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<br />dated by flows of less than 2800 m'l
<br />s were devoid of vegetarian (Turner
<br />and Karpiscak 1980, Webb 1996).
<br />Perennial riparian vegetation exiMed
<br />only as a linear band above this Mage
<br />on terraces and in tributarv canvons
<br />(Figure 5). This vegetation'consisted
<br />primarily of mesquite (Prosopis
<br />glandulosal, catclaw (Acacia greggii),
<br />Apache plume (Fa//llgi.1 paradoxa),
<br />desert broom (Baccharis sara.
<br />throidesl, and native willows (Salrx
<br />gooddmgil and Salt_,- eXlgua; Turner
<br />and Karpiscak 19S0,Johnson 1991).
<br />Photographs first show non-nam'e
<br />salrcedar (Ta/1/<lTlx spp.) near lees
<br />Ferry in 1938 (Webb 1996), bur salr-
<br />cedar may have arrived earlier (Graf
<br />1978). B}' 1962, saltcedar was well
<br />esrablished in small, high-density
<br />stands in pans of the Grand Canyon
<br />and was also common in tributary
<br />canyons.
<br />River-corridor vegetation now
<br />occurs in four distincr zones (Figure
<br />5). ~1arshes, which were nor present
<br />before the dam, occur ar elevarions
<br />inundated by average power.p]ant
<br />operarions (Stevens et al. 1995). The
<br />lower riparian zone occupies for-
<br />merly barren channel banks and
<br />sandbars that are inundared bv dIS-
<br />charges of 700-2800 ml!s. Th; pre-
<br />dam perennial riparian vegetation
<br />now comprises the upper riparian
<br />zone, and desert vegetation occurs
<br />on high terraces 3nd slopes that have
<br />not been inundated in more than a
<br />century (Carothers et al. 1979,
<br />Turner and Karpiscak 1980, Stevens
<br />1989, Carothers and Brown 1991,
<br />Johnson 1991, Webb 1996). Flood
<br />control by Glen Canyon Dam has
<br />reduced upper riparian zone recrUlt-
<br />me nt, productivity, and sun-i\'orsbip
<br />through increased drought stress
<br />(Anderson and Ruffner 1987).
<br />The lower riparian zont':, which is
<br />composed of a diverse assemblage of
<br />native and non-native plant spt':cies
<br />(e.g.. T<lm.lrix spp.. S.1lix spp..
<br />8,lCCh,uis srp., TeSS,Jri,J 5ericel1
<br />[arrowweedJ), is highly producrive
<br />and well establtshed (johnson 1991).
<br />Soil nutrient concentr3rions are re-
<br />duced because many pose-dam de-
<br />pOSIt!t have less silt and clay than do
<br />pre-dam depoSIts. ~t1Cshes are rhe
<br />most productlve Jssemblages of rhe
<br />lower riparian zone; they increase in
<br />area under high fluctuating flows
<br />but are scoured by tlows that exceed
<br />
<br />September 1998
<br />
<br />..
<br />..
<br />
<br />.
<br />
<br />Alanagcment resources and related processes of the
<br />Colorado River in the Grand Canyon
<br />
<br />Processes and resources that are relicts of the pre-dam riwr:
<br />
<br />Seasonally fluctuating discharge, sediment rransport, turbidity
<br />Seasonally changing temperature
<br />Large, unvegetated sandbars that are emergent ar low discharge
<br />Rapids dominated by large boulders
<br />~ative fish assemblage, including species that are now endangered
<br />or extirpated
<br />:--;ative upper riparian zone vegetation, native terrestrial
<br />species richness
<br />Archeological and historical sites
<br />
<br />Processes and resources that arc arrifacts of the post-dam river:
<br />
<br />Low variability in annual discharge
<br />Substantial hourly variation of discharge in some years
<br />Low sediment transport .and turbidity
<br />Const.anr low remperature
<br />Constricted rapids
<br />Blue-ribbon non-Ilative trOUt fishery
<br />Biologic.ally diverse marshes
<br />Dense lower rip.ari.an zone vegetarian
<br />Endangered snail and bird species and other regionally significant
<br />populations occupying non-natiVe riparian vegeration
<br />Hydroelectric power
<br />
<br />I
<br />
<br />power-plant capacity (Stevens et a!.
<br />1995). Terresrrial invertebrate and
<br />vertebrate populations, such as
<br />warerbird:'>, have increased in the
<br />lower riparian zone (USDI 1995,
<br />Steven:,> et al. 1997a). The endan-I
<br />gered southwestern willow f1ycatche~
<br />and Kanab ambersnail have ex-
<br />panded their ranges into marshes
<br />and lower riparian-zone vegetation,
<br />respecrively.
<br />
<br />The porcnrial for
<br />river rcsrorarion
<br />
<br />Possible Colorado River ecos\'Stem
<br />management goals range 'from
<br />tr3dirional, market-dri\-en dam
<br />management to full restorarion of
<br />the pristine river ecosystem. The iden-
<br />tification of desired goals can lead [0
<br />the ~elec[ion at appropriate engi-
<br />neering approaches_ Pursuit of a par-,
<br />ticular gOJ.1 wil] change the status of
<br />llldividual resources and the direc-
<br />tion of ecosystem development by
<br />aherlOg ecosystem processes. Some
<br />resource:'> ,lOd pro.:esses of the present
<br />(lver corridor are pre-dam relicts.
<br />others are post-dam artifact:'>. 3nd a
<br />
<br />,
<br />
<br />"
<br />>
<br />
<br />, ,~
<br />'",",;
<br />t,',
<br />
<br />,,-,:
<br />
<br />few include elements of borh (See
<br />box this page). This array of re-
<br />sources of differing origm means rhat
<br />alrering ecosystem processes will in-
<br />volve rradeoffs, because pre-dam and
<br />post-dam resources respond differ.
<br />ently to specific engineering ap-
<br />proaches"
<br />From a continuum of possibilities,
<br />we identify five management ap-
<br />proaches: traditional flver manage-
<br />ment: managmg the river as a naru-
<br />ralized eco:,>ystem; rehabilitating it
<br />as a simulated natura] ecosystem;
<br />rehabilit,Hing It as a substantial]y
<br />restored ecosysrt'm; and reestablish-
<br />ing a fully restored ecosystem IT able
<br />31. Each approach alters the tralec-
<br />tory of this open and dynamic eco-
<br />svstem, and the success of anv one
<br />a'pproach is not assured. .
<br />
<br />~ ,,-',
<br />'.<:
<br />
<br />;,.
<br />
<br />. Pursuit of Irl1.1,tio1l.11 techniques of
<br />river management uses existing fa-
<br />cilities to maximize power re'"enues
<br />and to optimize the mandared trans-
<br />fer of ",'atre within the basin. ThIS
<br />approach accepts the ri\"er as a trans-
<br />formed ecosystem. in which the goal
<br />of river man3gement is rhe efficient
<br />
<br />741
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