Chapter 3. Geomorphology
75
features presented in Table 7 will be present at a given site, but using multiple
features is more reliable than using single features for establishing the relative
length of time that different areas have been isolated from fluvial processes
(McFadden et al. 1989). The degree of development of each feature can be
quantified and a threshold value established that could be used to determine if the
area was safely above the OHWM.
A number of sedimentary structures are associated with ephemeral streams
(Table 11), and the presence of fresh undisturbed features at a given location
would be evidence of recent fluvial activity. Whether the structures present were
deposited by normal low flows or by an extreme flood may be difficult to
ascertain, so their occurrence would only be definitive evidence of being below
the limits of the geomorphically effective flood. They would represent only
possible evidence for being below the extent of the average annual flow unless
their presence is coupled with additional information indicating that no signifi-
cant flow events had occurred in the watershed for a significant period of time.
The features listed in Table 11 are easily disturbed, so if no extreme flow events
had occurred in the past decade or so, their presence in pristine condition would
likely be the result of low flows. Determinations of whether an area is defini-
tively below the OHWM would be strengthened by the presence of multiple
sedimentary structures. Many of the features in Table 11 are common to all of the
five stream system types described above and thus would be widely applicable.
Delineating the OHWM by this exclusionary approach would require identi-
fying along a given river cross section those areas that are definitively above the
OHWM (Table 7) and those areas definitively below the OHWM (Table 11). The
OHWM itself would then be located in the zone between those areas excluded.
The area not excluded would likely encompass areas inundated by extreme
floods since these zones would not possess fresh sedimentary structures (assum-
ing that no recent extreme flood has occurred) nor would they have well-
developed weathering features that form only when fluvial activity ceased
thousands of years before.
The exclusionary approach will not lead to precise determinations but will
provide a definitive way of identifying zones within which the OHWM is located
and could serve as a check on other techniques; the OHWM delineated by
another method should fall within the zone identified using the exclusionary
approach. In some cases delineation of the OHWM using the exclusionary
approach will constrain the location of the OHWM to a fairly narrow zone, such
as in narrow valleys or where there is large relief between river terraces and the
active flood zone. In other areas the zone might be quite wide and perhaps of
limited usefulness in reasonably locating the OHWM. A wide zone of uncer-
tainty, however, may be an accurate reflection of the uncertainties present on the
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