Geoscience Reference
In-Depth Information
Figure 12.1
Photographs showing different types of dryland rivers: (a) the exogenous perennial Colorado River in the Grand
Canyon, Arizona, southwest USA (flow direction from upper right to middle left); (b) an endogenous ephemeral river in the Eastern
Desert, Egypt, which emerges from rugged uplands into a piedmont setting (flow direction towards camera). These photographs
illustrate many of the 'textbook' characteristics of dryland fluvial landscapes (see Table 12.1).
other climatic settings, particularly (but not exclusively)
the seasonal tropics (e.g. Knighton and Nanson, 1997;
Tooth, 2000a; Nanson, Tooth and Knighton, 2002; Powell,
2009).
In the previous edition of this topic, Knighton and
Nanson (1997) provided an assessment of the distinc-
tive, diverse and unique characteristics of dryland rivers.
They recognised that dryland rivers exist along a contin-
uum from ephemeral to perennial flow, but their proposi-
tion that dryland rivers as a group can be both distinctive
and internally diverse may be seen by some as a pos-
sible inconsistency. Drawing on research advances over
the past 10-15 years, this chapter has three main aims:
(1) to outline why claims for the distinctiveness of dry-
land rivers have arisen, while also drawing attention to
their still underappreciated diversity; (2) to expand the
global assessment of dryland river diversity initiated by
Knighton and Nanson (1997), Tooth (2000a) and Nanson,
Tooth and Knighton (2002) by focusing on three contrast-
Africa and Australia); and (3) in the light of this revised
assessment, to ask whether we can make any sound gener-
alisations regarding the distinctiveness of dryland rivers.
The emphasis in this chapter is mainly on the catchment-
and reach-scale attributes of dryland rivers, particularly
river patterns and floodplains. The short reach, cross-
sectional or bed-scale hydrological, hydraulic, geometric
and sedimentological attributes of dryland rivers, particu-
larly those associated with endogenous ephemeral rivers,
are the subject of Chapter 13.
12.2
Distinctiveness of dryland rivers
Until the mid 1980s, the bulk of dryland fluvial research
was conducted in a limited range of environmental con-
texts, principally parts of the American southwest, the
Mediterranean and adjacent areas of Israel and Kenya (e.g.
Leopold, Wolman and Miller, 1964; Reid and Frostick,
