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PREFACE This profile report covers the lower Colorado River system from Davis Dam in southern Nevada to the border of Mexico. It is a synthesis of all existing information, to the extent possible, describing the ecology of the lower Colo- rado River and its adjacent riparian ecosystem. In a sense, it is a historical as well as an ecological document. Historically, the flow of the river decided the nature of the ecology of the lower Colorado River. The lower Colorado River is presently completely controlled by a series of upstream impoundments that regulate the flow of the river. This control provides man with the oppor- tunity to manipulate the river and its adjoining floodplain to an extent probably not realized on any other river system in the United States. As a consequence, there is little remaining of the "natural" systems that formerly flourished in and along the river prior to its settlement by emigrants of Euro- pean descent. The river and its adjacent riparian borders are and will be what the citizens of the southwestern United States and, to some extent, the whole nation want it to be. This report will facilitate efforts of those managers, ecologists, politicians, and other interested participants in deciding what kinds of environment we want along the lower Colorado River. The authors have tried to show how hydrology and vegetation formerly set the stage for fish and wildlife habitats and populations along the river, and how land and water use practices currently control fish and w i l d l i fe habitats and populations. The report is designed to provide the reader with easy access to information on a variety of subjects related to the hydrology and ecology of the river over time. There is some redundance for this reason; however, this decision was a conscious one to provide internal clarity within different sections of the report. Native floodplain vegetation, for example, is con- trolled by overbank flooding and groundwater, and is discussed in those terms. On the other hand, vegetation controls wildlife and it is also described in terms of wildlife habitat. The senior author had overall responsibility for assigning subjects for review and synthesis and for integrating the various components of the report. Questions and comments on the technical and scientific contents of the report should be addressed to the authors. Requests for copies of the report should be addressed to: Information Management Section National Ecology Research Center U.S. Fish and Wildlife Service Creekside One Building 2627 Redwing Road Fort Collins, CO 80526-2899