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ERDC TR-04-1
The downstream decrease in discharge arising from transmission losses (i.e.
water infiltrating into the channel bed) typical of ephemeral streams (Reid and
Frostick 1997, Tooth 2000) means that the sediment load cannot be transported
equally through each reach during a single flow (Bull 1997). This results in a
buildup of the channel-floor elevation, especially during low- and intermediate-
stage flows (Packard 1974), and gives rise to the distinctiveness of discontinuous
ephemeral stream systems. The optimal hydrometeorological conditions for their
formation appear to be 100500 mm of annual rainfall (Bull 1997). The semi-
arid climate maximizes sediment yield (Tooth 2000) while allowing for sufficient
vegetation growth to trap sediment on sheetflood zones (Packard 1974). With
transmission losses further inducing deposition, sheetflood zones and headcuts
migrate upstream over time. Sediment has a tendency to move through dryland
rivers episodically (Schumm and Hadley 1957), as a series of waves (Lekach and
Schick 1983, Schick et al. 1987, Graf 1988b). Consequently, scour and fill are
unlikely to be synchronized, with erosion occurring in one reach while another
reach is simultaneously aggrading (Tooth 2000).
The morphological changes observed in a downstream direction along dis-
continuous ephemeral stream systems are the same changes that occur at a single
point through time as the various reaches migrate headward. Consequently,
temporal variations in morphology are extreme. Channel backfilling caused by
the headward migration of aggradational reaches can transform a deep channel
into an area of sheetflooding over periods of tens to hundreds of years (Bull
1997). The buildup of slope resulting from aggradation ultimately leads to
retrenching of the same area once the threshold of critical power is crossed
(Patton and Schumm 1975, Bull 1979). Numerous studies in the Southwest have
documented the repeated cutting and filling of arroyos along discontinuous
ephemeral stream systems during the Holocene (Antevs 1952, Patton and
Schumm 1981, Waters and Haynes 2001).
Vertical-walled arroyos, entrenched ephemeral streams that form in desert
environments (Fig. 15) (Waters and Haynes 2001), are best understood as
erosional channels in discontinuous ephemeral stream systems (Fig. 11). Where
rates of erosion exceed those of aggradation, continuous arroyos can form on
valley floors because headcuts will advance headward faster than the aggrading
sheetflood zones. This situation can arise when conditions result in a long-term
increase in water:sediment ratios (Packard 1974) associated with climate change,
destruction of vegetation cover, or artificial flow concentration (Cooke et al.
1993). Arroyos evolve and backfill over time, as has been the case for many
arroyos in the 20th century (Emmett 1974). As the initial vertical walls and
narrow channel of the arroyo begin to widen, stream power is lost,
sediment:water ratios decrease, and vegetation takes hold in the channel,
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