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ERDC TR-04-1
various gradients, sediment sizes, and volumes and rates of discharge. Field
(Chapter 3) listed a suite of potential OHWM indicators below, at, and above
OHW. No literature specifically discussed the relationship of landform types to
these indicators, but these stream types are typically located within specific
landform settings and they may sort along landform types. The distribution of
OHWM field indicators by landform may play an important role in helping to
increase the reliability of the indicators.
Vegetation
Riparian vegetation is one of the most dynamic plant communities within the
arid landscape. Dramatic changes in vegetation patterns can occur over short time
scales because of periodic cycles of destruction and regrowth from flooding
events. As a result of these disturbances, the ability of riparian vegetation to have
"pure stands" or "climax" vegetation is limited in these environments. Periodic
flooding can quickly change the distribution and species composition and reset
the disturbancerecovery cycle. It is within these natural cycles that patterns of
species composition, age, succession, or physical characteristics develop that can
be useful indicators of OHW.
Climatic patterns, elevation, and other influences operating at both regional
and landscape scales strongly influence riparian plant assemblages (Bendix 1994,
Minckley and Brown 1994). At the reach scale, effects of geology, floodplain
soil chemistry, sediment particle size, disturbance, and land management are all
superimposed on the primary hydrologic effects (Chapter 4). Vegetation pattern
responses are also tied to lateral gradients in inundation duration and frequency,
floodwater depth, velocity and unit stream power, and depth to groundwater that
vary with distance from the stream channel (Stromberg 1993, Auble et al. 1994,
Stromberg et al. 1996, Stromberg 1998, Bendix 1999).
Several major factors distinguish riparian plant community arrangements
along arid streams. Elevation is associated with increases of precipitation, an
increase in the abundance of vegetation, and the selection of certain assemblages
of species. For example, Arctic-Boreal wetlands in subalpine environments and
Interior Riparian Deciduous Forest at mid-elevations are dominated by species
that are associated with more mesic conditions than those on lower slopes and
valley floors (Minckley and Brown 1994). Another factor influencing the
arrangement and distribution of riparian communities is local moisture
availability. Johnson et al. (1984) recommend a classification scheme based on
three distinct vegetation types associated with different hydrologic regimes--
hydroriparian, mesoriparian, and xeroriparian. This scheme recognizes variation
in available water within riparian corridors and the resulting vegetation
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