Status of Wetlands Supported by Agricultural Drainage Water in the Colorado River Delta, Mexico

Abstract

Eddleman, 1989). The indigenous Cocopa people still use the riparian zone of the delta for subsistence (Richardson and Carrier, 1992; Williams, 1983), and the wetlands and reforested areas provide incomes to ejido residents who gather fuelwood and serve as guides to hunters, fishermen, and ecotourists (Payne et al., 1992). In recognition of their importance, portions of the delta wetlands were incorporated into La Biosfera del Alto Gulfo de California y el Delta del Rio Colorado, an international biosphere reserve recognized by the United Nations international biosphere program and declared by President Salinas of Mexico in 1993 (Diario Oficial, 1993; Morales-Abril, 1994). The remaining wetlands are a collection of natural and anthropogenic marshes, supported by tide water, artesian springs, and agricultural drainage water. The wetlands are greatly impacted by water management decisions in the United States and Mexico. The safety of the wetlands for wildlife is dependent upon the water quality of the inflow waters. Until recently, however, they have not been managed or protected. With the creation of the biosphere reserve, international interest in preserving the delta wetlands has developed (Congdon and Luecke, 1996; Morrison et al., 1996). Furthermore, since Lake Powell filled in 1981, occasional flood flows are again a feature of the lower Colorado River, providing an augmented water supply that has reforested much of the river floodplain (Glenn et al., 1996). The delta of the Colorado River historically covered 780,000 ha in Mexico and the United States, encompassing most of what are now the Imperial, Mexicali, Yuma, and San Luis irrigation districts, as well as two below-sea-level depressions, the Salton Sea and Laguna Salada (Sykes, 1937) (Fig. 1). Before the 20th century, the lower delta was a vast network of riparian and wetland ecosystems in the most arid part of the Sonoran Desert (Leopold, 1949; McDougal, 1904; Sykes, 1937). Although most of the former wetlands and gallery forests have been converted to agricultural or urban use, there are still large wetland and riparian areas of conservation value in the lower delta in Mexico (Glenn et al., 1992, 1996; Morrison et al., 1996; Payne et al., 1992) (Fig. 1). Drainage water produced by horticultural industries in southwestern Arizona contributes to sustaining these ecosystems. The remaining wetlands provide critical habitat for shorebirds and migratory waterfowl (Abarca et al., 1993; Mellink et al., 1997) and support the largest remaining populations of two endangered species, the desert pupfish (Hendrickson and Varela-Romero, 1989; Zengel and Glenn, 1996) and the Yuma clapper rail (Abarca et al., 1993;