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824 NATURALRESOURCES/OURNAL [Vol. 40 Fall 2000] MANAGING ECOSYSTEM CONSERVATION 825 approximately 150,000 acres, within which the river periodically floods. In the center of the Delta, about 50 river miles south of the sm, the Rio Hardy joins the Colorado River from the northwest. A local tributary, the Rio Hardy is about 16 miles long and drains ~bout 13~ squar~ miles ~elow the nearby Cucapa Mountains. Most of the Rio Hardy s flow 15 brackIsh water that drains from surrounding agricultural fields.11 East of the Colorado's mainstem, the Main Outlet Drain Extension canal delivers additional agricultural wastewater to the Delta from southern ~rizona ~ th~ United States. At the end of its course, the Colorado River empties mto the northern end of the Gulf of California. Outside of the levees, the Delta is surrounded by the agricultural valleys of Mexicali and San Luis Rio Colorado and the Sonoran Desert. These farmlands comprise some 500,000 acres irrigated with a portion of Mexico's share of Colorado River water delivered from Morelos Dam via the Central Canal.12 Beyond the irrigated landscape lies the Sonoran Desert ecosystem, dominated by arid soils and low shrubs. During the twentieth century, river flows into the Delta have been reduced nearly 75 percent;13 in 24 of the past 40 years, less than two percent of the Colorado River's estimated undepleted flow reached the Delta. This reduction in water brought less silt, fewer nutrients, higher salinity, and higher concentrations of pollutants, resulting in major changes to the Delta.14 Erosion-rather than accretion-is now the dominant phrical process in the Delta, a highly unusual condition for a river delta. I Like other river deltas at risk, such as the Nile's, the Colorado's delta has actually decreased in size.16 The loss of freshwater. flows to the Delta over the past century, combined with land use changes, has reduced Delta wetlands and riparian . areas to about five percent of their original extentP Non-native species, better adapted to high-saline, low-flow conditions, have further compromised the ecological value of the region. Native forests of cottonwood and willow, which supported greater species richness and density than any other desert habitat,!8 have yielded to non-native salt cedar and iodinebush, decreasing the habitat value of the riparian corridor.19 B. The Undisturbed Delta: Before Upstream Development The Colorado River delta ecosystem's pre-development conditions c' provide a context for understanding the current ecosystem, as well as for 0 understanding the goals for ecological restoration. Undisturbed river deltas ~ tend to be highly productive and diverse ecosystems,20 and the Colorado i:.J River delta was no exception. Until the 1930s, highly variable flood cycles CoD on the Colorado created a dynamic delta nearly twice the size of Rhode Island, populated by a rich array of adaptable and resilient plant and animal species, as well" as human communities that lived off this bounty. . Historically, as much as 70 percent of the Colorado River's silt load was carried to the Delta,21 importing nutrients and extending the Delta ever wider into the upper Gulf of California. These sediments and nutrients created a fertile delta that once supported an estimated 200 to 400 species of vascular plants. 22 The Delta's richness was further increased by the action of tides typically ten feet or more in amplitude, an unusually high ebb and flow that extended the tidal estuary 35 miles upriver.23 The interaction of these tidal flows with freshwater from the Colorado River created a rich breeding ground for the marine life of the Gulf of California. The Delta was also home to a local people known as the Cucapa, or "the people of the river.,,24 Descendants of Yuman-speaking Native Americans, the Cucapa have inhabited the Delta for nearly a thousand years and used the Delta floodplain extensively, harvesting Palmer's saltgrass (a wild grain), and cultivating com, beans, and squash. Other foods included 11. Total dissolved solids in the water of the Rio Hardy have been documented at 4,000- 5,000 parts per mi.lli.on. See Edward P. Glenn et aL, Status o/Wetlands Supported by Agricultural Drainage Water in the Colortulo River Delta, Mexico, 34 HoR'lSclENCE 16,18-19 (1999). 72. See CARLos V ALDa5-CASILLAS ET AL, lNPoRMATION DATABASE AND lOCAL OUlREACH PROGRAM POR mE REsToRATION OF THE HARD\' RIvER WJm.ANDS, loWER COLORADO RIvER DELTA, BAJA CAUFORNIA AND SoNORA. MExIco 10 (1998). 13. See GleM et aL, supra note 11, at 16. 14. The natural ecology of most of the world's1arge river systems has been disrupted by dlUIl8, flow diversions, channelization of riverbeds, and alteration of riparian zones by agricultural activities that in tumreduce flows, silt accretion, and nutrient loads to their deltas. 15. See Glenn et aL, supra note 3, at 1117. 16. See Stanley &: Warne, supra note 5, at 628. 11. See GleM et al., supra note 3, at 1181. 18. See Jake Rice et aL, Comparison of the Importance of Different HRbitat Attributes to Avian Community Organization, 48 J. WILDLIFE MGMT. 895, 905-09 (1984). 19. See MARKK. BRIGGS &:STEVECORNEUUS, DEFENoERSOFWILDLIFE, OpPORTUNmES POR EcOLOGICAL IMPROVEMENT ALONG THE loWER COLORADO RIvER AND DELTA: FINAL REPoRT 4 (1997), 20. See EDWARD J. KoMOND\', CONCEPTS OF EcoLOGY 378 (4th ed. 1996). 21. Between 45 million and 455 million metric tons of silt per year were transported through the Grand Canyon between 1922 and 1935. See W. 1.. Minckley, Native Fishes o/the Grand Canyon Region: An Obituary?, in COLORADO RIVER EcoLOGY AND DAM MANAGEMENT 124, 126 (National Research Council ed.,1991). 22. See Exequiel Ezcurra et aL, Freshwater Islands in /l Desert S/lnd Sea: The Hydrology, Flor/l, /lnd Phytogeography of the Gran Desierto Oases of Northwestern Mexico, 9 DEsERTPt.ANJs 35 (1988). 23. See JACK M P AYNB ET AL, DUCKS UNlJMITED, INc., FEASlBIIJTY STUD\' FOR THE PossIBLE ENHANCEMENTOFTHECOLORADO DELTA WE'1'l.AND>, BAJACALIFORNIANORTe, MExICO 8 (1992). 24. See Sandra Postel et al, AllOCllting Fresh Water to Aqllllttc Ecosystems: The Case of the Colortulo River Delt/l, 23 WATERINT'L 119, 121 (1998).