The Role of Geomorphic Features and Hydrologic Processes on Sediment Clusters in Gravel-Bed Rivers, Washington: A Field-Based Approach

Ross Richard Hendrick
August 2005


This project investigated the movement and evolution of sediment clusters after four separate flood events at two geomorphically different sites along the Entiat River, Washington. Clusters are defined as an obstacle or anchor clast(s) that impede 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 on gravel bars regularly covered by high flow events. Data were collected during low-flow conditions, and clusters were re- examined and re-photographed after each flow event. Clusters were examined to determine whether they changed form, remained stable or were completely destroyed. Individual particles within clusters were also tracked, and the velocity and critical shear stress required to entrain particles were calculated to determine the possible effects of clusters on the entrainment of sediment. An improved understanding of how clusters develop, evolve and affect the entrainment of sediment under various hydrologic processes will aid in assessment of sediment transport processes, bed stability, and in- stream habitat conditions.

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