IDENTIFYING THE ORDINARY HIGH WATER MARK
ON DESERT RIVERS
The OHWM is defined as "the line on the shore established by the
fluctuations of water and indicated by physical characteristics such as a clear,
natural line impressed on the bank, shelving, changes in the character of the soil,
destruction of terrestrial vegetation or the presence of litter and debris" (U.S.
Army Corps 2001). This definition does not refer to the frequency with which the
"fluctuations of water" occur, but the 1.5-year recurrence interval flow is gener-
ally the dominant discharge in humid regions and therefore primarily responsible
for the formation of the physical features associated with the OHWM.
The morphology of many rivers in humid regions is adjusted to the flows
most commonly experienced, that is, the bankfull flow (Wolman and Miller
1960). Numerous features have been reported in the literature to identify the
bankfull stage of the river (Table 9), which in many instances is equivalent to or
a close approximation of the OHWM. However, the geomorphic effectiveness of
floods in arid regions means that morphological features associated with the
OHWM in these climates are more likely the result of extreme events, not the
1.5-year flow. This presents difficulties for defining what is meant by "ordinary"
and for accurately delineating the OHWM.
Table 9. Physical features that have been used to identify the
bankfull stage by previous workers.
Middle bench for rivers with multiple overflow surfaces
Most prominent bench
Highest surfaces of the channel bars
Lower limit of perennial vegetation
Upper limit of sand-sized particles
Top of point bars
Break in slope of banks
Change in particle size distribution
Staining of rocks
Exposed root hairs below an intact soil layer