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progressively deposit sediment (point bars) on the insides of bends. Point bars
provide seedling establishment sites that become increasingly protected from
flooding as the river migrates farther away and as sediment deposition builds up
the bar elevation (Everitt 1968, Nanson and Beach 1977, Bradley and Smith
1986). Where perennial meandering rivers occur in arid regions (e.g., in some
alluvial mountain valleys, Cooper et al. 2003), these processes will be important
in determining riparian vegetation patterns. However, many arid-region rivers are
not meandering, and in these cases other processes will predominate.
Along braided rivers or sand-bed streams typical of many arid regions,
seedling establishment of pioneer species tends to occur on the channel bed
during a period of channel narrowing. Channel narrowing occurs when the
stream abandons part of the former channel bed, making the surface available for
vegetation establishment. The moist alluvium provides ideal conditions for
cottonwood seedlings, and the absence of flood disturbance allows long-term
survival. Narrowing can occur following flood-induced channel widening
(Schumm and Lichty 1963, Friedman et al. 1996, Katz 2001), in association with
climate conditions producing several years of low flow (Nadler and Schumm
1981), or as a result of upstream dams (Howe and Knopf 1991, Johnson 1994,
Friedman et al. 1998, Shafroth et al. 2002). Riparian trees established during an
episode of narrowing tend to be broadly even-aged, with establishment occurring
over a period of several years to a few decades (Friedman and Lee 2002).
Where rivers are constrained in narrow valleys, flood deposition and
scouring are the most important processes creating recruitment sites for pioneer
species (Scott et al. 1996). In these settings, lateral channel movement may be
prevented by bedrock, large boulder banks, or debris fans, and infrequent large
floods can deposit sediment at high elevations on the floodplain (Nanson 1986,
Scott et al. 1997, Cooper et al. 2003). Woody plants may also become established
on low-elevation gravel bars, islands, and debris fans in canyons during multi-
year droughts (Cooper et al. 2003).
Growth and Survival
Riparian plants occur in habitats subjected to extreme hydrologic conditions
that can strongly influence plant performance and survival. Similar to riparian
species in more humid regions, arid-region riparian plants may be subjected to
occasional flooding. Woody riparian species exhibit a variety of adaptations to
inundation and associated anoxia, including techniques to facilitate oxygen
uptake, development of aerenchyma tissues that allow enhanced oxygen
diffusion, development of adventitious roots, and metabolic adjustments
(Kozlowski 2002). Not all species exhibit the same adaptations. For example, P.
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