Prioritizing Watersheds for Management: Quantifying the Impacts Fine-Grained Sediment on Salmon Egg Survival in Stream Ecosystems Impacted by Multiple Environmental Stressors
In Idaho, as well as other western states, watershed conditions and forest practices, such as timber extraction, road construction, grazing, land sliding and forest fires, significantly increase fine grain sediment into stream ecosystems. Differentiating natural from anthropogenic causes of erosion is difficult due to the punctuated nature and large-scales over which these process act. Moreover, assessing real impact compared to assumed impact to the stream ecology is difficult given the range of forest management and biotic conditions. Therefore, to effectively evaluate the impact of forest practices on stream ecosystem health, managers need effective tools that couple physical processes models of flow and sediment influx with the biota they aim to protect. Considering their economic and cultural importance, salmon survival, specifically
egg-alevin survival to emergence, is a strong indicator of a properly functioning watershed (Everest et al. 1987, Scrivener and Brownlee 1989, Montgomery et al. 1996). Although a single silver bullet fix probably does not exist, stream gravel conditions are thought to be a primary limiting factor in many streams due to their impact on spawning success, a critical life history stage for salmonids (Quinn 2005, Greig et al. 2007). Gravel conditions in spawning areas in North America are impacted both by increased fluxes of fine-grained sediment and the reduction of gravel downstream from dams (Phillips et al. 1975, Scrivener and Brownlee 1989, Kondolf 1997, Meyer et al. 2005). The watershed conditions under which fine-grained sediment limit salmon egg-alevin survival remains unclear due to the variable nature of watershed and biotic processes. While forest and road activities (timber
extraction, road building, surface maintenance and culvert condition) increase sediment entering the stream (Scrivener and Brownlee 1989, Jones et al. 2000), the impact of fine-grained sediment on egg survival, given the environmental condition, is not well mechanistically understood (Lisle 1989, Gottesfeld et al. 2004). That is, under some stream conditions (stream flow and channel gradient) high fluxes of sediment might have limited impact; while in other conditions, a small increase in fine-grained sediment could decrease egg survival markedly. Hence, researchers have suggested that a threshold might exist between fine-grained sediment accumulation and egg survival (Montgomery et al. 1996, Hassan et al. 2008). If this non-linear relationship exists, this would provide a valuable tool for identifying watersheds with high fine-grained sediment loads. In particular, researchers have
identified that sediment size distribution, which is altered by fine-grained sediment supply rates, is a strong predictor of egg-alevin survival (Cooper 1965, Koski 1966, Tappel and Bjornn 1983, Meyer et al. 2005). For this reason we propose a more detailed study of how sediment flux from surrounding catchments, watershed hydrology and redd building activities impact stream bed geomorphology. By quantifying the processes that transport sediment, we expect to be able to identify specific streams with sediment loads that impact salmon survival.