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ERDC TR-04-1
Compound Channels
Compound channels consist of a single, low-flow meandering channel inset
into a wider braided flood zone (Fig. 16) (Graf 1988a). Dramatic channel
widening and activation of braided conditions accompany extreme flow events
along dryland rivers, with a meandering form developing after a decades-long
sequence of lower flows (Kondolf and Curry 1986, Pearthree and Baker 1987,
Kresan 1988, Graf 1988a, Friedman and Lee 2002). Similar compound channels
are found in more humid climates as well (Hickin and Sichingabula 1988). An
anastomosing channel pattern forms as the low-flow channel under certain
conditions (Nanson et al. 1986) instead of a meandering channel. Discontinuous
ephemeral stream systems probably also occur within compound channels, but no
published report of this exists to date. While the braided channel pattern is often
considered the most common in drylands (Tooth 2000), single-thread, wide and
shallow, planar-bed channels are also characteristic of drylands (Fig. 13) (Reid
and Frostick 1997) and reflect the same discharge and bank conditions that
produce braiding (Tooth 2000).
The transformation of the channel back to a narrower meandering pattern
occurs by the attachment of midchannel braid bars to the channel banks by
sediment accumulation and vegetation growth (Pearthree and Baker 1987).
Vegetation plays a key role in the channel recovery process in drylands (Tooth
2000), with low flows enabling the establishment of vegetation on the channel
bed (Friedman and Lee 2002), which, in turn, helps to trap fine sediment
(Osterkamp and Costa 1987). The narrowing process is accelerated where a
reliable moisture supply increases the density and growth rate of vegetation
(Friedman and Lee 2002). Valley floor deposition by relatively frequent minor
floods is interrupted at long intervals by large floods that clear out sediment
(Schick 1974) and return the channel to a braided form.
The width:depth ratios of channels are directly related to bank composition
(Schumm 1960). Channel widening and braiding are more pronounced where the
sand content in the channel banks is higher. Silt and clay are less prevalent in the
banks of desert streams compared to more humid regions, giving rise to wide,
shallow channels with ill-defined banks and a braided pattern (Leopold et al.
1966, Baker 1977, Graf 1988a, Abdullatif 1989, Cooke et al. 1993). Vegetation
lowers the width:depth ratio of channels and increases channel sinuosity (Graf
1981), so not surprisingly the removal of vegetation can lead to braiding
(Kondolf and Curry 1986).
The geomorphic effectiveness of large floods in arid climates is well docu-
mented worldwide (Schick 1974, Baker 1977, Wolman and Gerson 1978, Graf
1988a, Cooke et al. 1993, Knighton and Nanson 1997, Tooth 2000, Kale 2002).
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