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ooosno T DELTA IN CONTEXT I have never gone back to the Delta of the Colorado since my brother and I explored it, btJ canoe, in 1922.... For all we could tell, the Delta had lain forgotten since Hernando de Alarcon landed there in 1540.... On the map the Delta was bisected by the river, but in fact the river was nowhere and everywhere, for we could not decide which of a hundred green lagoons offered the most pleasant and least speedy path to the Gulf. -Aldo Leopold, A Sand County Almanac, 1948 OVERVIEW Prior to the construction of major dams along its route, the Colorado River fed one of the greatest desert estu- aries in the world. Spread across the northernmost end of the Gulf of California,l the Colorado River delta's vast riparian, freshwater, brackish, and tidal wetlands once covered 1,930,000 acres (780,000 ha) and supported a legendary richness of plant, bird, and marine life. Because most of the river's flow reached the delta at that time, its freshwater, silt, and nutrients helped create a complex system of wetlands that provided feeding and nesting grounds for birds, and spawning habitat for fish and crustaceans (Glenn et aI., 1996). The Gulf's estuary was a source of fertility reaching far from its shallows, and the legendary richness of the entire Gulf can be laid to the delta's productivity as well as its capacity to sup- port marine and bird life. In contrast to the surrounding Sonoran Desert, the Colorado River delta's abundance was striking. Today, conditions in the delta are changed. Like other desert river deltas, such as the Nile (Stanley and Warne, 1993) and the Indus (Snead, 1987; Leichenko and Wescoat, 1993), the Colorado River delta has been greatly altered by human activity. Decades of dam con- struction and water diversions in the United States and Mexico have reduced the delta to a remnant system of small wetlands and brackish mudflats. As reservoirs filled behind dams and captured floodwaters, freshwa- ter flows no longer reached the delta. Now that the reservoirs are full, the periodic flood flows that do reach the delta are constricted by Ievees.2 In the 1970' sand 1980' s, the popular and scientific press referred to the delta as a "dewatered" or "dead" ecosystem (Spamer, 1990).3 Since 1981, however, the delta has begun to make a slow comeback. From 1980 to 1998, total water releases to the delta have amounted to an estimated 20 percent of the Colorado's total flows over the same period. While most of this is either flood- water or agricultural and municipal wastewater, these flows are proving beneficial. Although flood flows are extremely unreliable and irregular, and wastewater is high in salinity and pollutants, this water has begun to restore some areas of the delta. Current conditions have allowed wetlands and riparian vegetation to flourish on about 150,000 acres (60,000 ha). This report documents recent scientific findings about the delta's partial recovery and makes recommendations for managing existing flows to further benefit the delta's ecosystems (see Chapter 3). The authors believe that key areas of the delta might be saved through deliberate management of water that now flows into the delta. This could be accomplished without adverse effects on other Colorado basin water users. While demand for water by irrigators, cities, and other important constituencies makes it unlikely that pre development flows can be restored to the delta in the short term, the purposeful management of existing water resources such as agricultural drainage, wastewater, and floodwater could make a significant difference. Although the delta's ecosystems deserve greater consideration in the alloca- tion of Colorado River resources, the delta's minimum requirements are surprisingly modest. Even in its present state, the delta is the most significant wetland system in the American Southwest, and a very productive estuary. Increasing population throughout 1 The Gulf of California is also known as the Sea of Cortez. 2 For the purposes of this report, a flood is any volume of water that crosses the United States-Mexico international border, either as a result of releases from U.S. reservoirs for flood control purposes (or other reasons), or directly as a result of flooding in the U.S. (e.g., flooding in the Gila basin), in excess of the U.S. delivery obligation to Mexico, and that is delivered at a rate that exceeds Mexico's diversion capacity and inundates land (either within the levees or beyond) that is normally dry. 3 Reports of the delta's demise include accounts in the popular press such as Philip Fradkin's A River No More (1981), as well as scientific publications.