Chapter 3. Geomorphology
67
Figure 22. Anastomosing channels along Cooper Creek, Australia. Note the
primary channel that is active during low flows running from left to right
across the center of the photo. (Copyright, Colin P. North, University of
Aberdeen, Scotland; used by permission.)
also form in sand-bed rivers where vegetation serves as a stabilizing influence
(Wende and Nanson 1998, Tooth and Nanson 2000b). While anastomosing rivers
are mostly reported from different climates in Australia (Tooth 2000), they are
present in the arid western U.S. (Schumann 1989, Malisce 1993) but are not
common in the Southwest, given the necessary prerequisites of high suspended
load and/or dense vegetation.
Channel avulsions are an important process in the development of anasto-
mosing rivers, with overland flow from the main channel enlarging and
ultimately capturing the smaller anabranches (Schumann 1989). While avulsions
on braided rivers tend to occur by lateral migration of banks (Cazanacli et al.
2002), avulsions on anastomosing systems are generally the result of the head-
ward extension of anabranches (Schumann 1989). Processes within the main
channel may be similar to meandering rivers, with the formation of cut banks and
point bars, but the relief of such features is generally lower in arid climates
(Malisce 1993). The morphological evolution of anastomosing river channels and
their relationship to avulsions share many similarities to discontinuous ephemeral
streams on alluvial fans (Fig. 19) (Field 2001). The development of benches at
the base of the main channel due to bank sloughing leads to greater overland
flow, which drives the avulsion process (Schumann 1989). As on alluvial fans,
the dramatic temporal and spatial changes in channel morphology and position
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