Chapter 3. Geomorphology
69
3.4
DIFFERENCES AMONG STREAM TYPES
A number of factors contribute to the development of differences between
dryland and humid-region rivers (Table 8) and among stream types within
drylands. Channel morphology and position are highly variable, both spatially
and temporally, on most dryland rivers due to limited vegetation, unstable sandy
banks, transmission losses, and high interannual variability in peak discharges.
Smaller drainage basins have a greater sensitivity to large floods, especially in
arid climates, where stream widths remain largely unchanged for drainage areas
exceeding 50 km2 due to transmission losses (Wolman and Gerson 1978). Dis-
continuous ephemeral stream systems and alluvial fans are most prevalent in, but
not restricted to, piedmont settings. Compound channels, anastomosing rivers,
and single-thread channels with adjacent floodplains generally occupy the valley
bottoms.
Bedrock lithology and soils play a strong role in the size of sediment
available and consequently the processes that occur in dryland streams.
Discontinuous ephemeral steam processes are enhanced in granitic terrains and
with other lithologies that weather to sand and finer particles, as this ensures that
sediment is mobilized during even low to moderate flows (Wells 1997, Blair
1999, Field 2001). Drainage basins with mixed lithologies such as dolomite,
quartzite, and shale produce clay to boulder-sized particles necessary for the
formation of debris flows, a dominant process on some alluvial fans (Blair 1999).
Table 8. Comparison of hydrologic and channel characteristics of arid- and humid-region
rivers. (After Knighton and Nanson 1997.)
Characteristics
Arid-region rivers
Humid-region rivers
Hydrologic
Low and unreliable
Relatively high and dependable
characteristics
Long-duration precipitation of variable
Limited duration but high-intensity storms
intensity
Extreme annual variability in peak
Less temporal variability in peak
discharges
discharges
Precipitation events of limited spatial
Large areas generally affected
extent
Transmission losses downstream
Tributary inputs downstream
Ephemeral flow
Perennial flow
Sharply peaked runoff hydrograph
Runoff hydrographs have lower amplitude
Channel
Floods as major channel formers
Channels adjusted to frequent discharges
characteristics
Long recovery time after disturbance
Quicker channel recovery
Transient behavior dominant
Tendency for channels to reach equilibrium
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