74
ERDC TR-04-1
While many of the same features used to identify bankfull features could be
used in this approach (e.g., highest surface of bars, rock staining, change in
particle size distribution), others might not apply (e.g., lower limit of perennial
vegetation, exposed root hairs, active floodplain). Additional features not
associated with a bankfull condition might also be useful for identifying the
limits of extreme events in arid regions (e.g., outermost or highest scour lines and
silt lines, micro-terraces). While morphological features associated with low
flows may develop within a compound channel, these features would be
continually changing as the channel narrows after the passing of a large flood.
The value of identifying the limits of geomorphically effective events would
be to remove the uncertainty of whether the low-flow channels represent physical
features associated with more "ordinary" flow conditions or some intermediate
condition between the ordinary and extreme events. Given that arid-region rivers
generally respond to large floods by dramatically widening their banks, the limits
of the geomorphically effective event will likely be much more extensive than
the limits of a low-flow channel inset into a compound channel. Consequently, if
the OHWM is set at the outer limits of the extreme event, the designated
"waters" will encompass a much greater area than is occupied by more ordinary
flows.
Exclusion of Areas Definitively Above and Below the OHWM
Precise location of the OHWM is especially difficult in arid regions where
the morphological features present on the landscape are possibly the result of:
Flow events of various magnitude (e.g., high-flow and low-flow channels
of compound channels);
Multiple processes (e.g., sheetfloods and streamflows on discontinuous
ephemeral streams); or
No longer active processes (e.g., channels abandoned by avulsion on
alluvial fans and anastomosing streams).
Given these uncertainties, a more effective approach to locating the OHWM
may be to identify areas on the land surface that are definitively above or below
the OHWM. Vast portions of piedmonts and valley bottoms in the Southwest
have been removed from active fluvial processes for thousands, in many places
hundreds of thousands, of years (Field and Pearthree 1997). Once isolated from
the influence of flooding, a distinctive suite of weathering, erosional, and
pedogenic features develop that become more pronounced with age (Table 7).
Areas where these features are found can be considered above the OHWM since
they have not been inundated by floods for several millennia. Not all of the
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