WOOD AND VEGETATION EFFECTS ON SEDIMENT CONNECTIVITY IN THE BITTERROOT RIVER BASIN, MONTANA

The movement of sediment from hillslopes to channels and downstream through river networks in mountain watersheds influences landscape evolution, ecosystem function, and resource management. Sediment routing depends on not only abiotic controls such as slope, but also on biotic influences. We investigate the role of vegetation and instream wood on sediment connectivity in watersheds with varying topography and climate history, within a broader semiarid, snowmelt-dominated, and fire-prone watershed, the Bitterroot River basin in western Montana. With respect to how instream wood influences sediment connectivity, we found that the geomorphic impact and influence of wood on sediment storage varies substantially by channel type. Wood volume and sediment storage in a mixed bedrock-alluvial study reach were 50^% and 15^%, respectively, of those measured in an adjacent alluvial reach. Most wood in these reaches was organized into jams, and 50^% and 80^% of the reach-averaged wood and sediment volume, respectively, was concentrated in two channel-spanning jams. We also showed that on burned hillslopes, downed coarse woody debris can store significant sediment volumes, mediating post-fire sediment delivery to the fluvial system. On hillslopes draining to the instream-wood study reaches, we documented greater soil cover and maximum soil thickness in forested versus non-forested sample sites, providing another line of evidence for how biota influences sediment storage and transfer in steep landscapes. The ecogeomorphic perspective on sediment connectivity provided by this work is relevant to management and restoration in forested, fire-prone mountain landscapes; assessment of vulnerability to sediment-related disturbances and climate change; and aquatic habitat in mountain streams.