Desert rivers - Climatic Change in Deserts - page 162

Table 10.1. Some attributes of fluvial systems in arid and in humid regions

Humid Regions Arid Regions

Abundant precipitation Limited precipitation

Dense plant cover Bare/sparse plant cover

Abundant shallow and deep groundwater Limited shallow groundwater

Rapid, complete weathering Slow, incomplete weathering

Deep cohesive slope mantles Shallow, non-cohesive slope mantles

High infiltration, low run-off Low infiltration, high run-off

High base flow Low base flow

Regular perennial stream flow Ephemeral/seasonal flow, flash floods

Low sediment yield (silt, clay) High sediment yield (sand, gravel)

Suspension-load channel Bed-load channel

Stable sinuous channel Unstable shifting channel

Deep and narrow channel Wide and shallow channel

Low stream gradient Steep stream gradient

Low rate of flood-plain build-up Rapid rates of local aggradation

External outlet to drainage basin Often no outlet and internally drained

of past environmental change and to see whether unambiguous climatic signals can
be discerned by studying the history of desert rivers. Before we do this, it is worth
considering some of the more important differences between river systems in humid
regions and those in deserts ( Table 10.1 ).

10.2 Some attributes of desert river systems
DeMartonne andAufrere's ( 1928 ) threefold classification of river systems as endoreic,
exoreic and areic, mentioned in Chapter 4 , remains a useful one, provided that we
remain aware that river status may change over time as a result of tectonic, volcanic or
climatic events (Frostick and Reid, 1987b ; Vita-Finzi, 2012a ; Vita-Finzi, 2012b ). For
example, an exoreic river system that once flowed to the sea may become blocked by
tectonic uplift and diverted inland, thereby becoming endoreic. Climatic desiccation
may then lead to disintegration of the drainage network, converting it from endoreic
to areic.
An equally important distinction is that recognised recently between two quite
different types of river system, one termed a distributary fluvial system (Weissmann
et al., 2010 ) and the other the conventional axial drainage system with its main chan-
nel ( Figures 10.3 and 10.4 ), levees, flood-plains, terraces (abandoned flood-plains:
Figures 10.2 and 10.5 ) and back swamps. These latter systems have been invest-
igated in detail by hydrologists and fluvial geomorphologists during the past fifty
and more years (Baulig, 1950 ; Leopold et al., 1964 ; Gregory and Walling, 1973 ;
Gregory, 1977 ; Schumm, 1977 ;Gregoryetal., 1995 ; Anderson et al., 1996 ;Inam
et al., 2007 ; Singh, 2007 ). The distinction between a distributive fluvial system and an