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ID: 2464

Gaeuman, D. 2020. WY2016-2017 Trinity River gravel augmentation monitoring report. Report for the Trinity River Restoration Program (TRRP). Yurok Tribal Fisheries Program, Klamath, California. Available: https://www.trrp.net/library/document?id=2464.

Gravel augmentation is a method for restoring gravel-bed stream reaches in which coarse bed material that would otherwise be delivered from upstream is captured in impoundments behind dams. The Trinity River, located in far northern California, is regulated by two dams that divert a portion of its flow to the Sacramento River as part of California’s Central Valley Project. This report describes gravel augmentations implemented in the Trinity River by the Trinity River Restoration Program in water years 2016 and 2017, and presents the results of 2016 and 2017 monitoring activities designed to assess how those augmentations affected gravel transport and bed morphology in the reaches downstream from the augmentation points. Augmentations in those years were performed at the Diversion Pool augmentation site and at two locations at Lowden Ranch during spring high flow releases from Lewiston Dam. Monitoring conducted in the reaches downstream from those sites consisted of repeated topographic surveys, bedload transport sampling, and relocation of bedload tracer particles. The topographic data support development of morphology-based gravel budgets, whereas the bedload measurements provide boundary conditions that support estimation of gravel fluxes throughout the study reaches. Tracer locations delineate gravel transport distances and transport pathways, and provide insight into the dynamics of mass exchanges between the augmentation material and the native substrate. At 12,000 ft3/s, the peak discharge attained by the 2017 flow release was 26% greater than the peak of 9,500 ft3/s reach during the 2016 release. Sediment budget results for 2016 show that a large share of the gravel introduced into the river that year was deposited close to the augmentation points. The 2017 budgets indicate that gravel fluxes were between 50 and 170% greater in 2017 than in 2016, and that the areas immediately downstream from the augmentations experienced net erosion. Deposition of the 2016 and 2017 augmentation material occurred farther downstream, and significant gravel fluxes exited the downstream boundaries of both study reaches. The increase in gravel mobility during the 2017 flow release relative to the 2016 release appears to have been due in large part to the larger magnitude of the 2017 release peak. The tracer data, however, suggest that vertical and lateral sorting processes may have prevented the augmentation gravel from fully participating in the downstream transport. This report also presents data quantifying depth changes over a six-year period in 105 pool locations distributed between Lewiston Dam and the North Fork Trinity River. Those data reveal that the depths of most pools remained essentially constant between 2011 and 2017.

First Posted: 2020-02-25 18:05:11

Post Updated: 2020-02-25 18:22:56