Chapter 3. Geomorphology
77
in arid regions and the high sensitivity of arid-region rivers to change, dryland
rivers rarely reach a state of equilibrium (Graf 1988a, Tooth and Nanson 2000a).
Rivers in arid regions respond to large floods by widening their banks. During
periods of low flow, these rivers reduce their channel's size to attain equilibrium
with the reduced discharge. A constantly transitory state results because the time
needed to establish equilibrium with the low-flow conditions is greater than the
frequency of large floods that create a larger channel in equilibrium with the high
discharge. Immediately after a large flood, the channel will begin to narrow, and
the trend will be towards a smaller and smaller channel. Thus, the morphological
features generally associated with the OHWM (Table 9) will in successive years
be inset into features emplaced previously, as long as a large flow event does not
occur. Therefore, the actual location of the OHWM, the one that would be estab-
lished if the river reached an equilibrium condition, should be considered to be at
some point below the current position of the physical features normally associ-
ated with the OHWM (Table 9).
With this approach the physical features associated with the OHWM will
continue to develop and become more pronounced through time until the next
subsequent extreme event reworks the emerging OHWM features. The physical
features identified on the ground would be considered to be at some level above
the OHWM, and as such, this approach provides a means of establishing the
maximum possible extent of "waters." If a large flood has recently passed
through the river system, then the physical features mapped on the ground would
be high above or outside the actual OHWM. Conversely, if a long time has
passed since an extreme event, then the physical features mapped would be close
to the OHWM as the channel continues to shrink in the absence of an extreme
event. Studies of flood histories and historical aerial photographs could help
establish whether the channel is approaching an equilibrium condition or just
beginning the recovery process after passage of a large flood.
Application of Hydrologic and Hydraulic Methods
If the OHWM is assumed to occur at a level reached by the discharge at a
specified recurrence interval, then hydrologic and hydraulic methods can be used
to establish to what level along the channel this discharge would reach, regard-
less of whether any physical features on the ground are associated with this
synthetically determined stage. Rainfallrunoff models (e.g., HEC-1 and HEC-
HMS) can be used to establish the discharge that would result from a specified
rainfall amount in a given watershed. Gauge records can also be used to establish
the discharge at a certain recurrence interval to be used in hydraulic models.
Hydraulic models (e.g., HEC-RAS) can then be used to determine the stage of
the river at various points along its length for the specified discharge. A more
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