Introduction to the Colorado River Delta
by Edward Glenn, Christopher Lee, Carlos Valdes-Casillas: 2001

    The delta, meanders through 140-km passage from the US-Mexico border through the heart of the Sonoran Desert, to the sea. This region receives less rainfall than any other location in North America. There has been a renewed interest in the delta natural areas following 20 years of water releases from the US to Mexico. These so-called waste flows have had an unanticipated effect; they have revived wetland and riparian habitats which have become rare on upstream portions of the river (Glenn et al., 1996). The water sources, the habitats, and their water requirements probe policy issues affecting their future. Much of the research on the delta has been presented at a public symposium held in Riverside, California, in October 2000 "...to the Sea of Cortez: Nature, Water, Culture, and Livelihood in the Lower Colorado River Basin and Delta." Its sponsors included the US Fish and Wildlife Service, the University of Arizona's Udall Center, and the University of California UC-MEXUS Program.

    Godfrey Sykes, engineer, cowboy, explorer, and author of several early studies of the Colorado River Delta in the first half of the century, opened one paper with the  statement "It can truly be said of the Colorado that it is the most seen but least known of all the great Western rivers" (Sykes, 1937). This comment was made in advance of  the era of dam building that radically transformed the untamed flow of the Colorado and at the same time provided significant information as to the river's characteristics. However, the enigmatic quality attributed to the river by Sykes is still applicable to the river's delta. In an age of increasing access to, and knowledge of, much of the Earth's surface, the Colorado River Delta remains an area that is visited by few, and understood by still fewer.

    The great sediment transport system that was the untamed Colorado has been repeatedly dammed and it's discharge heavily managed. An area referred to as a 'jaguar-infested jungle' by Aldo Leopold in the 1920's (Leopold, 1949) became a barren,  salt encrusted tidal flat by the 1960's only to spawn new wetlands and riparian forests in the 1970's that are now the largest such habitats in the Sonoran Desert. Before dams, river discharges reached 7000m3/s and transported sediment at a rate that has resulted in deposits 5km thick in some areas. During the Quaternary alone, a cone of sediments covering more than 7700 km2 formed at the mouth of the river. These same flow rates interacted with tidal ranges of up the 10 m in the Gulf of California to create tidal bores that more than once sank large ships operating in the northern reaches of Gulf. Yet, today much less water and little new sediment reaches the Delta. The tidal bores are largely gone and sediments deposited before the regulation of the Colorado are now reworked primarily by the monumental tides. Still, the area exhibits some of the most dynamic hydrologic processes in North America.

    Superimposed on this impressive physical system is an extensive area of agriculture that exists north of the northernmost incursion of Gulf tides. These agricultural areas, located on the ancient deposits of the Colorado which straddle the US-Mexican border are some of the most productive in North America. Since these areas rely on water from the Colorado, they are also indirect contributors to the current hydrologic budget. The delta has also experienced urbanization; nearly 2 million people now live in Mexicali, San Luis, and the US border communities in Yuma and Imperial counties.

    Following construction of Hoover Dam (1935) and Glen Canyon Dam (1964), little or no water flowed to the Gulf of California. Excess water in the watershed ws simply stored behind the dams, as the reservoirs (Lake Mead and Lake Powell, respectively) were still filling. In 1981, however, Lake Powell finally reached capacity. The Colorado River Delta now experiences a different hydrological regime. During years of excess precipitation in the watershed, as during the El Nino cycles of 1983-86, 1993, 1997-99, water is spilled from the dam system to the sea, revitalizing the delta habitats along the way. Augmenting these flood flows, the US has discharged appreciable quantities of agriculture return flows into the southeastern part of the Delta since 1977, creating a large, brackish marsh (Cienega de Santa Clara). Local agricultural return flows in Mexico have created smaller wetlands in the Delta.

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