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<br />E. P. GLENN ET AL
<br />
<br />
<br />12
<br />
<br />channel of the Colorado River (Cohen et al., 2001). Hence, the heal~ of the del~
<br />natural areas is almost entirely dependent on water management declslo.ns made. m
<br />the United States and Mexico. Yet, these natural areas have no offic~al stan~g
<br />in the water management strategies of either country (Pitt, 2001). Their ecological
<br />importance, even their existence, was largely unknown p~or to 1992, when the opera-
<br />tion of the Yuma Desalting Plant threatened to destroy CIenega de Santa Clara (Glenn
<br />et al., 1992). In the United States, maintenance of ~nviro~ental ass~ts ~ Mexico are
<br />not among the criteria the U.S. Bureau of Reclamation uses In managmg nver flows. In
<br />Mexico, large areas of cottonwoods and willows are routinely cleared from the channels
<br />following flood releases, to facilitate the ~ovement of water to the sea., .
<br />A key question is, 'How much water IS needed to restore the delta? (PItt, 20~1).
<br />Urbanization on both sides of the border is increasing the demands for Colorado River
<br />water which is already considered to be over-apportioned among the seven basin states
<br />and Mexico. The answer emerging from these studies is that surprisingl~ little water
<br />might suffice to conserve the existing riparian and wetland ecosystems ID the delta.
<br />Two findings support this hypothesis: (1) a water balance study suggests that even when
<br />there are no flood waters released to the delta, vegetation including native trees aI?-d
<br />marsh plants are supported by agricultural return flows which recharge the allUVial 11',:.
<br />aquifer and wetlands (Cohen et al., 2001); (2) ev~n modest flood releases ~re suffi-
<br />cient to induce overbank flooding and to germmate new cohorts of native trees
<br />(Zamora-Arroyo et al., 2001). Once e~tablished, these phreatophytic species extract r
<br />water from the aquifer and do not require surface flows. ~
<br />Zamora-Arroyo et al. (2001), analysing the v~etation respons; t~ past flow ev;n~l ~
<br />determined that a once-in-4-years, 3-months spnng flow of 3 x 10m at 80-120 m s ~
<br />was sufficient to establish new cohorts of native trees in the Salt Cedar/Wil- ~
<br />low/Cottonwood Zone. Pitt et al. (2000) recommended that ~n a~dition ~o thi~ pulse ~
<br />flood, a smaller, perrenial flow of 4 x 107 m3 was neede~ to maIDtaln aquatic habitat for f
<br />birds, fish and insects using this zone. The total (annualized) water req~ement of ab~ut 1
<br />lOs.m3yr-1 is only 0'5% of the mean annual flow of the Colorado River. Yet, policy I
<br />makers anticipate substantial difficulty in securing even this ~mall ~mount of water t
<br />as an environmental allotment, given human demands on the nver (Pl~, 2001} Varady i
<br />et al., 2001). A continuing water source for the Cienega de Santa Clara IS also ID .do~bt,!
<br />as the water entering in the M.O.D.E. canal might be diverted to the Yuma Desalinanon !
<br />Plant, and replaced with hypersaline brine (Glenn et al., 1992, 1996; Zengel et al.,
<br />
<br />1995). . . . d
<br />The effect of flood flows on the marine environment an~ the ~uannnes reqwre
<br />to boost productivity are, presently, unknown. Oceanographic studies suggest that the
<br />upper Gulf of California is not nutrient limited (Hernandez~Ayon et. al., 1993;. San-
<br />tamaria del Angel et al., 1996), hence river flows are not reqwred to snmulate p~ary
<br />productivity. On the other hand, the work of Kowalewski et al. (2000), and Rodnguez
<br />et al. (200Ia, b) suggests that the former brackish mollusk beds an~ the unknown fauna
<br />that may have depended on them will not re~rn without su~s~nal annual flows. The
<br />shrimp catch in the upper Gulf of California responds ~oslnvely even to the modest
<br />releases which have occurred since Lake Powell filled (Galindo-Bect et al., 2000). M';1ch
<br />more study is required on the estuarine and marine ecosystem before water require-
<br />ments can be estimated.
<br />
<br />Conclusions
<br />
<br />Recent studies have shown that the basis for a resurge~ce in ecosy~teD? function in the
<br />lower Colorado River basin exists due to the reestablis~ent o~ npanan and wetland
<br />vegetation in the delta. This resurgence depends on connnued discharge of flood water
<br />and agricultural drainage water from the United States to Mexico. The few faunal
<br />
<br />ECOLOGY AND CONSERVATION BIOLOGY OF COLORADO RNER DELTA 13
<br />
<br />studies, mostly of endangered species, show that the habitat revival has had positive
<br />effect on wildlife. Yet, there is little information on the most of the populations of
<br />fish, reptiles, mammals and birds that use the delta and its marine zone. There have been
<br />no studies at all of movement of species between the United States and Mexico, even
<br />though a. species revival in the delta could help repopulate upstream habitats. This
<br />region still can be described as a scientific 'blank spot' on the map of North America
<br />deserving much more study to inform those who make decisions about its future. '
<br />
<br />r-<"~
<br />-"."
<br />
<br />References
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<br />Augustinus, P,G.E.F. (1989), Cheniers and chenier plains: a general introduction. Marine
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