SPATIAL SCALES OF JAM-INDUCED CHANNEL BIFURCATIONS

Physical heterogeneity caused by downed wood in river channels drives diverse river corridor morphology across spatiotemporal scales. An extensive literature describes wood in active channels yet we lack quantitative underpinning toward understanding the role of jams in mediating channel adjustments and bifurcations. Logjams can induce lateral or downstream bifurcations. As a logjam forces overbank flow and bank erosion, multithread reaches can form via lateral bifurcation. A channel-spanning logjam can also create lee deposition that is then stabilized by woody vegetation, forming a relatively short segment of split flow via downstream bifurcation. We surveyed logjams and bifurcations along four rivers of varying size within the montane and mesic montane zone of the Rocky Mountains - the Swan River and two of its tributaries in Montana and North Saint Vrain Creek in Colorado. Preliminary results suggest that tributaries to the Swan River primarily experience local downstream bifurcation because of limited transport capacity for large wood. The larger Swan River experiences lateral bifurcation over greater lengths because of its greater erosive energy and transport capacity for large wood. North Saint Vrain Creek experiences lateral bifurcation due to alternating reaches of strings (relatively steep, narrow river segments with high transport capacity for LW and minimal bank erodibility) and beads (retention zones along wider, lower gradient segments of the river with more erodible banks and lower wood transport capacity). We expect that interactions among wood transport capacity, bank erodibility, key piece size relative to channel geometry, and the proportion of flow obstructed by a jam strongly influence channel bifurcations in forested river corridors. Quantifying the drivers of jam-induced channel bifurcations at different spatial scales has important implications for understanding the mechanisms of wood-induced heterogeneity that drives the evolution of channel form at varying scales in forested river corridors.