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6 E, P. GLENN ET AL habitat in the Sonoran Desert (Ohmart et aI., 1988; Glenn et al., 1996). Over ~e past 100 years diversion of water for human use, alteration of the natural flow regune ~nd invasion ~f exotic plants and animals has negatively impacted the ~ower Colorad? River ecoregion, such that 45 species on .the United States stretch of nver are now hste~ as endangered, threatened or sensitive (Ohmart et aI., 1988; U.S. Bureau of Recla~ano~, 1996). This review discusses the terminus of the river, the delta of the Colorado River m Mexico which has had a resurgence in vegetation since the filling of the dam system o~ the riv;r in 1981 (Glenn et aI., 1996). It is part of a special issue of Journ~I of And Environments devoted to recent scientific and policy studies of the delta, and It sets the stage for the articles that follow, Historically the Colorado River delta encompassed several million hectares of land near or belm': sea level in the United States and Mexico, including two evaporation basins the Salton Depression (now the Salton Sea) and the Laguna Salada (Sykes, 1937/ Much of the historic delta has been converted to irrigated agriculture or towns and cities. In Mexico, however, there remain approximately 170,000 ha of natural area, containing riparian, brackish wetland and intertidal habitats, running from the ~ o~erly International Boundary with the United States to the mouth of the Colorado River m the northern Gulf of California (Glenn et aI., 1996). Much of this land, and a large portion of the adjoining marine zone, are now protected in the Biosp~ere Reserve of th~ Gu~f of California and Delta of the Colorado River (Morales-Abril, 1994). We will bnefly i describe these habitats in terms of vegetation and wildlife values, and review the ~ ",01001<01 '"" ",mer..tioo ;,... whi'" will d""mUn. thrk fu""".i Although not treated here, other natural areas within the historic delta region are also & key components in the lower Colorado Riv~r ~coregio~. The Salton Sea !s now the .~ object of a major restoration effort and sClenofic studies to unde~tand Its ecol~gy ~ have been initiated but not yet published (Cohn, 2000). The detenorated ecolOgIcal ~ status of the lower Colorado River from Davis Dam to Morelos Dam in the United t States was documented by Ohmart et aI. (1988) and in subsequent studies by others ; (Busch & Smith, 1995; Stromberg, 2001). The lower delta of Colorado River has never been thoroughlY studied. D. T. MacDou- gal of the New York Botanical Garden briefly describe~ the .vegetation of the area on several short excursions from Yuma to the Gulf of Califorma or the Salton Sea from 1904 to 1907 (MacDougal, 1905, 1907). Aldo Leopold, describing a camping trip he made with his brother in the 1920s, called the delta the last great blank spot on the map of North America' (Leopold, 1949). Both MacDougal. and Le~pold pon;rayed ~e ~el~ as a vast gallery forest of cottonwood (Popul~ [remontz~) and willow (Salve K,ooddtngu) m the north, interspersed with wetlands contammg cattail.(Typha domengens~) and com- mon reed (Phragmites austraIis) in low areas and mesquIte bosques (Prosopu, glanduIosa and P. pubescens) on higher terraces. Large expanses of salt tolerant vegetaoon suc~ as salt bush (Atrip1ex spp. ), salt grass (Distichlis spicata) and arrowweed (PIuchea sencea) were found throughout the delta, as the Colorado .River .carries salts leac~ed from upstream soils. The endemic s~lt grass, p. paImen, dommated the .e~tuanne zone. Beaver, jaguars and deer were snll found m the delta when Leopold V1slte~. In 1937 Godfrey Sykes published The Colorado Delta, a record of his personal explorations of the delta by small boat over a period of years. He predicted that the vast, lush delta viewed by early visitors would be drastically altered by Hoover Dam, completed in 1932. True enough, from 1932 to 1981, little water flowed to the delta and the Gulf of California. Lake Mead, behind Hoover Dam, was filling from 1935 to 1957, and Lake Powell, behind Glen Canyon Dam, was filling from 1 ~64 to 1981 (Glenn et al., 1996). Excess water in the watershed was simply captured behmd the dams ra~er than transmitted to the delta and the sea. Much of the delta was developed for a~nculture, and the perception arose that what was left was a dead ecosystem (e.~. Fradkin, 1981). Research interest in the delta was minimal for many years, but has mcreased recently as scientists, envirorunental organizations and natural resource managers have become ECOLOGY AND CONSERVATION BIOLOGY OF COLORADO RIVER DELTA 7 IIware that the 'dead ,delta' perception is no longer accurate. The remaining delta ecosystems have rich conservation potential (Glenn et aI., 1996; Pitt, 2001; Pitt et ai., 2000; Varady et aI., 2001). From 1955 to 1989, Science Citation Index lists only five publications on the Colorado River delta; from 1990 to 1997 there were 10, and from 1998 to 2001 there were 23. The 14 papers in the present collection add to our knowledge of the delta's water budget (Cohen et aI., 2001) and water quality (Garcia- Hernandez et aI., 2001a), species diversity (Garcia-Hernandez et aI., 2001b; Hinojosa':' Huerta et aI., 2001), vegetation dynamics as affected by flows from the United States (Zamora-Arroyo et aI., 2001) and connections between floods and the ecology of tJ1e marine zone (Rodriguez et aI., 2001a). They discuss possible mechanisms for managing a binational resource like the delta, where the critical habitats are in one country (Mexico) but a key sustaining resource, water, flows from another country (the United States) (Pitt, 2001 and Varady et aI., 2001). c::, W l' -~ CO t+'- (""'- ....,.. Ecozones in the colorado river delta Using information from summer Thematic Mapper images, combined with ground and low-level aerial surveys, we divided the natural areas of the delta into four terrestrial ecozones plus the marine zone (Fig. 1, Table 1). Using the normalized difference vegetation index (NDVI) to classify satellite images, we calculated the percentage of vegetation cover in each zone, averaged over the years 1992-1999 (see Nagler et aI., 2001 and Zamora-Arroyo et aI., 2001, and Valdes-Casillas et aI., 1998, for methods and details of vegetation surveys). The vegetation composition of the delta is not complex. The present vegetation communities, though much reduced in area compared to their historic proportions, are basically similar to those observed by MacDougal, with the remarkable exception of salt cedar (Tamarix ramosissima). TIlls salt-tolerant shurb or small tree, an exotic from Eurasia not yet introduced to the delta at the turn of the century, now dominates the riparian corridor except in the most saline locations (the intertidal zone) and in emergent wetlands. Salt Cedar/Willow/Cottonwood Zone The first ecozone, which we designated the Salt Cedar/Willow/Cottonwood Zone, is a narrow stretch of habitat between earthen levees, that runs for 100 km (14,000 ha), from Morelos Dam Oast diversion point for water on the river) to the junction of the Colorado River with the Hardy River. This river stretch is not perennial, but flows when surplus water is released from the United States. Since the filling of Lake Powell, water' has flowed down this stretch then to the sea in 10 of 20 years, representing about 20% of the total river flow (Zamora-Arroyo et aI., 2001). This stretch is appro~ately 45% vegetated, with the remainder consisting of unvegetated sand bars in the river channels and bare earth between plants on the terraces. The vegetation is dominated by T. ramosissima, as elsewhere on the river, but cohorts of native trees were established following rivers flows associated with El Niiio/La Niiia events in 1983-1988, 1993 and 1997-1999. In 1999, P. fremontii and S. gooddingii trees, sometimes growing in gallery forests, composed 23% of the vegetation along this stretch (Zamora-Arroyo et ai., 2001). The other common plant in this zone is the salt-tolerant, native shrub, P. seriaa which often grows in dense stands which exclude other species (Zamora-Arroyo et aI., 2001). The return of native trees to this stretch illustrates the importance of pulse floods in restoring the ecology of western United States rivers (Stromberg, 2001). Ecophysiologi- cnl studies show that native trees tend to be superior to T. ramosissima in tolerance to flooding (Vandersandae et al., 2001) and siltation (Levine & Stromberg, 2001) and in nutrient recovery (Marler et aI., 2001), but are inferior in salt tolerance (Glenn el ai"