Structural Control of Bedrock Canyon Alignment and Morphology along the Fraser River, British Columbia, Canada

Bedrock rivers work to incise terrain down to base level. Their geomorphic work is accomplished by the transfer of momentum from flow to particles that strike the bed and walls, or, more effectively, to pluck fracture-bound blocks from the boundary. Through this latter process, bedrock fractures can control bedrock river morphology at the reach scale by dictating local vertical and lateral incision rates. At the watershed scale, regional structure patterns partly control channel alignment by providing fractured rock along discontinuities where incision rates are enhanced. We investigated the structural controls on the alignment of bedrock canyons along the Fraser River, British Columbia, Canada, and controls on the reach-scale morphology of one of its major canyons. We used topographic data and optical imagery derived from multibeam echo-soundings, drones, airborne LiDAR, and satellite sensors. We produced a digital elevation model that includes bedrock walls both above and below the water surface. Our analysis of these data shows that the canyon geometry of the Fraser River is controlled by a combination of large landslides and faults and reveals that epigenetic gorges created by landslides have distinct geometries and morphologies from those canyons associated with faults. Kinematic rock slope analysis of the walls of one major canyon reveals that reach-scale geometry is partly controlled by the orientation of the major joint sets. Vertical and horizontal joints appear to dictate the style of rock erosion and the propensity for abrasion, undercutting, and plucking, and through this control the spatial distribution of constrictions and pools.