Monitoring the Effectiveness of Gravel Augmentations Downstream from Dams for Habitat Improvements
Major gravel augmentation efforts intended to improve salmonid habitat are currently underway in a number of streams where natural recruitment of gravel is impeded by Bureau of Reclamation (Reclamation) dams, including those on Clear Creek, and the Trinity, Sacramento, American, and Stanislaus Rivers in California. Despite the widespread use of gravel augmentations in stream management, the full range of habitat improvements that can be expected from the practice have not been clearly defined. Improved habitat conditions and increased spawning success in the immediate vicinity of gravel introductions are relatively simple to document, but the potential for gravel additions to improve conditions in downstream reaches is more difficult to predict. Some specific questions relevant to the management of gravel augmentation programs include:
* How far downstream from a gravel augmentation point will habitat benefits be realized?
* How long will it take to achieve downstream habitat benefits?
* How long will habitat benefits at a specific location persist?
* How much added gravel is needed to achieve habitat benefits at specific downstream locations?
In the context of gravel augmentation, the answers to these questions are determined, to a large extent, on how gravel slugs (i.e., localized large inputs of gravel-sized sediments) evolve as the material is transported downstream. A number of recent geomorphic studies have considered whether gravel slugs tend to translate downstream as a coherent wave or whether they tend to simply disperse with minimal translation of the slug from its original location.
Need and Benefit
As noted above, numerous gravel augmentation programs are currently being implemented in the western U.S.. These programs are typically managed by Reclamation and funded through Reclamation or its constituents. For example, several augmentation programs in California are implemented with funds derived from the Central Valley Improvement Act. However, basic characteristics of the long-term behavior of coarse sediment slugs needed to optimize augmentation design and management remain poorly understood.
A number of recent studies conducted in laboratory flumes (Lisle et al. 1997; Cui et al. 2003a; Sklar et al. in review) or by numerical simulation of transport in prismatic channels (Lisle et al. 2001; Cui et al. 2003b; Cui and Parker 2005; Greimann et al. 2006) strongly suggest that the evolution of gravels slugs is dominated by dispersion in place. However, these studies do not consider the possibility that geomorphic structures found in natural streams may cause dispersed slugs to be reconstituted at downstream storage areas. For example, dispersed gravel may reconstitute into a coherent slug where channel curvature or obstructions produce fixed bar forms or in so-called "response reaches" where incoming sediment interacts strongly with the existing channel morphology (Wathen and Hoey 1998).
Grass Valley Creek is exceptionally well suited to investigate reach-scale gravel dynamics for a number of reasons. Gravel augmentations are currently underway in Grass Valley Creek in two closely spaced reaches below Buckhorn Dam, a Reclamation dam constructed in the early 1990s. These augmentations will impound fine sediment generated in the watershed. Although the stream is large enough to transport gravel particles in the range used for spawning by anadromous fishes, it is small enough to make particle relocation practical. At base flow, Grass Valley Creek is 100 percent wadeable. The stream is relatively remote and mostly accessible only through private land, so public traffic and the potential for particle disturbance by the public are low. The study segment has a clear downstream boundary at Hamilton Ponds, a set of dredged ponds immediately upstream from the creek's confluence with the Trinity River. Any tracers that pass through the entire study segment will likely be recovered during pond dredging operations. Grass Valley Creek is biologically important as it contains spawning habitats used by Coho salmon, a listed species.
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Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Monitoring the Effectiveness of Gravel Augmentations for Salmonid Habitat Improvement Downstream from Dams (final, PDF, 1.5MB)
By David Gaeuman
Report completed on January 15, 2013