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<br />24
<br />
<br />J. C. STROMBERG
<br />
<br />}uncus IOrreyi, Typha domingensis, and Scirpus americanus (Grace, 1989; Stromberg
<br />el ai., 1996, 1997), Beavers, keystone species in riparian ecosystems, also engineer the
<br />marshy conditions favoured by these obligate and facultative wetland plants.
<br />Summer and winter floods maintain high productivity rates of riparian forests by
<br />providing water for ground-water recharge, wetting flood plain soils, depositing nutri-
<br />ents, and flushing salts. Growth rate of the phreatophytic tree Platanus wrighlii, for
<br />example, increases not only with size of the winter floods (a primary source of ground-
<br />water recharge), but also with the frequency of small summer floods, which increase
<br />nutrient availability (Grimm & Fisher, 1986; Stromberg, 2001). Several plant species,
<br />including Populus fremonlii and Salix gooddingii, do not tolerate high concentrations of
<br />salts (Shafroth et al., 1995; Glenn el al., 1998) and decline in germination rate and
<br />growth-rate if salt concentrations are not flushed by flood flows. Large floods also
<br />function to fire-proof riparian ecosystems (Ellis, 2001). Without floods, plant debris and
<br />litter accumulates, plant water content decreases, and fires become larger and more
<br />frequent (Busch, 1995). Fires favour species that are clonal or readily resprout from the
<br />root crown, such as Tessaria sericea, Chloracantha spinosa and Tamarix ramosissima,
<br />Populus fremontii, in contrast, is readily killed by summer burns.
<br />
<br />Restoration constraints and compromises
<br />
<br />Ultimately, full restoration of riparian ecosystems hinges on removing impediments to
<br />the natural flows of water and sediments (Schmidt el al., 1998). There are cases in which
<br />flow regimes have been fully restored, in response to changing societal goals. In central
<br />Arizona, for example, a decision was made to decommission the hydro-power dam on
<br />Fossil Creek and restore full flows to the stream. Benefits from restoring downstream
<br />aquatic and riparian habitat were believed to outweigh the small loss of hydro-power
<br />production and loss of habitat developed above the dam. There are other cases where
<br />full naturalization of fluvial processes is not desired by all stakeholders, In such cases,
<br />how do we make compromises between water needs of the riparian and aquatic
<br />ecosystems and direct human water demands? Can we maintain or restore ecosystem
<br />integrity while accommodating some degree of water extraction, hydro-power produc-
<br />tion, flood control, and/or flood plain agriculture (Schmidt el al., 1998)? Generally,
<br />management emphasis on the production of commodities requires. that one accept
<br />ecological costs of reduced site potential and functional abilities of the riparian ecosys-
<br />tem. However, there are many changes that can be made to restore a greater degree of
<br />riparian ecosystem structure and function. Some changes are described below, organ-
<br />ized by ecological stress factors.
<br />
<br />Loss oj sl7'eam flows or declines in ground water
<br />
<br />There are several sustainable solutions for restoring stream flows and raising water tables
<br />to levels that allow for recovery of hydrophytic and mesophytic vegetation types such as
<br />Populus-Salix forests or riverine marshlands, while also allowing for water extraction for
<br />human consumption. Water can be stored in aquifers rather than reservoirs, municipal
<br />water can be recycled and released into stream channels, stream channels rather than
<br />canals can be used for water delivery, efficiency of municipal, agricultural, and
<br />industrial water-use can be increased, and extraction demands can be reduced. Ulti-
<br />mately, integrated, watershed-based approaches to water management are needed to
<br />reverse adverse effects of ground water mining and surface water diversions. All
<br />water users, municipal, agricultural, or industrial, need to work together and address
<br />water overdraft problems.
<br />In the arid south-west, where open water evaporation rates are greater than
<br />2,7 m year- I, it seems more ecologically advantageous to store water in aquifers than in
<br />
<br />RESTORATION OF RIPARIAN VEGETATION OF FLOW REGIME
<br />
<br />25
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<br />surface impoundments. A case in point involves the Agua Fria River in central Arizona,
<br />Lake Pleasant Reservoir, behind New Waddell Dam, stores Colorado River water that is
<br />delivered through Central Arizona Project canals and pipelines. The reservoir loses
<br />~ore water to evaporation per year than arrives from the in-flows of the Agua Fria River
<br />Itself. The reach downstream of the dam is completely dewatered. A modeling study
<br />showed that a several mile stream reach below the dam could be used as a conduit for
<br />delivery of water to a ground water recharge and recovery site (Springer et ai., 1999), If
<br />Central Arizona Project water was released from the dam, the shallow bedrock layer
<br />would allow water in the aquifer to rise to levels that would sustain riverine marshland
<br />Populus-Salix forest and Prosopis woodland. No more water would be released to th~
<br />atmosphere through evapotranspiration than if the water were stored in the reservoir,
<br />and there would be substantial increases in riparian habitat.
<br />Ma~y cities, such as.1;logales, Ariz~na and Phoenix, Arizona are recycling water by
<br />releasmg treated mumclpal effluent 111to stream channels. With increased planning
<br />efforts, more, ~ipa~an ~orridors could ~enefit from such a process. For example,
<br />a recent declSlon m Pima County, Arizona allows the county to buy reclaimed
<br />water (municipal effluent) for riparian restoration projects. Projects that secure
<br />endorsement by the U.S. Fish and Wildlife Service will be eligible for a portion of
<br />a 5000 acre-foot pool for each of the first 5 years of conservation efforts. A
<br />key c?ncern is where to utilize the water to maximize its habitat value. Regional
<br />planmng efforts are underway to identify sites that would maximize the environ-
<br />- mental benefits of reclaimed water. Hydrogeologic studies can identify sites where
<br />shallow water tables exist or are likely to develop, and thus sites able to support
<br />phreatophytic riparian vegetation. Ecological studies can identify sites that are conne<:~
<br />ted to high quality patches of riparian vegetation and thus more likely to have value as
<br />wildlife habitat.
<br />Agricultural return flows constitute yet another source of water for riparian restoration
<br />efforts. Return flows are being considered as a water source to maintain cotton-
<br />wood-willow habitat in the Limnitrophe area of the Lower Colorado River, to allow for
<br />survivorship of plants that established after the 1992-1993 winter floods (LCRBR,
<br />2000). Elsewhere in the lower Colorado River flood plain, agricultural return flows have
<br />been used to increase survivorship of riparian trees and shrubs planted as part of
<br />revegetation efforts (Briggs & Cornelius, 1998). When using such flows to maintain
<br />or restore riparian habitat, it seems prudent to restrict or minimize use of biocides and
<br />fertilizers on farm fields, periodically flush soils to reduce concentrations of salts, and
<br />provide a year-round water source. Multiple drainage ditches could be created to
<br />simulate the multi-channeled pattern of desert streams and provide the dense strips of
<br />riparian forests intermingled with forest edges required by some bird species.
<br />
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<br />Daily fluctuations in water levels
<br />
<br />A reach of the Salt River below Phoenix Arizona has been revitalized by the daily release
<br />of over 100 million gallons of treated municipal water into the channel (Rea, 1983).
<br />Water level in the channel, however, varies over the course of a day with the water use
<br />patterns of urbanites, To minimize diel fluctuations, restoration efforts are under-
<br />way through the Tres Rios project. Water will be released into side basins and wetland
<br />treatment cells and then into the stream channel. Research and adaptive management
<br />are needed to determine whether there also is a need to restore seasonal fluctuations in
<br />water level.
<br />Stream flows also can vary below hydro-power dams. Below Glen Canyon Dam on
<br />the CO,lorado River, the river stage once fluctuated up to 5 or 7 m day -I, in response to
<br />diel cycles of hydro-power demands, To minimize adverse impacts, fluctuations pres-
<br />endy are limited to about I m day - I, which constrains the ability of Glen Canyon Dam
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