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Morelos Dam, these native trees are rarely dominant. Only about 250 acres (100 ha) of cottonwood-willow forests remain in the Colorado's floodplain in the United States. The Rio Hardy jRio Colorado complex (Zones 1-5) provides critical habitat for wildlife [see AppendixA]. Zones 4 and 5, the Rio Hardy jRio Colorado wetlands, are the largest brackish wetland in the delta (23,719 acres [9600 hal)o At the turn of the century, this area was described as a gallery forest of cottonwoods and willow, transitioning to a tidal plain of salt grass and other halophytes interspersed with screw bean and mesquite trees (Glenn et aI., 1996). By 1977, with the elimination of freshwater flows and their replacement with brackish irrigation return flows, the gallery forest was gone and salt-tolerant plants dominated the vegetation. The Rio Hardy jRio Colorado wetlands have undergone significant changes in the last several decades. From 1947 to 1983, the wetlands covered approximately 45,000 acres (18,000 ha) and were sustained by geothermal springs and agricultural wastewater that backed up behind a natural sand dam in the channel. Floods in 1983 increased the size of the wetlands to 156,000 acres (63,000 ha) but finally broke through the dam, and the wetlands shrunk nearly in half to 79,000 acres (32,000 ha). In 1986, Mexico began to improve flood control systems in t~e Mexicali Valley, building up the levees along the maIn Colorado and creating drainage canals. These improve- ments further reduced the wetlands to only 2900 acres (1175 ha) of scattered marshlands. Floods on th~ Gila River in 1992 restored part of the northern portIon of the wetlands, but only temporarily. Zone 7 (which includes Ia Cienega de Santa Clara) is separate from the main channel of the Colorado, and its wetlands are fed mostly by agricultural drainage from the MODE and Riito canals and small artesian springs. Overall, Zone 7 contains 14,350 acres (5808 ha) of emer- gent, hydrophytic vegetation an~ 5620 a~res (22.74 ha) of R1 vegetation. The WI vegetatIon conSIsts maInly of dense cattail stands, while the W2 vegetation consists of sparse stands of cattail, bulrushes, and Palmer's ~alt grass on the salt-marsh fringes. A large area occupIed by the low-intensity R3 and R4 land-cover classes con- sists mainly of stunted tamarisk and iodine bush that have colonized large flats of wet, saline soil in the supralittoral zone. Zone 7 also contains 932 acres (377 ha) of Palmer's salt grass in the tidal area belo~ la Cienega de Santa Clara, which received both agrIcul- tural drain water exiting the marsh and tidewater entering from the Gulf of California. Based on this fieldwork, researchers believe that the delta can potentially support 68,000 resident and 49,000 nonresident summer birds in the R1 vegetation of Zones 1 to 5.34 In the United States, the entire Colorado River is estimated to support fewer than half as many birds (Ohmart et aI., 1988). Because the research team did not include other delta habitat classes, this estimate of the delta's capacity to support bird life is almost certainly an underestimate. 35 At 150,000 acres (60,000 ha), the Colorado's vegetated floodplain in Mexico is nearly twice the size of the river's vegetated floodplain in the United States (84,000 acres [34,096 hal) (Balogh, 1996). 34 This calculation is based on the work of Anderson and Ohmart (1986). . h III 35 The comparison between river reaches in the United States and Mexico is made to emphasize the importance of the delta region to t e overa ower Colorado River ecosystem. 24