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ERDC TR-04-1
Anastomosing patterns are enhanced when the clay and silt content in the channel
exceeds 80%, as can occur when the surrounding bedrock is largely claystones
and shale (Schumann 1989). Otherwise, soils in arid regions have limited clay
because hydrolysis of feldspars and ferromagnesium minerals occurs very slowly
with the limited moisture. Very coarse material on a streambed, as is produced by
fine-grained volcanics, quartzites, and carbonates in arid regions, increase the
geomorphic effectiveness of large floods because of the high response threshold
required to scour and rework bouldery deposits (Baker 1977). Consequently the
morphological features observed in such drainages, especially in arid climates,
are more likely the result of extreme floods and not smaller-magnitude events.
Floods in the arid Southwest usually result from one of three kinds of storms:
tropical cyclones with regional extent and intense rainfalls; convective thunder-
storms of limited extent but intense rainfall; and winter low-pressure frontal
storms from the Pacific Ocean with regional extent but lower rainfall intensities
(Ely 1997). Convective thunderstorms affect smaller drainage basins but are
generally too localized to produce significant flows on larger rivers. Winter
frontal storms, in contrast, can produce moderate flows on larger rivers but have
limited impact on small drainage basins that often respond only to flashier flows.
When widespread low-intensity rainfall from frontal storms is accompanied with
local high-intensity convective storms, large volumes of runoff may result
(Burkham 1988). Tropical cyclones have the potential to dramatically impact
both large and small drainages (Kresan 1988) but are less common than the other
storm types. In many deserts, particularly where precipitation intensities are high,
the runoff hydrograph characteristically rises very steeply as the result of limited
infiltration capacity and then falls sharply in response to transmission losses
(Cooke et al. 1993). The lower infiltration capacities and higher rainfall
intensities result in higher runoff coefficients (which express the proportion of
rainfall converted to runoff) in arid regions (Knighton and Nanson 1997). The
floods most likely to result in significant geomorphic change are those that
produce discharges many times above that normally experienced by the river,
that is, those with high ratios of maximum peak discharge to mean annual
discharge (Kochel 1988). Such high ratios are characteristic of arid regions (Graf
1988a, Knighton and Nanson 1997). Humid-region rivers tend to respond less
dramatically to floods because interannual variability in peak discharges is much
lower.
The sequencing of prior flood events is critical for understanding the channel
morphology observed at any given time (Graf 1988a). A series of low flows over
a number of years may produce a smaller channel inset into a compound channel
that would be particularly sensitive to morphological change during an extreme
event. When two large storms occur close in time, the second event is generally
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