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 U.S. Department of the Army under Contract No. W912DQ-08-D-0041. 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 organization or agency that provided support for the project.

The cover photo shows the mouth of the Niobrara River as it empties into Lewis and Clark Lake, the reservoir behind Gavins Point Dam, near Yankton, South Dakota. The Niobrara is a major sediment contributor to the reservoir, as illustrated by the sharp boundary between turbid Niobrara water and clear Missouri River water across the bottom of the image. The islands are the upper surface of the reservoir headwaters delta. Photo courtesy of William L. Graf, University of South Carolina.