Most of the highly productive habitat for salmon occurs in low gradient streams with broad valleys, yet there is limited understanding of what controls the width of valleys in mountainous landscapes.
C. May and colleagues used high-resolution topographic data in the Oregon Coast Range to explore controls on valley width and couple these findings with models of salmon habitat potential.
The first step in this inquiry was to determine how valley floor width varies with drainage area in a catchment that exhibits relatively uniform ridge-and-valley topography sculpted primarily by shallow landslides and debris flows.
In this steep and highly dissected terrain, valleys get systematically wider as one moves downstream according to a simple mathematical equation. This drainage area-valley width relation was our baseline for comparing valley width in a neighboring catchment with large deep-seated landslides.
In this terrain, anomalously wide valleys tend to occur upstream of ancient large landslides. According to habitat potential models for coho salmon, broad valley segments associated with deep-seated landsliding result in a larger proportion of the channel network with the capacity to provide productive habitat.
Because landslides in this area are structurally controlled, our findings suggest a strong link between geologic properties and aquatic habitat realized by geomorphic processes.
Controls on valley width in mountainous landscapes: The role of landsliding and implications for salmonid habitat, C. May et al., Dept. of Biology, James Madison University, Harrisonburg, Virginia 22807, USA. Posted online 7 Feb. 2013; DOI:10.1130/G33979.1