EXAMINING DOWNSTREAM TRENDS IN CHANNEL BED GRAIN SIZE AND COMPOSITION, SAN JOAQUIN RIVER, CA: TESTING THE EFFECTS OF HISTORICAL DAMMING AND RECENT RESTORATION EFFORTS

Over the last 70 years, the ecology of the San Joaquin River, in Central California, has been greatly altered due to damming and population growth. Flow rates and sediment supply have been substantially affected by these changes. Restoration efforts are in place to increase flows from Friant Dam to the river in order to support a thriving salmon population. Further work must be done to determine how increased flows will affect the system, especially in terms of bed material transport and deposition. The goal of this project is to record downstream trends in bed grain size and composition, and to examine these trends in the context of human modifications to the river.

Three sites were selected at various distances from Friant Dam; bed samples were collected across the channel at each site and were then sieved and weighed for grain size analysis. Sand and gravel sized sediments were analyzed independently to identify trends within and between the two size classes. The river at the base of the dam represents a bimodal system with large populations of gravel and sand-sized sediment. Moving downstream, the river exhibits a continuous bed evolution resulting in a unimodal system that has distinctly smaller grain sizes, by weight percent, than those found at the base of the dam. These results suggest that the sand-sized sediment is actively transported through the system, while gravel appears to be less mobile under modern flow conditions. These findings indicate that the effects of the dam on sediment supply and discharge can still be recognized in the system, despite recent restoration flow increases. Sediment composition was determined by the Gazzi-Dickinson Point-Counting Method as well as X-Ray Diffraction. Gravel samples are compositionally identical across all sampling sites and are composed of a mixture of igneous lithologies. The composition of the sand is relatively consistent through each site, consisting of mostly quartz with various lithic fragments. These results suggests that the sand fraction has undergone greater chemical weathering and may have a distinct origin compared to the gravel in the river. By constraining the transport potential and origin of sand and gravel fractions in the San Joaquin River, below Friant Dam, this study can inform future management decisions and restoration flow targets.