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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 committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

Support for this project was provided by the Army Research Office, National Aeronautics and Space Administration, National Oceanic and Atmospheric Administration, National Science Foundation (under Grant No. EAR-9814582), National Weather Service, and U.S. Geological Survey. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors.

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Cover by Van Nguyen, National Academy Press. Adapted from painting Niagara Falls by Frederick Church, 1857.

Copyright 1999 by the National Academy of Sciences. All rights reserved.

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