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Investigating Downstream Trends in Bed Sediment Texture and Composition in The San Joaquin River, Central California


The San Joaquin River, deemed the most endangered river in the United States of America, has been been greatly altered due to damming and population growth over the last century. The goal of this project is to document downstream trends in bed grain sizes of the modern river and to analyze the composition of the sediment to determine its provenance. The downstream trends will be analyzed to identify any disconnect between expected grain texture, composition, and modern flow rates. This study will distinguish whether restoring flows to pre-dam conditions will contribute to increased mobility of channel bed sediments. If the bed sediment grain size reflects a local source from the adjacent floodplain, composed of Pleistocene fluvial fan deposits influenced by high discharge during glacial outwash, then even with restoration flows, the channel bed sediments may be relatively immobile. However, if the bed sediments are mobile under modern flow conditions, then the river may be more easily restored to pre-dam conditions that favor a full salmon reintroduction. Various sites were selected at different distances from Friant Dam near Fresno, California. Sediment samples were collected across the channel at each site and were then sieved and weighed for grain size analysis. Sand and gravel-size sediments were analyzed independently to identify trends within and between the two size classes. Sediment composition was determined by applying the Gazzi-Dickinson Point-Counting Method to thin sections and loose sediment of collected samples from each site. Composition was also determined through X-Ray Diffraction. These results can contribute to our understanding of extent of the human impact on the river system as well as the factors that influence bed mobility, grain texture, and composition. Evaluating the effects of local sediment sources versus channel flow conditions will also inform how channel processes and conditions are recorded in the stratigraphic record.