Geoscience Reference
In-Depth Information
13
Channel form, flows and sediments of
endogenous ephemeral rivers in deserts
Ian Reid and Lynne E. Frostick
13.1
Introduction
derstatement to declare that the explosive increase in both
rainfall-runoff and sediment-transport data for perennial
systems of temperate latitudes has not been matched by
that collected in drylands. As a result, there is little ba-
sis for stochastic or other forms of modelling of flows in
ephemeral rivers. Each piece of information, each mea-
sured flood, is fairly unique and has to be treasured as a
gem that gives insight into the workings of these myste-
rious elements of these vast landscapes. For the geomor-
phologist, hydrologist and sedimentologist, this remains
pioneering territory, even at the start of the twenty-first
century.
Notwithstanding the small total information base for
ephemeral river systems, they have figured greatly in
the development of process-orientated geomorphology
through the incorporation of the results of work done
in the southwestern United States and collated in the
timely text of Leopold, Wolman and Miller (1964). How-
ever, the impetus that this text might have given to work on
desert streams per se was lost in its adoption as a general
statement about river processes by a generation whose
imagination it had caught. There was also the fact that the
rash of process studies that broke out during the 1960s
and 1970s inevitably focused on the backyards of those
concerned. These backyards were (and still are) predom-
inantly in humid environments where water catchments
are (or could be) fully clothed with vegetation, where the
soil fauna ensure a perforated medium that accommodates
infiltering rainfall and where the stream at the bottom of
the hill has a perennial flow regime.
More recently, attempts have been made to redress the
continuing imbalance in emphasis on windblown sedi-
ments in desert environments by giving equal attention
Water is a significant agent of erosion in arid lands. Upon
entering a desert or semi-desert area, it soon becomes ob-
vious that access is often facilitated or hindered by the
river system. Yet, until recently, the role of rivers in shap-
ing desert landscapes has generally been underestimated
by geomorphologists. The reasons for this are several.
First, there has been a tendency to concentrate attention on
processes and forms that are thought to be more peculiar
to drylands. Seminal texts such as that of Bagnold (1941)
and collections of papers such as those of McKee (1979)
and Brookfield and Ahlbrandt (1983) have highlighted
the action of wind and given prominence to windblown
dunes. This has been reinforced by popular portrayals of
the desert in novels and on film. Second, the infrequence of
rainfall and runoff in drylands has made data acquisition
an expensive proposition for the fluvial geomorphologist.
Few have had either the financial resources or the patience
to collect long-term records that consist largely of non-
information and that are punctuated only spasmodically
with a frenzy of relevant data. Notable exceptions are Re-
nard and Keppel (1966) and Stone
et al
. (2008) in Arizona
and Schick (1970), Reid, Laronne and Powell (1995) and
Alexandrov
et al
., 2009 in Israel. Indeed, many of those
who have examined ephemeral streams have done so over
shorter periods of time. However, to do this, they have
moved towards semi-arid areas where rivers are no less
dry and where higher rainfall gives a greater likelihood of
flash floods (see, for example, Leopold and Miller, 1956;
Thornes, 1976; Frostick and Reid, 1977).
There is, as a consequence, a dearth of information
about processes in ephemeral rivers. It would be an un-
