LATERAL RESPONSE OF A GRAVEL-BED BRAIDED RIVER TO A SEDIMENT PULSE GENERATED BY A LARGE LANDSLIDE

The downstream routing of clastic material episodically delivered to a channel network by various landscape disturbances may lead to profound changes in the behavior and morphology of the fluvial system. The geomorphic response to such sediment pulses is of great scientific and practical interest because it can disrupt the associated freshwater and riparian ecosystems and exacerbate natural hazards for humans living along the river corridor.

Although much progress has been made in understanding the vertical and textural adjustments of river channels to the passage of sediment pulses, less is known about their lateral responses. This study investigates the lateral response of the gravel-bed braided Lillooet River, southeastern British Columbia, Canada, to the downstream progression of a landslide-generated sediment pulse. In 2010, a catastrophic landslide released ~50 Mm3 of sediment into the upper Lillooet Valley. To understand the adjustment of the river, we analyzed PlanetScope and RapidEye satellite imagery (resolution of 3-5 m) and examined interannual changes in basic metrics of channel planform morphology and behavior along a >30km study reach during a period of 12 years (2009-2021). Although the dynamic nature of the Lillooet River and its complex morphology pose a challenge for detecting the signal of disturbance, our preliminary results reveal a marked response in active channel width. The time since disturbance and the sequencing of floods determine the temporal pattern of response. Mean active channel width changes (𝞓MACW) doubled during the year following the landslide, then declined exponentially, returning to pre-landslide conditions within a decade. Variations around this trend, indicating a gradual depletion of the readily mobilized supply, were controlled by annual peak flow magnitude. The longitudinal pattern of 𝞓MACW suggests that the downstream evolution of the sediment pulse was also strongly affected by valley morphology and localized avulsions. This research provides insights into how lateral channel dynamics in active, mountain rivers respond to low frequency, high magnitude, sediment supply events, and yields knowledge that benefits river management in landslide-prone mountain landscapes.