42
ERDC TR-04-1
2.4
CONCLUSIONS
Variable moisture sources and the complex physiography in the Southwest-
ern U.S. results in a highly variable spatial and temporal distribution of precipi-
tation and stream flow. Differences in watershed size and relief ensure that the
stream response to a single precipitation event can vary between watersheds even
within the same physiographic or ecological subregion. Consequently, regional
generalizations about runoff patterns are difficult to make, and watershed-
specific studies will be necessary to accurately delineate the OHWM.
Physical features associated with the OHWM are more readily identified
using geomorphic methods, but hydraulic and hydrologic studies are potentially
valuable for establishing the discharge recurrence intervals and runoff conditions,
respectively, responsible for forming the OHWM features. Numerous hydrologic
(Table 5) and hydraulic (Table 6) models are available for characterizing runoff
and flow conditions associated with the OHWM, but no single model is likely to
be superior in all situations. Comparing discharge recurrence intervals responsi-
ble for the development of the OHWM between watersheds holds the greatest
promise for concluding whether OHWM features throughout the Southwest, or
within specific subregions, are consistently formed by events of the same mag-
nitude (i.e., frequent or extreme events) or whether such OHWM features are
formed by events of varying magnitude because of the overriding influence of
watershed characteristics, channel boundary conditions, and highly variable
rainfall intensities.
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