River basins in the Northwestern U.S. are shaped by a variety of natural processes and disturbances operating across multiple temporal and spatial scales. Snowmelt and peak flows, rain-on-snow events, earthquakes, avalanches, windstorms, wildfires, rainstorms, landslides, and debris flows create and alter aquatic habitat complexity, diversity, and connectivity. To persist in such dynamic environments, salmon and other species have developed multiple life history strategies that utilize diverse habitats across time and space. Because human activities have altered disturbance regimes in many watersheds, our ability to examine natural processes and species responses has been compromised. Our objective was to examine wild Chinook salmon responses to natural disturbances in the Middle Fork Salmon River, Idaho (MFSR); a large wilderness basin where natural processes function relatively unimpeded by humans. Since 1990, fires have burned >53% of the MFSR basin. Post-fire rainstorms and rain-on-snow events created debris flows and avalanches that recruited wood, carbon, nutrients, and sediment to streams. Since 2003, we have geo-referenced debris flows and log jams in reaches of Pistol Creek and the mainstem MFSR. During these 15 years, we also geo-referenced all Chinook salmon redds. We merge these datasets and apply them to improve understanding of the landscape and biophysical processes that influence aquatic habitat and salmon distributions. Field data are integrated with sediment routing models to explore the consequences of these dynamic processes for salmon habitat.
Results/Conclusions
In this paper, we describe the mechanisms of debris flow creation and sediment routing and illustrate temporal and spatial responses by spawning Chinook salmon to natural processes and habitat disturbances. Results illustrate that spawning salmon rapidly colonized gravels replenished by post-fire debris flows and avalanches. Our observations emphasize the importance of salmon dispersal and habitat connectivity. We also discuss how a changing climate may alter natural landscape dynamics. Warming temperatures are expected to increase fire frequency and subsequent debris flows and increased rain-on-snow events may cause more frequent avalanches. Inspection of sedimentary deposits and large-scale stream and basin morphology confirms that fires and debris flows have shaped this landscape for millennia by altering channel gradient, stream width, and associated salmon habitats. Consequently, although these disturbance processes are not new, geomorphically or biologically, their frequency and spatial extent are likely being altered by climate change. Although salmon have evolved with these disturbance processes, a critical question remains whether adaptation of salmon will keep pace with accelerated rates of climate change and associated disturbance regimes.