Desert rivers - Climatic Change in Deserts - page 159

During the early 1940s, two young German geologists, HennoMartin and Hermann
Korn, spent two-and-a-half years living in the remote gorges of the Namib Desert and
witnessed several flash floods, one triggered by a mere fifteen minute downpour:
The whole thing [rainstorm in the distant mountains] had lasted perhaps a quarter of an hour,
and an hour later we heard the roaring of water rushing down the main gorge. We ran quickly
to see it as it swirled along, a tumultuous frothing bore of brown water over two metres
high, flattening the tough bushes in its path, uprooting trees and tossing them into the air like
matchsticks. It seemed almost incredible that a short downpour could produce such a volume
of water. (Martin, 1983 , p. 270)
Not all desert rivers are presently as ephemeral as the Alashan and Namib examples
just mentioned. The Nile is a well-known example of a desert river that originates
well-outside the desert, bringing sediments from its mountainous headwaters to be
deposited along its narrow flood-plain in hyper-arid Egypt. The ever-perceptive Hero-
dotus (ca. 485-425 BC) commented more than 2,500 years ago on the Nile silt in
Egypt, noting that 'the soil of Egypt does not resemble that of the neighbouring coun-
tryofArabia,orofLibya,orevenofSyria...butisblackandfriable as one would
expect of an alluvial soil formed of the silt brought down by the river from Ethiopia'
(translated by Aubrey de Selincourt, 1960 , p. 106).
In fact, many desert rivers flow from well-watered uplands adjoining the desert and
remain perennial today, like the Awash River that flows from the highlands of Ethiopia
into the hyper-arid Afar Desert, finally disappearing into Lake Abhe, which is now
a vast and very shallow lake. The spectacular algal limestone pillars that rise several
score metres above lake floor level bear witness to times of higher lake level (Fontes
and Pouchan, 1975 ; Gasse, 1975 ; Gasse, 1976 ) when flow in the Awash was greater
than it is today. The Pliocene and Pleistocene alluvium in the now arid Middle Awash
Valley ( Figure 10.1 ) contains a wealth of fossils, including the well-known hominid
fossils of Australopithecus afarensis and cognate discoveries (see Chapter 17 ).
Likewise, the Cooper and Diamantina rivers that flow from the Eastern Highlands
of Australia towards Lake Eyre are but a shadow of their Pleistocene ancestors, as
shown by their deep and extensive distal alluvial sands (Nanson et al., 1992 ; Cohen
et al., 2010a ) and, indirectly, by the high lake strandlines abutting the present salt
lake (Magee, 1998 ; Magee et al., 1995 ; Magee and Miller, 1998 ; Magee et al., 2004 ).
Today Lake Eyre only receives water from its feeder channels during exceptionally
wet years, most notably during extreme La Ni na events (Allan, 1985 ;Kotwicki, 1986 ;
Allan et al., 1996 ; Kotwicki and Allan, 1998 ), including in 2011 and 2012. Indeed,
once extensive freshwater lakes fed by formerly active river systems are a feature of
many deserts (Mabbutt, 1977 , pp. 262-271; Cooke et al., 1993 , pp. 202-219). The
extensive linear network of salt lakes in western Australia (Van de Graaff et al., 1977 )
provides an enduring legacy of the rivers that flowed when Australia and Antarctica
were part of a single supercontinent more than 45 million years ago. Similar now