Chapter 2. Hydrology
41
with the elevation of the identified OHWM, can be used to reconstruct the dis-
charge necessary to emplace the given OHWM. This could be accomplished with
one-dimensional or two-dimensional hydraulic models, depending on the degree
of accuracy and computational effort desired. Once the discharge is estimated,
the recurrence interval for that flow could be established to determine if the
identified OHWM is the result of an extreme event or more frequently occurring
flows. Conversely, if a recurrence interval is established that represents the
"ordinary" flow, then the depth of flow associated with the OHWM along any
given channel could be established. Such an approach, however, does not guar-
antee that the river stage associated with the chosen "ordinary" discharge will be
consistent with the level at which the physical OHWM features are found. Once
the flow conditions associated with the OHWM are established, various combi-
nations of rainfall conditions (e.g., amount, intensity) and physical watershed
parameters (e.g., drainage area, antecedent moisture conditions) necessary to
create the given discharge could be modeled.
Comparisons of hydraulic model results from different watersheds and eco-
regions could be used to determine if there is a general agreement in the recur-
rence interval of flows associated with identified OHWM features. Given the
highly variable temporal and spatial distribution of rainfall in the Southwest and
the slow channel recovery times after extreme events, congruence between
recurrence intervals over wide areas seems unlikely. Hydrology and hydraulic
studies will be extremely valuable, however, for identifying those physiographic,
climatic, or ecological subregions where there is consistency in the recurrence
interval of the "ordinary" flow (i.e., the flow responsible for creating the
OHWM) from one watershed to the next.
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