Chapter 3. Geomorphology
73
Table 10. Advantages and disadvantages of different methods for defining the OHWM in
arid regions.
Method for
defining OHWM
Advantages
Disadvantages
Geomorphically
- Less ambiguous to delineate in the
- Extraordinary events delineated, not
effective event
field
ordinary
- Good for establishing probable
maximum flood
- Provides only range of area within
Exclusion of areas
- Definitively removes areas from
which the OHWM is located, so less
above and below
consideration as either above or
precise
OHWM
below the OHWM
- Less likely to lead to false positives
- Provides a means of checking the
accuracy of determinations
established by another method
Recognition of the
- Accounts for dominant process in arid
- Temporal in nature and difficult to
transitory nature of
regions
define spatially
desert rivers
- Maximum limit of the OHWM
- Imprecise
delineated
- More precise than geomorphically
effective approach when long time
since last flood
Hydrologic/hydraulic
- Precise elevation of the OHWM
- Uncertainties in use of gauges in
methods
established
desert regions
- Possible inconsistencies with
physical features
Consequently four approaches for delineating the OHWM in arid regions are
described below with a description of the potential advantages and disadvantages
of each approach (Table 10): identification of the limits of the geomorphically
effective event; exclusion of areas definitively above and below the OHWM;
recognition of the transitory tendency of arid-region rivers; and application of
hydrologic and hydraulic methods.
Identification of Limits of Geomorphically Effective Events
In humid regions the geomorphically effective event, or the flow that leaves
the most lasting impression on the landscape, is the bankfull flow. Thus, using
the limits of the geomorphically effective event to delineate the OHWM is
essentially the current practice in humid regions. Applying the same concept to
arid regions would be straightforward in terms of identifying physical features on
the ground, but the outer limits of extreme flows would be delineated, not the
annual flow.
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