VALLEY ARCHITECTURE CONTROLS ON HILLSLOPE-CHANNEL CONNECTIVITY IN THE FRASER CANYON

In steep landscapes, hillslope mass wasting can have an outsized impact on long-term landscape evolution and short-term hazards. However, the impact of mass wasting on fluvial processes is limited by the connectivity of sediment production on the hillslope and the fluvial network. Characterizing how connectivity varies within landscapes is particularly challenging within landscapes resulting from complex histories of both glacial and fluvial processes, as hillslope-channel connectivity is disrupted by glaciofluvial terraces, hanging tributary valleys, and local inner gorges.

Here, we investigate how hillslope morphology, valley architecture, and connectivity vary across the Fraser Canyon in British Columbia using a new suite of high-resolution airborne LiDAR. The Fraser Canyon is a 375-kilometer reach of the Fraser River, with a valley that is relatively confined between the Coast Mountains and Cascade Mountains. The Fraser Canyon has a regional gradient in valley morphology from upstream to downstream, varying from comparatively wide valleys with abundant terraces upstream to tight valleys with minimal sediment storage downstream. We describe hillslope morphology by mapping the distribution of bedrock exposure, identifying the relief and slope of these exposed cliffs, and distinguishing how relief is partitioned within the hillslopes. We characterize valley width across three different process domains: 1) the near channel floodplain, which determines local hillslope inputs to the channel, 2) the terraced extent of the valley, which separates the channel from bedrock hillslopes, and 3) the width between adjacent ridgelines which defines the region of water and sediment input to the Fraser mainstem. Preliminary results indicate that both the degree of bedrock exposure and the proximity of cliffs to the channel generally increases from upstream to downstream. While the river has high connectivity to steep slopes throughout most of the downstream region, zones of high connectivity in the upstream region are primarily localized to regions where either the valley locally narrows, or the river abuts the valley wall. This suggests that different processes can control connectivity between mass wasting on the hillslopes and fluvial networks even within a single landscape.