The lasting legacy of megaflood boulder deposition in mountain rivers

Abstract

Infrequent, large-magnitude discharge (>10 6 m 3/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.

Citation

Morey, S. M.; Shobe, C. M.; Huntington, K. W.; Lang, K. A.; Johnson, A. G.; Duvall, A. R. 2024. The lasting legacy of megaflood boulder deposition in mountain rivers. Geophysical Research Letters. 51: e2023GL105066.