Chapter 4. Vegetation Literature Review for Ordinary
High Water Mark Indicators in the Arid Southwest
GABRIELLE KATZ
4.1
INTRODUCTION
Riparian ecosystems are terrestrial ecosystems that occur alongside rivers in
zones that are influenced by both surface and subsurface hydrology. In arid
regions the moisture subsidy provided in river valleys often allows for the occur-
rence of gallery forests, which may be the only naturally occurring forest patches
in low-elevation landscapes (Malanson 1993, Patten 1998). These linear
corridors of forest contrast sharply with the surrounding upland deserts or grass-
lands, provide important structural habitat for a variety of wildlife species, play
an important role in the dispersal of both animals and plants, and also shade and
stabilize fluvial environments, providing habitat for aquatic organisms (Naiman
et al. 1993, Patten 1998). Where hydrologic conditions do not support riparian
forests, riparian zones may still support vegetation communities distinct in com-
position or structure from nearby uplands (Stromberg et al. 1993, Evans 2001).
The degree of contrast between riparian and upland vegetation communities in
the western U.S. is more distinct at low elevations because at high elevations
differences in moisture availability are less pronounced (Patten 1998).
The river flow regime, characterized by patterns of variation in surface flow
able" shaping aquatic and riparian environments (Poff et al. 1997). In arid
regions, however, subsurface hydrologic conditions may be equally important in
determining vegetation patterns; the moisture subsidy provided by streams results
from both surface and subsurface hydrologic conditions and their interaction. For
example, surface streamflows (e.g., annual flow volume) (Stromberg and Patten
1990), overbank flows that replenish floodplain soil moisture (Reily and Johnson
1982, Friedman and Auble 2000), and the level of the subsurface alluvial aquifer
(Stromberg et al. 1993, 1996, Scott et al. 1999) have all been identified as
hydrologic conditions that strongly influence riparian vegetation.
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