THE EFFECTS OF DEEP-SEATED LANDSLIDES ON THE WIDTH OF VALLEYS AND THE HABITAT FOR SALMON

A fundamental yet unresolved question in fluvial geomorphology is what controls the width of valley floors in mountainous terrain. Because the most productive habitats for salmon often occur in low gradient streams with broad valley floors, establishing a predictive relation for valley width is critical for realizing linkages between aquatic ecology and geomorphology. Understanding spatial variation in valley floor width is important because broad valley floors can accommodate sinuous channels, provide off-channel habitats during floods, and dampen episodic inputs of sediment by providing space for the formation of debris flow fans. Although hydraulic geometry relations for channel width exist in many settings, trends in valley width have not been well-characterized. For our study sites in the steep and densely forested central Oregon Coast Range, we used 10m DEMS and 1-m airborne lidar data to explore controls on valley width and couple these findings with previously published models of salmon habitat potential. The first step in this inquiry was to determine how valley floor width varies with drainage area in catchments that exhibit relatively uniform ridge-and-valley topography sculpted primarily by shallow landslides and debris flows. In this steep and highly dissected terrain, valley width varies as a power-law function of drainage area with an exponent of ~0.5. We used this area-valley width relation as a baseline for comparing how valley width varies in catchments subject to large deep-seated landslides. In this terrain, anomalously wide valleys tend to occur upstream of and adjacent to ancient large landslides while valleys downstream of slope failures were narrower than predicted from our power-law area-valley width relation. According to habitat potential models developed for Coho salmon, broad valley segments associated with deep-seated landsliding resulted in a large proportion of the channel network hosting productive habitat. Because landslides in this area are controlled by structural variations in the underlying Tyee Formation, our findings suggest a strong link between geologic properties and aquatic habitat realized by geomorphic processes.