle and Mainguet, 1993). Moreover, many of the irrigated soils have
become saline and have been abandoned (Beaumont, 1993).