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<br />
<br />and alkali bulrush (Scirpus paludosus) are prevalent. On the lower Colorado
<br />River, reeds (Phragmites communis) are well developed. In more mesic systems,
<br />rushes (Juncus spp,), sedges (Carex spp.), bulrushes (Scirpus spp.) and cattails
<br />(Typha spp.) occur around open waters containing hydrophytes (e,g. Pota-
<br />mogeton spp" Zannichellia palustris, Elodea canadensis; cf. Minckley 1973;
<br />Carothers & Minckley 1981).
<br />
<br />Desert canyon scrub
<br />
<br />Riparian plants occur in the desert canyons above and below Lake Powell, a
<br />habitat that historically was reliant on seasonal flooding. Fast-growing species
<br />like arrow-weed (Pluchea sericca), groundsel (Baccharis spp.) and willows
<br />occupy the flood zone, Beyond is a terrace created by extreme floods (Fig. 7),
<br />populated by Apache plume (Fallugia paradoza), redbud (Cercis occidentalis),
<br />hackberry (Celtis reticulata), mesquite (Prosopis juliflora) and acacia (Acacia
<br />greggi), Typical desert plants (e.g. brittIebrush, Enceliaforinosh; cacti, Opuntia
<br />spp.; creosote bush, Larrea tridentala) intergrade with riparian plants on the
<br />canyon walls (Carothers et al. 1979).
<br />
<br />Impacts of stream regulation and salt cedar invasion
<br />
<br />The pristine associations have been affected by over-grazing (cf. Dobyns 1978),
<br />logging of cottonwoods to fuel river steamers (Ohmart et al. 1977) and stream
<br />regulation. Now there are merely over-mature remnants of the great cotton-
<br />wood galleries (particularly Populusfremontii), once more than a kilometre wide
<br />on either side of the lower Colorado and on islands in the delta (Ohmart et aI,
<br />1977; Stanford & Ward 1984). Most mesquite bosques have disappeared as
<br />water tables recede along the lower Gila River (Brown et al. 1977). By far the
<br />greatest change has been the invasion of a deciduous Eurasian tree, the salt
<br />cedar (Tamarix chinensis), This species is adapted to saline soils, and in 50 years
<br />has invaded all but montane riparian habitats. The trees grow to 10---12 m, often
<br />as dense stands, and the seedlings are hardy and readily displace native plants
<br />on scoured floodplains, The interactions with native species are not well under-
<br />stood (Everitt 1980), and are complicated by flow regulation.
<br />Photographs ofriparian sites in the Grand Canyon between 1870---1930 show
<br />that salt cedar invaded the canyon before construction of Lake Powell. How-
<br />ever, post-regulation discharges have greatly encouraged its proliferation
<br />(Turner & Karpiscak 1980). Prior to regulation, seasonal flooding scoured the
<br />riparian land and, although many species could colonise the land between
<br />floods, none assumed dominance. Since regulation, flows have allowed salt
<br />cedar to assume dominance or subdominance (Fig. 7a). This is seen also in
<br />the Upper Basin, where tangles of salt cedar and willow predominate, Old
<br />
<br />368
<br />
<br />,.~
<br />
<br />
<br />(0)
<br />
<br />ZONE 3
<br />
<br />(b)
<br />
<br />~
<br />~ 10
<br />::;;
<br />
<br />Flood
<br />
<br />o
<br />60
<br />
<br />ZONE 1 - Typical vegetotion (desert), uninfluenced
<br />by river regime (stobie community)
<br />ZONE 2 - High flo?d zone, woody ,vegetation
<br />Prosopls, Acacio, Cere/s, Celfis (stable)
<br />ZONE :3 - Zone of shorr li"ed invasion species
<br />A/hagl; Salsola, OesclJrainio, Bromus,
<br />Fesfuca (unstoble)
<br />ZONE 4 - New riparian zon,. Tam~rix, ,Sol/x, Plvcheo,
<br />Baccharis (rapId prollferaflOn)
<br />
<br />,
<br />
<br />ZONE 4
<br />
<br />, "
<br />
<br />ZONE 1 - Typical veoerotion {desert)j uninfluenced
<br />by river regime (stable community)
<br />
<br />ZONE 2 . High flood lone, woody vegetation
<br />Prosopis. Acacio, Cerc/s. Celtis (stobie)
<br />ZONE 3 - Ephemeral plant zone, periodically scoured
<br />(unstable)
<br />
<br />ZONE 3
<br />
<br />;'1 '
<br />
<br />1
<br />
<br />Pre dam flood zone
<br />
<br />Zone
<br />
<br />
<br />30
<br />Metres
<br />
<br />Figure 7. Impact of stream regulation on the riparian ecosystem in the Grand Canyon of the
<br />Colorado River: (a) after regulation by Lake Powell, (b) pristine conditions (after Carothers el at.
<br />1979),
<br />cottonwoods remain but are over-mature, with seedlings unable to mature in the
<br />salt cedar-willow understory (cf. Stanford & Ward 1984).
<br />These changes also impact the riparian fauna: some species are extirpated or
<br />reduced in numbers, while others gain advantages from the dense vegetation of
<br />stabilised streambanks (thus beavers, Castor canadensis, now are abundant in
<br />the Grand Canyon: Carothers & Johnson 1983). Increased aquatic productivity
<br />imposed by regulation also may provide supplementary food for terrestrial
<br />animals. In the Grand Canyon, algae and amphipods (Gammarus) are entrained
<br />as discharges change with power operations. The amphipods are caught in pools
<br />and backwaters, providing food for birds and, surprisingly, a desert lizard
<br />(Cnemidophorus spp,; Carothers & Johnson 1983).
<br />
<br />369
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