TRACKING THE EVOLUTION OF SEDIMENT CLUSTERS AFTER HIGH FLOW EVENTS, AND THEIR AFFECTS ON SEDIMENT TRANSPORT: ENTIAT RIVER, WA. A FIELD BASED APPROACH

This project investigated the movement and evolution of sediment clusters during three separate flood events at two geomorphically different sites along the Entiat River, WA. Clusters are defined as an obstacle or anchor clast(s) that impedes the progress of two or more sediment particles, and are believed to be an important characteristic of the variable bed topography of gravel-bed rivers. Detailed field descriptions and digital photographs of clusters were used to determine the characteristics of clusters at chosen locations within the thalweg, riffle and high-flow gravel bars. The data were collected during low-flow conditions, and the clusters were re-examined and re-photographed after high flow events in fall 2003, winter 2004, and spring 2004. Clusters were examined to determine whether they changed form, remained stable or were completely destroyed. Individual particles within clusters were also tracked to determine the possible affects of clusters on the entrainment of sediment. Data from a USGS gaging station, located between the two sites, were used in combination with cross-sectional data to calculate the velocity and average bed shear stress associated with the entrainment of particles within clusters.

Results from this project will be compared with cluster patterns along the American River, WA. which features contrasting flow events, sediment and cluster sizes, and geomorphic settings. These field investigations also will support collaborative research in laboratory flume experiments performed by the University of Iowa in which cluster evolution was simulated under varying hydrologic settings and changing sediment fluxes, sizes, and densities. An improved understanding of how clusters develop, evolve and affect the entrainment of sediment under varying hydrologic conditions will aid in assessment of sediment transport processes, bed stability, and in-stream habitat conditions in natural, healthy functioning streams. This improved understanding can then be applied to regulated rivers and restoration projects aimed at sustaining in-stream flows that maintain optimal sediment transport conditions for enhanced in-stream habitat.