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largest stream in the region, has been transformed from a free-flowing, dynamic waterway to a string of storage pools. Its stream flow has been greatly reduced and only rarely reaches the Colorado River delta. Multiple impoundments also disrupt patterns of water and sediment flow on major and minor tributaries of the,Colo~do River (Graf, 1999). Diversion structures, including Granite Reef Dam on the Salt River, and New Waddell Dam on Arizona's Agua Fria River, completely reroute stream flow into canals, leaving dry river beds below (Graf, 1982; Springer et al., 1999). Other streams, such as the Santa Cruz in Arizona, have been. dewatered as a result of ground-water pumping. l:iround-water mining, stream flow diversion, and damming in combination have caused ground-water tables along a portion of the Gila River to plummet by 200 m in the last century Qudd et al., 1971). Concentrations of dissolved solids (salinity levels) have increased in many streams, a result of high rates of evaporation from expansive, shallow impoundments, and run-off from agricul- tural fields. Vast areas of flood plains have been converted to agricultural fields and urban settlements, necessitating construction of dikes and levees to constrain flood ~ , waters. ~ These physical changes have led to biological losses and various types of biological ~ change (Rood & Mahoney, 1990; Patten, 1998; Johnson, 1.998). Al~ng ~ome. reaches in i which floods have been suppressed but stream flows not diverted, npanan biomass has I Success rates for effecting desired ecosystem changes vary widely. Our ability to increas. ed. For example, riparian vegetation has increased significantl. y along the Bill I' restore riparian ecosystems sometimes falls short of our desire, despite some assertions Williams River, a small tributary to the Colorado, since closure of Alamo Dam in 1968. . to the contrary (Hey & Phillipi, 1999). Sometimes, efforts fail because of an Most of the new vegetation, however, is composed of the exotic woody plant Tamarix' inability to implement or sustain necessary changes. In other cases, the. failure lies with ramosissima and the dense riparian forests have become fire-prone (Busch, 1995; ~ inadequate scientific recommendations. Too often, restoration efforts fail because Shafroth, 1999). Along the Colorado River, narrow strips of marshland and riparian i the underlying factors that caused the riparian degradation are not addressed (Briggs, forest have developed in areas of the narrow Glen Marble and Grand Canyon reaches ~ 1996; Briggs et al., 1994; Hobbs & Norton, 1996; Alpert et al., 1999). For example, that formerly were scoured by annual floods (Stevens et al., 1994). Along much of the l a decade ago in the south-western United States, riparian restoration was synonymous lower Colorado River and major tributaries, however, riparian vegetation has declined I with cottonwood pole planting. Many of these planting efforts failed, because and the vegetation mosaic in the flood plain has simplified. Much of the lower Colorado j no-one asked, 'What factors caused the loss of the cottonwoods?'. Today, removal of River flood plain is now vegetated by Tamarix ramosissima and Pluchea sericea (Ohmart 1 exotic plant species seemingly has become a restoration panacea, but again, few are et al., 1988). Both are reproductively opportunistic; stress-tolerant shrubs, that can ; asking 'What factors have allowed for establishment of the exotics?'. Additions or withstand saline soils, drought, and repeated burning. Near the delta of the Colora.do ) removals of plants can sometimes suffice as the sole restoration effort. More River, the cumulative effects of river use have nearly obliterated the most extensIve" frequently, though, th,ere is a need to 'dig deeper' to identify and reverse the root causes wetlands in the south-western United States and northern Mexico, leaving only a few j of ecosystem degradation (Wissmar & Beschta, 1998). Because we do not yet have all scattered ecological remnants amidst an expanse of dry, salty, unvegetated land (Glenn ~ the answers, restoration projects should be designed as science-based experiments or at et al., 1992, 1996; Briggs & Cornelius, 1998). i least integrate some degree of experimentation. Adaptive research and management Generally, in arid regions, riparian species diversity is lower along stre~s that ~o n?tl policies should be integral to the process. . flow perennially (fabacchi et a/., 1996). As streams are dewatered, spec~es decline In, Primary root causes of riparian loss and degradation are alteration of herbIVOry number because the wetland to upland species continuum is truncated: obligate wetland .; regimes, disruption of hydrologic regimes, and direct conversion to irrigated cropland species that depend upon shallow water tables or perennial stream flows decline, while! and urban areas. Restoration measures to reverse impacts of livestock grazing generally upland species that grow on drier portions of the flood plain may be unaffected orj require exclusion of the grazing animals or large declines in their stocking rates in expand into areas once occupied by riparian obligates (Stromberg et al., 1996). Lo~er~d; riparian zones as well as upland sites throughout the watershed (Williams et al., 19~7). ground water levels have killed phreatophytic riparian plants alon~ the. Ca~el Ri~er In ,\ Active intervention may be required to reverse l~ng-term effec~ of ~)Ver~mg, California, Coal Creek in Colorado, and the Santa Cruz and Gila Rivers In Arizona, ~ such as compacted soils and lowered water-tables. With respect to direct dlsrupno.ns of among others (Bryan, 1928; Rea, 1983; Groene~eld & ~riepeI?trog, 198?; Scott ~t a.l.,I. hydrologic regimes, there is growing recognition that res~oring na~ fl~w ~epmes, 1999). Fremont cottonwood (Populus fremonui)-Godding willow (SallX. gooddln~t) ; including natural patterns of flood disturbance and flUVIal dynarmsm, .IS CDtI;cal to forest associations, dependent on ample amounts of fresh water and speCific flooding .l restoration success (Briggs, 1996; Middleton, 1999; Graf et al., 2001)'. ProJects With the patterns, have become globally imperiled (Fenner et al.) 1985; Busch & Smith, 1995)..: greatest chance for sustainable improvement encompass an 'ecosystem managem~nt Riverine marshlands have become rare (Hendrickson & Minckley, 1984): .~ Ilgenda' of restoring ecological processes, as a basis for restoring ecosystem co~pleXlty Declines in species diversity also can occur if flood disturbance becomes Infrequent or .~ (Whisenant, 1999). Active restoration techniques such as planting trees or shapmg land if the temporal and spatial diversity of flood disturbance declines (pollock et al., 1998). ~ forms sometimes are used to jumpstart the recovery but the ultimate goal is to allow Flood suppression may be a key reason why plant diversity is low in the dense thickets of t, process restoration to naturally drive ecosystem recovery (Kauffman et ai., 1997). Tamarix that develop on flow-regulated streams (Brock, 1994). Although theseJ One such ecosystem management effort is underway in California. Los Angel~s declines have been attributed to changes produced by Tamarix, the ultimate cause may;, Department of Water and Power, in conjunction with Mono County, is involved In be river damming and flood suppression. Plant biodiversity levels can be equally high in! II multi-year effort to restore the Owens River gorge. Restoration of flood pulses and 18 J. C. STROMBERG RESTORATION Of RIPARIAN VEGETATION OF FLOW REGIME 19 understories of the patchy forests of Populus and Tamarix that occur along free-flowing streams (Stromberg, 1998a). The combined effects of physical and vegetational changes in riparian habitats has endangered many riparian-dependent animal species, including south-western wil- low flycatcher (Empidonax traillii extimus), Yuma clapper rail (Rallus longirostris yumanensis), and least Bell's vireo (Vireo bellii pusillus). Large percentages of native fish species have become endangered or extinct (Minckley & Deacon, 1991; Richteret al., 1997 a). Restoration of habitat for endangered speci~s has been one, impetus Jor restora- tion projects in the south-western U.S.A. (Kus, 1998). Other restoration goals expressed by managers of Nature Conservancy preserves, National Wildlife Refuges, flood control districts, and other sites, include increasing the abundance of rare native riparian vegetation types, shifting dominance from exotic plant communities to native, improv- ing wildlife habitat and recreational amenities, and reducing the size of flood peaks that have increased due to activities such as road construction and urbanization. C;) C? W I-~ "" <:::::> Approaches to riparian ecosystem recovery