Chapter 4. Vegetation
93
For identifying reliable vegetative indicators of the ordinary high water mark,
it is important to distinguish between the effects of surface flows and the effects
of groundwater levels on vegetation patterns. This task is likely to be challenging
for at least two reasons. First, surface and ground waters are almost always
connected and interacting, which means that the two variables do not vary inde-
pendently in riparian settings. Riparian sites with shallow groundwater levels are
much more likely to be characterized by perennial or near-perennial stream flow
than are sites characterized by deeper alluvial water tables. Because these two
key hydrologic characteristics co-vary, in some cases it may be difficult to tease
apart the specific effects of surface flow on vegetation characteristics at the site
scale. Second, riparian sites are characterized by complex environmental gradi-
ents, along which multiple physical factors may vary in tandem. Of particular
importance are the lateral gradients in inundation duration and frequency, flood-
water depth, velocity and unit stream power, and depth to groundwater that occur
with distance from the stream channel at many arid-region riparian sites
(Stromberg 1993c, Auble et al. 1994, Stromberg et al. 1996, Stromberg 1998,
Bendix 1999). That is, in general at a single riparian site, near-channel locations
are likely to be characterized by higher inundation frequencies, floodwater
depths, velocities, and stream power and also by shallower groundwater than are
locations farther from the stream channel. Although these variables are likely to
be highly correlated, the relationships are not always consistent. Bendix (1999)
found that stream power of the 20-year flood and floodplain surface height (i.e.,
depth to groundwater) were poorly correlated at sites in California. Arid-region
riparian vegetation patterns can be viewed as integrating the physical character-
istics of the entire complex floodplain gradient, not solely the surface flow char-
acteristics. Efforts to isolate the effects of surface flow are most pertinent for the
present context and will be emphasized in this review.
Any attempt to understand the effects of hydrologic conditions on riparian
vegetation must consider that non-hydrologic factors may also strongly impact
riparian vegetation patterns and processes. At the regional and landscape scales,
climatic patterns and elevation strongly influence riparian plant assemblages
(Bendix 1994, Minckley and Brown 1994). At the reach scale, effects of geology,
floodplain soil chemistry and sediment particle size, disturbance, and land
management are superimposed on the primary hydrologic effects. For example,
fire (Bendix 1994, Busch 1995) and livestock grazing (Belsky et al. 1999) may
have profound and long-lasting impacts on riparian vegetation patterns in arid
regions.
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