The formation of sand bars and other types of depositional landforms along stream channels is a function of many factors, especially the amount and nature of sediments carried by the stream and the volume and velocity of stream water. A factor often overlooked in the analysis of depositional dynamics is the fluctuation of water flows. The Colorado River flowing through the Grand Canyon of Arizona offers a unique setting for the analysis of changes in depositional dynamics resulting from changing fluctuations in stream flows. Prior to construction of the Glen Canyon Dam above the canyon in the early 1960s, the Colorado's flow tended to follow annual cycles, with large floods in spring followed by long periods of minimal flow. The dam now intercepts all upstream sediments, however, and discharges usually made on a regular basis for power generation and flood control now produce tidal-like daily pulses of water. The lower-level but more frequent fluctuations have had an adverse impact on the number and size of sand bars along the canyon floor. This project will examine the stability of sand bars in the Grand Canyon under the current water discharge regime, focusing special attention on the erosive effects of waves below major rapids. Research methods used in this project will include a number of approaches and measurement techniques commonly employed in coastal studies but rarely used to study fluvial systems. Sensitive instruments that measure water pressures and current speeds and directions will gather data to test beach equilibrium models that relate sediment erosion and accretion to wave parameters. NSF funding will complement support for this project provided by the U.S. Geological Survey. This project will provide valuable data and new insights into the dynamics of sand bar development in the Grand Canyon, a topic of great practical importance because of the critical role of sand bars as wildlife habitat and as recreational sites for people on float trips down the Colorado. The data collected in this project and the refined models will have broad scientific significance, too. They will expand understandings of the complex dynamics of depositional features in stream channels under different flow regimes and will provide the first detailed examination of backwash wave impacts on sand bars in major streams.
