WORKING DEFINITION: OHW IN THE ARID SOUTHWEST
We need a working definition of OHW for the arid Southwest to allow the
selection of OHWM indicators. The definition must recognize that stream
channels in the arid Southwest have greater flow magnitudes, more rapid
response to rainfall, and shorter-duration flows than those in the humid East,
where precipitation is more evenly distributed both temporally and spatially.
Channel morphology in arid regions is mostly a result of either recent "ordinary"
events or the most recent extreme event, which cannot be generalized across a
landscape because of frequent localized storms and diverse topography. The
great variability in hydrologic conditions can be expected to result in an even
more tenuous relationship between flow recurrence interval and channel charac-
teristics (such as bankfull discharge) than exists in less arid areas. Consequently
the working definition does not identify OHW as a function of a flood recurrence
interval. Rather, it focuses on the dominant processes in a channel that result in
the development of potential OHWM indicators.
We proposed the following definition of OHW in arid stream channels:
That part of the active channel where sediment transport is due to the
most frequent or repeating hydrologic discharges, resulting in the
development of bed and bank or other physical features, including
vegetation, representing long-term trends in either storm or annual
discharge events. This definition recognizes that, in some instances,
extreme events may have developed the outermost physical features of
the active channel and that the current "ordinary" limits may occur
within these features.