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 study was supported by the U.S. Geological Survey, Department of the Interior, under assistance award No. 1434-HQ-97-AG-01819. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. government.

Cover: Illustrations courtesy of the U.S. Geological Survey published in the National Atlas of the United States of America™, 1970.