114
ERDC TR-04-1
4.9
IDENTIFYING THE ORDINARY HIGH WATER MARK:
SUMMARY AND SYNTHESIS
Because of the close association between riparian vegetation and stream
hydrology, there is considerable potential for using vegetation patterns to assist in
identifying the ordinary high water mark in the arid Southwest. To this end,
vegetative evidence can be considered for two cases: evidence of locations below
the OHWM (Table 13) and evidence of locations above the OHWM or outside of
the OHW zone (Table 14).
Several points become clear upon examination of these tables. First, the
ecology of some riparian systems is not well understood, and these systems are
therefore under-represented. In particular, very little has been published on
xeroriparian systems, yielding only speculative entries in the tables. More
information regarding the role of streamflow in influencing spatial and temporal
patterns of xeroriparian species is needed before the observed patterns can be
reliably used to indicate specific levels of surface flow. Also, mid- to high-
elevation mixed deciduous forests have not been nearly as well studied as have
the lower-elevation Populus-, Salix-, and Tamarix-dominated systems. While the
higher-elevation riparian forests contain pioneer tree species (i.e. Populus, Salix,
Platanus) whose ecology is relatively well understood, they also contain a variety
of seral species with different life history traits and environmental tolerances.
Efforts to understand the role of hydrology in influencing rates and patterns of
ecological succession in these systems would provide helpful information for
delineating areas beyond the ordinary high water level.
Second, many of the lines of evidence listed in the tables may be applicable
beyond the specific settings addressed in the published examples. For example,
fluvial marsh taxa should indicate locations below the OHWM in high-elevation
systems, as well as in the low-elevation systems for which examples are
provided. For each potential indicator listed, the likely elevation range (mid- to
high elevation, low elevation) and riparian moisture type (Hydro-mesic riparian,
Mesoriparian, Xeroriparian) for which it is applicable is provided. For cases
where published examples are not listed, the potential indicators should be care-
fully tested.
Third, although analysis of the current scientific literature provides several
potential vegetation indicators of the OHWM, in many (or most) cases this was
not the purpose of the original research. Therefore, the use of specific indicators
based on a single published source is probably not advisable. Rather, these
findings should be taken as suggestions regarding the types of patterns that could
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