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<br />2 <br /> <br />E. p, GLENN ET AL <br /> <br /> <br />CALIFORNIA <br />.-..-..-...----..-.. <br />BAJA CALIFORNIA <br /> <br />......1-91... <br />so;.~1'''l <br />..o-9.q....... <br /> <br />..... <br /> <br />..... <br /> <br /> 1 <br />0 10 20 30 <br />I I I I <br /> km <br /> <br />Figure 1. The Colorado River delta from Morelos Dam at the United States-Mexico border <br />to the Gulf of California. The shaded area on the map shows the location of the natural <br />areas remaining in the delta. The main riparian corridor is confined between levees, with <br />agriculture on either side. The major brackish or freshwater wetland areas (El Indio, El Doctor and <br />Cienega de Santa Clara) are also shown. The photographic inset is a subscene of a Landsat <br />Thema~c Mapper Image acquired 18 June 1993..It is displayed as a False Color Composite <br />using bands 4 (NIR), 3 (VIS red), and 2 (VIS green). Vegetation appears in red. Water or <br />saturated soils appears black. This image was taken during a period of extensive water releases <br />from the United States to the Gulf of California, hence the riparian corridor appears Booded. Note <br />the contrast between the lush vegetation irrigated by the river, including riparian and wetland <br />habitats and irrigated fields, and the surrounding desert. This figure is available in colour online, at <br />www.idealibrary.com. <br /> <br />interacted with tidal ranges of up to ten meters in the Gulf of California to create tidal <br />bores that more than once sank large ships operating in the northern reaches of the Gulf. <br />Yet today much less water and little new sediment reaches the Delta. The tidal bores are <br />largely gone and sediments deposited before the regulation of the Colorado are now <br />reworked primarily by the monumental tides. Still, the area exhibits some of the most <br />dynamic hydrologic processes in North America. <br />Superimposed on this impressive physical system is an extensive area of agriculture <br />that exists north of the northernmost incursion of Gulf tides. These agricultural areas, <br />located on the ancient deposits of the Colorado which straddle the U.S. Mexican border, <br /> <br />INTRODUCTION <br /> <br />-' <br /> <br />Hn: some of the most productive in North America. Since these areas rely on watt'!" <br />11"'111 the Colorado, they are also indirect contributors to the current hydrologic budget. <br />'I'he delta has also experienced urbanization; nearly 2 million people now live in <br />.\1cxicali, San Luis, and the United States border communities in Yuma and Imperial <br />.:"unties. <br />Following construction of Hoover Dam (1935) and Glen Canyon Dam (1964), <br />lillle or no water flowed to the Gulf of California. Excess water in the watershed <br />was simply stored behind the dams, as the reservoirs (Lake Mead and Lake Powell, <br />n:spectively) were still filling. In 1981, however, Lake Powell finally reached capacity. <br />'I 'he Colorado River delta now experiences a different hydrological regime. During <br />)TarS of excess precipitation in the watershed, as during the El Nino cycles of <br />1983-1986, 1993 and 1997-1999, water is spilled from the dam system to the sea, <br />I'l'vitalizing the delta habitats along the way. Augmenting these flood flows, the United <br />Slates has discharged appreciable quantities of agricultural return flows into the south- <br />l'astern part of the delta since 1977, creating a large, brackish marsh (Cienega de Santa <br />(:Iara). Local agricultural return flows in Mexico have created smaller wetlands in the <br />delta (see Fig. 1). <br />It is this unique interplay of water, sediment, and vegetation, both prehistoric <br />and contemporary, unregulated for the majority of it's existence and modified in <br />striking ways during the last century, that lures geographers, hydrologists, and biologists <br />10 this incredible living laboratory. This volume presents the findings of some of <br />the scientists that have been drawn to this largely unknown area and hopefully serves <br />10 whet the appetite of a new generation of explorers and scientists eager to <br />help us understand the many aspects of the Colorado River Delta that have yet to be <br />l'xplained. <br />The papers in this volume are organized into four sections: Reviews, Hydrology and <br />Water Quality, Vegetation and Wildlife Dynamics and Policy Issues. There are two <br />review papers. The first, by Glenn et ai., reviews the literature on the Colorado River <br />delta over the past 100 years, including information in the present collection of papers. <br />This paper also describes the major ecozones in the delta today and identifies the water <br />sources that support them. The second, by Stromberg, reviews the critical effect of <br />How regime on vegetation along desert rivers. <br />The Hydrology and Water Quality section begins with a water balance study of the <br />delta during flood and non-flood years (Cohen et ai.). Glenn et ai. then document the <br />return of native cottonwood (Popuiusfremontil) and willow (Saiixgooddingii) trees to the <br />riparian corridor in response to pulse floods, and estimate the amount of water needed to <br />support this ecozone. Finally, Garcia-Hernandez, King et al. report on levels of toxic <br />substances in the sediments and biota of the delta ecozones, and compare their results to <br />those obtained on upstream river stretches in the United States. <br />The widespread replacement of native trees by the exotic shrub, salt cedar (Tamarisk <br />ramosissima), and the ecophysiological conditions that can help reestablish native trees, <br />is explored in three papers in the Vegetation and Wildlife Dynamics se~tio~, address~ng <br />the effects of salinity and water deficit (Vandersandae et ai.), slltatlon (Levme <br />& Stromberg) and nutrients (Marler et ai.) on seedling establishment under greenhouse <br />conditions. The distribution of two endangered bird species in the delta is described in <br />Garcia-Hernandez et ai. and Hinojosa-Huerta et ai, <br />Perceptions of the river by stakeholders on both sides of the border, especially by <br />residents of the delta, and prospects for conserving and enhancing the delta habitats are <br />explored in two papers in the Policy section. Varady et ai. summarize the poi~ts ofvi,ew <br />expressed at the 'to the Sea of Cortez' symposium, while Pitt addresses the big question <br />in 'Can the delta be restored?', The policy papers both emphasize the need for further <br />research to understand the delta ecosystems. Hence, this volume should be viewed as <br />a starting point rather than a source of answers for all the questions about the delta and <br />its connection to the larger ecoregion. <br /> <br />C', <br />C.J <br />W <br />I->~ -" <br />00 <br />l<..1 <br />