Presenter/s: Nick Southall
Symposium Session: 2020 - 10 Prioritizing restoration
Topics covered: beavers, fish passage, fish-cutthroat, fish-salmon, fish-steelhead, floodplain, hydraulics, lessons learned, modeling, and riparian
Today’s salmon runs in California’s Central Valley have decreased to a small fraction of their historic abundance (Yoshiyama, 1998). Salmonid populations have suffered due to the impacts of the 1849 California gold rush, subsequent settlement and agricultural development, overfishing, and the damming of nearly all Central Valley rivers. Rim dams on the major tributaries have substantially limited the extents of anadromy, regulated flows, and interrupted natural downstream processes. As part of the Central Valley Project, Folsom and Nimbus Dams were constructed in 1955 on the American River, limiting anadromy to the lower 23 miles of the American River.
Over the last 12 years, salmonid recovery efforts on the lower American River have focused on gravel augmentation efforts to improve spawning conditions. Following the completion of nine successful projects, the focus has evolved to include the enhancement and creation of rearing habitat. A key question for decision makers is how much rearing habitat is needed and where should it be located? This presentation will provide an overview of a rigorous science-based planning effort to quantitatively identify and prioritize potential rearing habitat enhancement projects.
Using the USFWS Anadromous Fish Restoration Program’s doubling goal for Chinook salmon on the lower American River, the overall spatial and temporal need for rearing habitat was estimated with the Emigrating Salmonid Habitat Estimator (ESHE) Model. Results from a detailed two-dimensional hydrodynamic model were combined with habitat suitability indices to estimate the spatial distribution of existing rearing habitat across a wide range of flows. The rearing habitat deficit was calculated as the difference between the habitat required to reach the doubling goal and the habitat currently available, which varies both spatially and temporally. A high-resolution digital elevation model of the river corridor and results of the hydrodynamic model were combined to calculate the relative elevation of floodplain areas above typical rearing season flows. Findings of this assessment were used to identify potential floodplain enhancement (lowering), side channel connection, backwater channel enhancement, and structural improvement (large wood and revegetation) opportunities.
Initial prioritization employed the methods described above and identified over 60 opportunities ranging in scale and type. Using various cost/benefit criteria, sites were ranked and the leading opportunities reviewed with stakeholders. The leading sites were subsequently refined and reprioritized through evaluation of grading boundaries, minimization of impacts to existing vegetation, optimization of rearing conditions for juvenile salmonids and evaluation of additional factors including the thermal regime, the quality of existing rearing habitat, and recent geomorphic evolution. In future phases of the project, the highest ranked sites will be advanced for further design, permitting and implementation.
The innovative application of the ESHE model, multi-dimensional hydrodynamic modeling, rearing habitat quantification and the relative elevation analysis, provides a quantifiable science-based approach to identify and prioritize a variety of rearing habitat restoration opportunities aimed to address limiting factors necessary to recover declining salmon stocks.
Ronald M. Yoshiyama, Frank W. Fisher & Peter B. Moyle (1998) Historical Abundance and Decline of Chinook Salmon in the Central Valley Region of California, NAJFM, 18:3, 487-521