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Exposed and active coarse sediment (>2 mm diameter) and fine sediment (≤2 mm diameter) deposits were mapped in summer 2023 on the Trinity River between Lewiston Dam and the North Fork Trinity River. The mapping was used to identify areas where fine sediment deposits are lacking, quantify the trajectory of change in coarse sediment storage since active bar mapping was last completed in 2014, and describe mechanisms for and limitations on coarse bar development. The upstream-most fine sediment deposit was mapped 2.04 river miles downstream of Lewiston Dam, indicating a deficit of fines in this reach. The deficit likely extends to river mile (RM) 89.92 near Steiner Flat, where areas of fine sediment deposit per unit channel distance triples relative to the upstream reach. Coarse bar areas increased 45% from 1,345,673 ft2 in 2014 to 1,957,437 ft2 in 2023; the number of mapped bars also increased from 302 to 392 in these years, which is a 30% increase that indicates bars were larger and more numerous in 2023 than the previous survey. Bar areas per unit channel distance increased from 29,262 ft2/mile in 2014 to 49,256 ft2/mile in 2023, and the cumulative areas of bars exhibited stepwise rises at stream junctions and gravel augmentation locations, suggesting gravel placements may effectively simulate natural supplies of sediment to the river. Coarse bars were most frequently associated with channel expansions followed by river bends, channel reconstruction projects, and bedrock forcings. Fine sediment deposits were commonly associated with the same forcings driving coarse deposits, except roughness from vegetation replaced constructed areas in importance. The main limitation on fine sediment deposits appears to be the under supply between Lewiston Dam and Steiner Flat, located 22.3 river miles downstream from the dam. Adding fine sediment near Lewiston and increasing channel complexity would help reverse the deficit by providing areas for deposits to reside within the active channel. Large wood additions and increased roughness from channel widening would also promote fine and coarse bar development. Large wood was the least frequently observed forcing mechanism for these sediment deposits due to its storage principally outside the active channel of the Trinity River.

Associated information:

• McSloy, J. and T. H. Buxton. 2024. Active Bar Monitoring Map Book. Trinity River Restoration Program (TRRP), Weaverville, California. Available: https://www.trrp.net/library/document?id=2655.
• Buxton, T. H. and J. McSloy. 2024. Active Gravel Bars 20231030 RM. Data Package for the Trinity River Restoration Program (TRRP). TRRP, Weaverville, California. Available: https://www.trrp.net/library/data?id=161.