intensities in the recent past, not just statistically predicted intensities for a given
time duration (e.g., 10-year 24-hour storm), would be useful for identifying those
areas where the channel morphology is potentially the result of a recent extreme
flood. Existing data on past rainfall and stream gauges are sparsely distributed
throughout the Southwest, so rainfall intensity maps may be difficult to construct.
Additionally, the accuracy of such maps would be difficult to determine, given
the highly localized nature of rainfall events in the Southwest (Pilgrim et al.
1988). For larger drainages influenced by more regional storms, such maps may
be of greater value.
Composite maps showing climate, physiography, and geology/soils would be
particularly valuable for identifying watersheds with the highest potential for
channel response to extreme events (e.g., areas that can experience intense rain-
fall, that have steep terrain, and where the rainfall flows through unconsolidated
materials). Extreme events dominate the morphology of channels in arid climates
because record peak flows are much greater than the average annual peak flow
(Graf 1988). Maps developed to show variations in the ratio between record and
average annual peak flows throughout the region have the potential to discrimi-
nate areas where the physical features associated with the OHWM are more
likely the result of extreme events. This pronounced discrepancy between record
and average peak flows is ameliorated at higher elevations, where snowfall
runoff leads to lower winter peaks and greater flows in the generally dryer
summer. The more consistent and persistent flow throughout the year leads to
channel features that are the result of more "ordinary" flows. Intense rains from
tropical storms are perhaps their strongest in the driest regions of the Southwest
(southeastern California and southwestern Arizona), resulting in the greatest
discrepancy between record and average peaks. Consequently, channel features
in these areas are more likely the result of extreme events.
Rainfallrunoff models account for both the meteorological and physi-
ographic factors that control the amount of runoff that ultimately flows in the
stream channel. Those models that consider the full hydrograph resulting from a
runoff event, not just the peak discharge, are more valuable since the duration of
a flow, not only the peak, is important in determining the impact of flows on
channel morphology (Costa and O'Connor 1995). Peak discharges for a particu-
lar watershed can be estimated from regional regression equations or a statistical
analysis of stream gauge data (Texas DOT 2002). Regional regression equations
do not apply to watersheds with mixed-population floods, such as in the South-
west, where many watersheds are affected by three distinct storm types: winter
frontal storms, summer convective thunderstorms, and fall tropical cyclones.