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.

This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

Support for this project was provided by the U.S. Geological Survey under Grant No. 1434-93-A-0982.

Copies of this report are available from the Water Science and Technology Board, National Research Council, 2101 Constitution Avenue, N.W., Washington, DC 20418.

Cover by Van Nguyen, National Academy Press, using photos provided by U.S. Geological Survey.