NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the panel responsible for the report were chosen for their special competences and with regard for appropriate balance.

Support for this study was provided by the National Research Council’s Day Fund, the U.S. Bureau of Reclamation under contract number 05PG303309, the California Department of Water Resources, the Metropolitan Water District of Southern California under contract number 77624, and the Southern Nevada Water Authority. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project.

Cover: Tree-ring cross section photograph courtesy of Connie Woodhouse. Glen Canyon Dam and Lake Powell (2004) photograph courtesy of Brad Udall. Skyline photograph of Las Vegas, NV provided by the U.S. Bureau of Reclamation at http://www.usbr.gov/dataweb/html/ griffith.html. Formal portrait of John Wesley Powell circa 1890’s from the U.S. National Park Service’s Grand Canyon National Park Museum Collection.