Abstract
The Lower American River has historically provided natural spawning habitat for approximately one third of Northern California’s salmon population. However, since the construction of Folsom and Nimbus Dams, downstream reaches have become sediment starved and periodic high outflow from the dam has caused channel armoring and incision, thereby degrading the natural spawning habitat. Restoration work on spawning sites in the Lower American River has consisted primarily of importing gravel to create riffles during periods of moderate flow. This is an effort to mitigate armoring of the riverbed and to rehabilitate salmonid spawning habitat by providing suitable grain size for all stages of spawning (redd construction, incubation, and emergence). Since restoration activities began, all rehabilitated sites have not been equally used for spawning. This study attempts to examine and compare the physical parameters of each site in order to ascertain which characteristic create more suitable rehabilitated habitat. To do this, we compared physical parameters of enhanced areas and a natural spawning area to redd density using principle component analysis and ANOVA statistical analysis. We found that some augmentation sites are more heterogeneous than others, and this correlates with higher spawning use (F=30.81, p=0.009). With time, salmonids alter the spawning sites, creating small ridges and valleys perpendicular to flow. This creates more variable subsurface flow and generates hyporheic flow through the new gravel. This may have an effect on spawning as the more seasoned additions have a higher frequency of spawning than the newer augmentations. In order to efficiently rehabilitate a site and expedite the “seasoning process”, creating variance through gravel contours during the gravel augmentation process may be effective as it mimics the small scale biophysical interactions.