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the Mexicali Valley annually carry 70,000 tons (64,000 metric tons) of fertilizer and 110,000 gallons (400,000 liters) of insecticide (Direccion General de Ecologia, in Valdes-Casillas et aI., 1998a). While agricultural wastewater may not be an ideal source of water, its ben- efits may - for the present - outweigh its liabilities, particularly since there are few other potential sources for restoring delta ecosystems. BOX 2. A RELATED ISSUE: LA CIENEGA DE SANTA CLARA One of the chief goals of recent research efforts in the delta has been to examine surviving ecosystems and learn about incidental water supplies, such as waste- water, that support wetlands. In fact, wastewater from geothermal wells forms the headwaters of the Rio Hardy. If wastewater can be deliberately managed, many areas of the delta might be sustained without any new dedicated flows. La Cienega de Santa Clara (cienaga means wetland in Spanish) is the major marsh wetland in the eastern region of the Colorado River delta. At 50,000 acres (20,000 ha), dominated by cattails, it is home to perhaps the largest remaining populations of the Yuma clapper rails and desert pupfish. It is a major stopover for migratory waterfowl on the Pacific Flyway, and supports guiding, hunting, fishing, and limited ecotourism activities for local communities. In 1992, Ia Cienega de Santa Clara was included in the core zone of the Biosphere Reserve of the Upper Gulf of California and the Delta of the Colorado River (see Chapter 1). Photo 4. La C1~nega de Santa Clara. Historically, an arm of the Colorado River was located where Ia Cienega de Santa Clara is today. The banks of the river were covered with willow and cottonwood. Over time, the river channel shifted westward, and af- ter the construction of Hoover Dam the river no longer entered the shallow depression (formed by a branch of the San Andreas fault) that defines the wetland. In 1973, the only water flowing into la Cienega was supplied by local artesian springs and agricultural drainage water from the Riito drain. La Cienega de Santa Clara was reduced to a mere 500 acres (200 ha) (Glenn et aI., 1996). --"~--1 Starting in 1977, brackish agricultural drainwater from Arizona has flowed 50 miles south into Mexico via the MODE canal, and drained into the Santa Clara depression. These flows created a wetland of up to 50,000 acres (20,000 ha) of water surface, of which 11,200 acres (4,500 ha) were thickly vegetated. This is considered to be the mature size of the wetland. Flow from the canal was interrupted for eight months in 1993 (following floods), and the vegetated wetland diminished again to 2,750 acres (1,100 ha). The resiliency of the delta marsh systems was demonstrated when flows returned to the MODE canal and the vegetated area assumed its former dimensions (Glenn et aI., 1996). The flows through the MODE were originally intended to be temporary. The U.S. Bureau of Reclamation made other plans for this water as a source for the Yuma Desalting Plant, completed in 1993. [See Box 4.] However, the plant is not yet operating. Any viable plan to operate the Yuma Desalting Plant must provide replacement water supplies and other assurances to protect and sustain the remnant wetlands of Ia Cienega de Santa Clara which now depend on MODE drainage outflows. 16