Chapter 3. Geomorphology
79
3.6
CONCLUSIONS
A review of the geomorphic literature shows that channel morphology on all
five arid-region stream system types is spatially and temporally variable. This is
unlike humid-region rivers, where an equilibrium condition is reached. Conse-
quently, traditional methods of delineating the OHWM, originally developed in
humid regions, are inadequate for the arid Southwest. Four alternative
approaches for defining and delineating the OHWM in arid regions each have
their own advantages and disadvantages (Table 10). Any method used should be
combined with an exclusionary approach because any surface with well-
developed weathering features (Table 7) should be considered definitively above
the OHWM. Mapping alluvial surfaces of different ages provides a relatively
simple method for narrowing the search for the OHWM since large areas of
desert piedmonts have been isolated from fluvial processes for tens of thousands
of years (Fig. 20) (Field and Pearthree 1997).
Whatever approach is taken to delineate the OHWM, the transitory nature of
desert rivers must be recognized and the flexibility provided to adjust OHWM
delineations over time. Avulsions on alluvial fans and anastomosing streams
change the location of subsequent flows, large and small. Along compound
channels, large floods obliterate existing flow paths, and low-flow channels may
be re-established in new locations. On discontinuous ephemeral streams,
morphological changes to the channel result not only from large floods but also
from aggradation caused by small floods.
The normal condition for desert-region rivers is a tendency towards estab-
lishing a meandering channel form (compound channels and anastomosing
streams) or a sheetflood zone (discontinuous ephemeral streams and alluvial
fans). In the absence of an extreme threshold crossing event that would create a
braided form, precipitate an avulsion, or initiate arroyo cutting, the features
marking the extent of the low-flow channel will become more pronounced, and
identifying the extent of the "ordinary" flow will be easier. In many areas,
though, the "ordinary" flow event is ineffective at erasing the morphological
impact of more extreme events. Consequently physical features associated with
the OHWM in these settings will be the result of extreme events.
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