Influence of Large Wood on Sediment Routing in a Mixed Bedrock-Alluvial Stream

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, with implications for fish habitat, flooding, and carbon storage. Transitional or mixed bedrock-alluvial channels represent important portions of mountain stream networks, yet the distribution and geomorphic impact of large wood within these streams are poorly understood. To determine how the distribution of large wood in a mixed bedrock-alluvial stream relates to sediment storage, we measured and characterized large wood, and surveyed the volume of associated sediment within a representative stream reach in the Bitterroot Mountains, Montana. The upstream portion of the study reach is predominantly alluvial and has an average bed slope of 0.0097 and average bankfull shear stress of 34 N/m². The downstream portion has significant bedrock exposure along the channel bed and banks and has an average bed slope of 0.031 and average bankfull shear stress of 180 N/m². This subreach contains 1.0*10² and 4.4 m³ of wood and sediment, respectively, per hectare of channel area, compared to 220 and 11 m³/ha of wood and sediment, respectively, in the alluvial subreach. At the reach scale, we measured 56 m³ of sediment stored by wood, which is an order of magnitude less than annual bedload transport of 600 m³, as estimated using Parker's bedload transport model. This finding contrasts with previous studies of instream wood which document storage between zero and 10 times greater than mean annual sediment yield. Together, these results suggest that even as wood may significantly alter local hydraulics and sediment transport, the geomorphic impact and influence of wood on sediment storage may vary substantially by channel type. Moreover, wood may have only a modest effect on sediment routing within mixed bedrock-alluvial channels, although its role in shaping aquatic habitat may be disproportionate given the overall paucity of sediment in such channels.