<br />2
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<br />E. p, GLENN ET AL
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<br />CALIFORNIA
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<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
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<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.
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