Large wood and sediment storage in a mixed bedrock-alluvial stream, western Montana, USA

Abstract

Sediment storage by instream wood in forested mountain streams mediates sediment movement from hillslopes through the channel network and can alter channel morphology at multiple spatial scales. Mixed bedrock-alluvial channels are prevalent in mountain stream networks, yet the distribution and geomorphic impact of large wood within these streams are poorly understood. To estimate how the distribution of large wood in a mixed bedrock-alluvial stream relates to sediment storage, we measured and characterized large wood, and we surveyed the volume of associated sediment within a stream in the Bitterroot Mountains, Montana. The upstream portion of the study reach is predominantly alluvial and the downstream portion has significant bedrock exposure along the channel bed and banks. Wood volume and sediment storage in the mixed bedrock-alluvial subreach are 50% and 15%, respectively, of those measured in the alluvial subreach. Most wood is organized into jams, and two channel-spanning jams within the upstream subreach account for 50% and 80% of the reach-averaged wood and sediment volume, respectively. The volume of sediment stored by wood in the full reach is the same order of magnitude as the estimated annual bedload export. Our study highlights the interactions among channel form and the fluxes and storage of wood and sediment. Wood that forms channel-spanning jams, which store a disproportionate amount of wood and sediment in the study reach, may accentuate differences between alluvial and bedrock-dominated subreaches in mountain stream networks, by promoting sediment deposition and storage upstream and by reducing sediment supply to downstream reaches.