THE ROLE OF GEOMORPHIC AND HYDROLOGIC FEATURES ON SEDIMENT CLUSTERS IN GRAVEL-BED STREAMS, WASHINGTON: A FIELD-BASED APPROACH

This project investigated the effects of geomorphic features in gravel-bed rivers on the formation, evolution and geometric features of cluster microforms. Clusters consist of two or more sediment particles that become interlocked within a stream bed, and are believed to be an important characteristic of the variable bed topography of gravel-bed rivers. Detailed field descriptions and photographs of clusters helped determine the characteristics of clusters at chosen locations within the thalweg, riffle, and high-flow gravel bars along the American and Entiat Rivers in central Washington. The data were collected during low-flow conditions and were re-examined after the spring/summer peak flows of 2003 in order to determine the evolution of the clusters during high-flow events. Stage data collected throughout the peak-flow period, in combination with cross-sectional data, were used to calculate the average bed shear stress associated with the growth, stability, or destruction of given clusters. Geomorphic settings within the channel, particle size distribution, gravel bar morphology, and cross-sectional information were examined in order to determine the geomorphic conditions that favor cluster formation.

Results from the American River were compared with preliminary results from similar data collected from two locations along the Entiat River during the fall of 2003. Heavy rain in late October created flows near historic peak-flow values in the Entiat River, which allowed for preliminary determinations of how clusters evolve during high-flow periods in this river system. These field investigations will support collaborative research in laboratory flume experiments performed by the University of Iowa in which hydrologic and cluster conditions will be simulated. Results from the field-based portion of the project will be used to calibrate and validate those experiments. An improved understanding of the clusters will help in assessment of sediment transport processes, bed stability, and in-stream habitat conditions.