The Lasting Legacy of Megaflood Boulder Deposition in Mountain Rivers

Abstract

Infrequent, large‐magnitude discharge (>106 m3/s) outburst floods—megafloods—can play a major role in landscape evolution. Prehistoric glacial lake outburst megafloods transported and deposited large boulders (≥4 m), yet few studies consider their potential lasting impact on river processes and form. We use a numerical model, constrained by observed boulder size distributions, to investigate the fluvial response to boulder deposition by megaflooding in the Yarlung‐Siang River, eastern Himalaya. Results show that boulder deposition changes local channel steepness (ksn) up to ∼180% compared to simulations without boulder bars, introducing >100 meter‐scale knickpoints to the channel that can be sustained for >20 kyr. Simulations demonstrate that deposition of boulders in a single megaflood can have a greater influence on ksn than another common source of fluvial boulders: incision‐rate‐dependent delivery of boulders from hillslopes. Through widespread boulder deposition, megafloods leave a lasting legacy of channel disequilibrium that compounds over multiple floods and persists for millennia.

Sediment Cover Modulates Landscape Erosion Patterns and Channel Steepness in Layered Rocks: Insights From the SPACE Model

“…Thaler and Covington (2016) found that channel steepness in their study area was correlated with boulder size and percent boulder coverage rather than underlying bedrock lithology. Some field studies have also sought to quantify the influence of immobile cover from boulders using UAV surveys and satellite imagery (Carr et al, 2023;Morey et al, 2024;Shobe et al, 2020). They map the presence of large, immobile boulders in the channel and then apply different quantitative metrics, such as boulder concentration (Shobe et al, 2020) or the percentage of the channel area covered by these boulders (Carr et al, 2023).…”