118
ERDC TR-04-1
4.10
CONCLUSION
Riparian vegetation is strongly influenced by fluvial processes. For many
species, key ecological processes depend on the hydrologic conditions and geo-
morphic changes associated with floods, and this dependence leads to vegetation
establishment patterns that may be useful in identifying the level of ordinary high
water. However, straightforward equilibrium interpretations of the relationship
between arid-region riparian vegetation assemblages and specific flood regimes
are only appropriate in certain settings. In particular, such interpretations are
potentially most useful for explaining the distributions of herbaceous species, and
possibly short-lived shrubs, on perennial rivers in arid regions (Auble et al. 1994,
Stevens et al. 1995). On intermittent and ephemeral streams, flooding may be
very infrequent, leading to non-equilibrium relationships between vegetation
patterns (particularly spatial distributions of long-lived trees) and surface flow
conditions. In these situations, which comprise the majority of arid-region
riparian zones, vegetation patterns may constitute evidence of past, but not
present, hydrologic conditions or may most strongly reflect the influence of sub-
surface hydrology or other factors.
We can conduct successful hydrogeomorphic interpretations of arid-region
riparian vegetation patterns if we are careful to consider site history and patterns
of change over time. In the absence of extreme flooding within the last decade,
the distributions of seedlings, saplings, and poles of pioneer riparian trees may be
useful in determining the location of the ordinary high water level. In addition,
herbaceous plant distributions may be useful in this regard, since the distributions
of short-lived species can be expected to reflect recent ecological conditions. The
spatial distributions of disturbance-adapted species with high moisture require-
ments may indicate the extent of recent fluvial disturbance, though more research
is needed on this topic. Finally, the distributional limits of upland plant species
that do not tolerate flood disturbance may provide some indication of the location
of the zone beyond the ordinary high water level.
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