Chapter 1. Introduction
7
the growing season. More abundant vegetation at higher elevations tends to
stretch the runoff over longer periods compared to low-lying areas (Chapter 2).
The cumulative effects of all these factors on runoff causes arid stream systems
to have greater flow magnitudes, more rapid responses to rainfall, and shorter-
duration flows than their humid-region counterparts (Chapter 2). These factors,
coupled with the highly variable climate and intense precipitation events, make
selection of reliable OHWM field indicators challenging.
Geomorphology
Channel bed morphology and sediment arrangement in arid streams retain
information about prior hydrologic events. Knowledge of these "fingerprints" has
a basis in the principles of fluvial geomorphology. As in all graded streams, the
river channel's form--cross section, planform, and gradient--is a result of
prevailing watershed conditions that control the amount of sediment and water
delivered to the channel (Leopold et al. 1964, Leopold and Bull 1979). The
dominant, or effective, discharge in a particular region generally controls the size
and shape of the channel (Chapter 3). More frequent events, in concert with
revegetation, tend to "heal" the impacts of larger flows and return the channel to
a size that is in equilibrium with the dominant sediment and water discharge.
OHW is sometimes interpreted as the bankfull position in an active channel
(Rosgen 1996). Bankfull discharges are reported as having a recurrence interval
of 1.5 years (Dunne and Leopold 1978, Rosgen 1996). Williams (1978) pointed
out exceptions to this rule, so one should not assume that this average value holds
for any given channel (Chapter 3).
Beyond the bankfull position in river channels are active floodplains and
abandoned terraces. These abandoned terraces, typically at higher elevations, are
a consequence of channel gradient changes resulting from either decreases or
increases in sediment loads. These changes isolate previous channels and flood-
plains above the newly established channel (Chapter 3). In arid stream systems,
abandoned terraces sometimes are easy to distinguish since they are high above
the active channel; however, in systems with limited relief it is difficult to distin-
guish the active floodplain (Dunne and Leopold 1978). This constant striving to
achieve equilibrium in arid regions with highly variable and intense discharges
further confounds the selection of OHWM indicators.
Landscape setting, size of the watershed, location in the watershed, elevation,
gradient, lithology, and hydrologic events all play an important role in develop-
ing different stream types in the arid Southwest. Stream types in this region are
classified as discontinuous, ephemeral, compound, alluvial fan, anastomosing,
and single-threaded channels. These various types are in part a response to
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