ABSTRACT
Human activity has disrupted most of the world’s large river ecosystems. Arid rivers are especially at risk due to their modest base flows and the many human demands on their waters. An example is the Colorado River, which is a major source of water for the desert region of the southwest U.S., northwestern Mexico, and the southern California coastal plain. Approximately 23 million people now use Colorado River water (Morrison et al., 1996). The delta of a regulated river system, below the last dams and diversions, is the natural collecting place for wastewaters and surplus flows exiting the watershed. The delta is the depositional section of the river, where the reduction in grade broadens the floodplain and permits the expansion of wetland and riparian habitats. It is also the mixing zone with the ocean, which is enriched by brackish and freshwater sources from land. River deltas may retain natural functions that have disappeared from upstream stretches. That is certainly true of the Colorado River delta, which supports a large riparian habitat (Glenn et al., 1996, 1999; Valdes-Casillas et al., 1998, 1999). The Colorado River delta is a binational resource, located mainly in Mexico, yet supplied primarily by water from the U.S.
