The Geomorphic Legacy of Megaflood-derived Boulders on River Processes and Form

The role of high-magnitude, low-frequency floods in shaping landscapes on Earth and other solar system bodies is an outstanding question in geomorphology. On Earth, glacial lake outburst floods, including exceptionally large megafloods (discharge ≥106 m3/s), carve canyons, mobilize large boulders, and initiate landslides, but the legacy of megaflood deposits on subsequent river evolution remains underexplored. Here, we present a numerical model that tests the impact of outsized (those that exceed the expected stream competence), megaflood-derived boulders on bedrock fluvial processes and form. We build on a previous 1D reach-scale model that uses a shear stress bedrock erosion rule to simulate the evolution of boulder-influenced bedrock rivers. This model accounts for two key effects of immobile boulders in the channel: bed armoring and hydraulic roughness/drag that reduces erosive stresses on the bed. We extend this model to simulate a full longitudinal river profile as it evolves after a megaflood-boulder depositional event. Model runs are inspired by the Yarlung-Siang River (YSR) in the eastern Himalaya, which hosts dozens of boulder bars that previous hydraulic simulations suggest were deposited during outburst floods. Simulated megaflood bed shear stresses can move boulders ≥4 meters in diameter that are immobile in subsequent, smaller discharge flows, and, thus, become semi-permanent fixtures in the modern landscape. We therefore modify the model to include large (≥4 m) megaflood-derived blocks, with sizes and longitudinal positions scaled to measurements of in-channel boulder bars along the YSR. The initial river profile is concave-up, has a constant channel width and uplift rate, and a monsoon influenced discharge distribution scaled to the YSR. We track bed elevation, erosion rate, and the number and size of boulders along the river profile. Results suggest that megaflood-derived boulders impede bedrock erosion, creating knickpoints where they are deposited. This causes a decrease in erosion rates in the upstream reaches of the river. The spatial distribution of megaflood-derived boulders likely influences average channel steepness in the river profile. These results suggest an important legacy of megaflood deposition on channel processes and form in highly erosive, steep mountain rivers.