EVOLUTION OF SEDIMENT CLUSTER MORPHOLOGIES ON THE AMERICAN RIVER, CASCADE MOUNTAINS, WA

The purpose of this project was to find patterns of the evolution of cluster morphologies on the American River, WA. Cluster bedforms are groups of sediment particles that consist of one or more anchor clasts which affect the movement of other sediment particles. Our focus was the evolution of the cluster morphologies during periods of moderate peak flows. It is our hypothesis that this evolution would only occur during moderate flow conditions because during high flow periods the clusters would be completey reorganized.

Six types of clusters were observed at the study site on the American River. The six types include: upstream triangle, downstream triangle, diamond, ring, line, and transverse line. In the field, the clusters were identified, surveyed, and photographs were taken of them before and after high flow periods. The photos were taken in April 2003, August 2003, and August 2004. The photos were then analyzed to track the movement of sediment within the clusters for each year. The 2003 peak flow was 31.5 m³/sec and the 2004 peak flow was 24 m³/sec, which are moderate-sized annual peaks compared to the historic record. Several aspects of sediment movement were analyzed: 1) the number of particles added or removed on the downstream side of the cluster, 2) the number of particles added or removed from the upstream side of each cluster and 3) the number of original particles moved within each cluster. The results were evaluated graphically and plotted on several maps.

From these findings preliminary conclusions were made regarding the evolution of cluster morphologies. No strong correlations between cluster movement and position on the gravel bar were found. Line clusters appear to be an incipient form that ultimately transforms into an upstream or downstream triangle. Upstream and downstream triangles appear to be intermediate cluster forms that ultimately form diamonds.