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8 E. p, GLENN ET AL LEGEND fl/lexlco" USA aoundaly water Dry SOIl wat sol <30%Vegetalk:ln 3()'49%vegetciflon 5Q~9% VegeroIIon ..!0-89%~ ......~.~etaIIon ....~l.Cl9%.~on Fleure 1. Terrestrial ecozones of the lower delta of the Colorad~ ~ver in Mexico. Zone~ are overlaid on a June 1998 Thematic Mapper image of delta. The area inSIde the zo~es was c1assl~ed using NDVl (Nagler Bt al., 2001: Zamora-Arroyo et aI., 2001) .to show ~ater, soil and ~egetanon cover as indicated in the Legend. Areas outside the zones are dIsplayed In false color u~mg the I~ band (red a vegetation). The marine zone begins at the bottom of the figure. ThIS figure IS available in colour online, at www.idealibrary.com. ECOLOGY AND CONS~RVATION BIOLOGY OF COLORADO RIVER DELTA 9 Table I. Area and vegetation cover of the major ecozones of the Colorado River delta, estimated by mean values calculated for summer ThematU: Mapper sateUite images (1992-1999) Ecozone Total Vegetated Vegetated (ha) (%) (ha) Salt Cedar/Willowf C.'-' Cottonwood 13,711 45'1 6814 c::.') '^-, Salt Cedar 40,861 23.1 9439 y .~ Salt Grass 78,897 1'6 1291 CO Cattail 35,788 11'5 4115 C..ll Total 169,257 13-0 21,659 Note. Percentage vegetation was calculated using NDVl values correlated with scenes of known .~I~..,... vegetation cover for a ground-truthed, 1997 image (Nagler el al. 2001; Zamora-Arroyo el al., 2001), Marine zone not included. ~: . i ~ .~ I ~ ~ ~ I': ',; ~ '( '~ ~ :1. Below the junction of the Colorado and Hardy Rivers, the river is perennial. It carries o saline agricultural return flows from the Mexicali Valley, and is tidally influenced, hence ~ the water and banksides are saline (Glenn et al., 1996; Valdes-Casillas et al., 1998). t The river spreads out in this zone and is divided into numerous, braided channels. We ~. designated this middle portion of the delta the Salt Cedar Zone, because much of t the area between channels is a vast monoculture of T. ramosissima thickets. Most of the water entering this section (in absence of flood discharges from the United States) are agricultural return flows from the Mexicali and San Luis Irrigation Districts. They enter in the Rio Hardy and from smaller drains discharging into the western portion of this stretch. Overall, this zone is only 23% vegetated, with the vegetation concentrated near the river channels. In addition to T. ramosissima, the emergent plants, P. australis and T. domengensis, grow along the river and canal banks and in wedand areas created by the discharge of agricultural drains onto the mud flats. There are very few native trees and less P. sericea than in the first zone, due to high salinity in the soil and alluvial aquifer (Zamora-Arroyo el al., 2001). Sediments and biota from the Salt Cedar Zone have higher levels of selenium in sediments and biota than other zones (Garcia-Hernandez el al., 2001a), perhaps due to the predominance of agricultural drainage in its water budget. In general, wildlife uS,e has not been adequately studied, hut some endangered Yuma Clapper Rails (RaUus wngirosr:ris yumanensis) are found in El Indio and other drain-fed wetlands in this zone (Hinojosa-Huerta el al., 2001), 1998; Vandersandae et al., 2001). Occasional overbank floods on this river stretch have washed salts from the banks and scoured out T. ramosissima, allowing the reestablish- ment of native trees. This stretch, and T. ramosissima-dominated habitat to the south, is apparendy used as a summer migration route for the endangered s,outhwestern willow Hycatcher (Empidonax traillil) and perhaps other neotropical migratory songbirds (Gar- eia-Hernandez et al., 2001). The Salt Cedar/WillowfCottonwood Zone in the delta contains the greatest amount of native tree habitat remaining on the lower Colorado River (Zamora-Arroyo et al., 2001). . Salt Cedar Zone