Trinity River Restoration Program Technical Report: TR-TRRP-2020-1 WY2016-2017 Trinity River Gravel Augmentation Monitoring Report Prepared by: David Gaeuman The Yurok Tribe Fisheries Department Klamath, California January 2020 Contents Abstract .................................................................................................................................................. 1 Introduction ............................................................................................................................................ 2 Geographic Context ................................................................................................................................ 3 Flow Management .................................................................................................................................. 4 Gravel Augmentations ............................................................................................................................ 5 The Diversion Pool ............................................................................................................................ 6 Lowden Ranch ................................................................................................................................... 7 The 2016 and 2017 Flow Releases .................................................................................................... 9 Methods ................................................................................................................................................ 12 Topographic Surveys ....................................................................................................................... 12 Topographic Change Analysis ......................................................................................................... 14 Sediment Monitoring ....................................................................................................................... 17 Uncertainty ....................................................................................................................................... 17 PIT-tagged Tracer Stones................................................................................................................. 21 Results of Reach-Scale Monitoring ...................................................................................................... 23 Gravel Budgets in the Lewiston Bridges Reach .............................................................................. 23 The 2016 Flow Release ................................................................................................................ 24 The 2017 Flow Release ................................................................................................................ 25 Gravel Budgets in the Lowden/THG Reach .................................................................................... 29 The 2016 Flow Release ................................................................................................................ 29 The 2017 Flow Release ................................................................................................................ 32 Gravel Activity in the Lewiston Bridges Reach .............................................................................. 36 Gravel Activity in the Lowden/THG Reach .................................................................................... 37 PIT-tagged Tracers ........................................................................................................................... 38 Tracer Recovery Efficiency ......................................................................................................... 38 Integration of Gravel Budget and Tracer Results ................................................................................. 45 Results of Pool Depth Monitoring ........................................................................................................ 51 Background ...................................................................................................................................... 51 Quantifying Changes in Pool Morphology ...................................................................................... 51 Pool Selection .................................................................................................................................. 52 Depth Changes by Time Interval ..................................................................................................... 56 Conclusions .......................................................................................................................................... 61 Recommendations ................................................................................................................................ 63 References ............................................................................................................................................ 64 Appendix A: Assessment of Elevation Bias in 2017 Sonar Data Appendix B: Bedload Sampling Uncertainty Appendix C: Morphodynamic Modeling Appendix D: Pool Polygon Location Maps Appendix E: Pool Depth Statistics Appendix F: Cumulative Pool Depth-frequency Curves Appendix G: Pool Depth Change Statistics i Technical Report TRRP-2020-1 Abstract 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. 1 Technical Report TRRP-2020-1 Introduction 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. Elimination of bed material inputs downstream from dams can lead to channel incision, coarsening of the streambed, decreased bed mobility, and a loss of topographic and habitat diversity (Williams and Wolman, 1984; Lisle et al. 1993; Church, 1995; Viparelli et al. 2011). In such instances, gravel augmentation has the potential to decrease bed surface particle sizes and increase bed mobility, thereby stimulating increased rates of geomorphic activity. Processes such as bed scour and fill, bank erosion, and channel migration create and maintain the pools, bars, and riffles that drive variations in flow depth, velocity, and direction that constitute the diversity in meso- and micro-scale habitats needed to support the range of species and life stages present in the aquatic ecosystem (Petts and Maddock 1996; Kondolf and Wilcock 1996; Stanford et al. 1996; Richter et al. 1997; Milhous 1998; Ock et al. 2015). Despite its potential for ecological benefit, however, there is also a risk for coarse sediment augmentations to produce unfavorable outcomes. There is considerable evidence that large increases in sediment supplies can cause topographic relief and physical habitat