Chapter 3. Geomorphology
71
less effective since the channel has already adjusted its morphology to
accommodate high discharges (Kochel 1988).
Vegetation exerts a strong influence on the morphology and stability of
stream channels (Millar 2000). Rivers with vegetated channel banks have
channel widths half that of their weakly vegetated equivalents (Hey and Thorne
1986). Hydraulic geometry relationships that reflect how channel width, depth,
and velocity change with varying discharges consistently show the channel width
changing more rapidly with discharge in drylands compared to humid regions
(Park 1977). Regional curves show the width increasing more rapidly with
drainage area in semi-arid climates (Rosgen 1996). Consequently, braiding (Fig.
16) and sheetflooding (Fig. 14) are more common in arid regions where dense
riparian vegetation is lacking. Lateral instability also arises with an absence of
bank vegetation (Millar 2000). By inhibiting widening and lateral migration,
vegetation leads to the vertical accretion of sediments and the development of
floodplains. The effectiveness of large floods is diminished when flow energy is
dissipitated on a floodplain, ensuring that the channel morphologies present
where vegetation is dominant are reflective of the frequent low flows in the
region. Understanding the different processes, patterns, and sensitivities to
change that result from various watershed characteristics (e.g., bank vegetation,
bank composition, drainage size and lithology, hydrology) is critical for
distinguishing between morphological features formed by extreme floods
compared with the more frequent low flows.
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