Abstract
Anthropogenic forces, including dams, artificial levees, channel modification and overall urbanization have led to streambed degradation and the decline of resident salmonid populations in the Lower American River. Approximately one third of natural salmonid spawning in Northern California occurs in the Lower American River, making the condition of this stream channel very important. Gravel restoration projects have been conducted on the river to offset the degradation of natural spawning areas for more than two decades. Many of the factors that limit successful natural spawning are part of the physical environment and depend on appropriate substrate size, water depth and velocity, temperature, dissolved oxygen content, and a variety of other more subtle factors like cover, upwelling or downwelling conditions, and hyporheic flow. This study analyzed data from four augmentation sites, restored between 2008 and 2012, to evaluate temporal changes at individual sites and compare results between sites. Additionally, researchers observed salmonid response to augmentation projects over time by examining the use of each restoration site during Fall-run Chinook salmon spawning seasons from 2007 to 2012. Analysis showed that gravel augmentation projects on the Lower American River are improving salmonid spawning habitats. Immediately following restoration, physical parameters showed significant improvement when compared to pre-restoration conditions, and all restoration sites showed an increase in spawning activity. Over time, these sites are continuing to provide an optimal environment for salmonid spawning. However, the sites are changing and showing signs of an eventual return to pre-restoration conditions. The longevity of restoration projects is still unclear, but four years after augmentation the oldest site examined had the highest number of spawning salmonids seen at the site since the addition of gravel. Further monitoring is necessary to establish the duration of habitat improvement. Furthermore, usage of the sites by spawning salmonids is highly variable. To understand why salmon are choosing one site over another, it is important to continue monitoring sites and identify variables that predict fish use or good habitat.