267-3 After the Flood: The Impacts of Megaflood Deposition on River Morphology in the Eastern Himalayas

Session: Advances in Fluvial Processes and Sediment Transport (Posters)

Poster Booth No.: 165

Presenting Author:

Gabriel Santana

Authors:

Santana, Gabriel G.¹, Huntington, Katharine W.², Shobe, Charles³, Lang, Karl A.⁴, Duvall, Alison R.⁵, Montgomery, David R.⁶

(1) Earth and Space Sciences, University of Washington, Seattle, WA, USA, (2) U Washington, Seattle, WA, USA, (3) U.S. Forest Service Rocky Mountain Research Station, Fort Collins, CO, USA; Department of Geosciences, Colorado State University, Fort Collins, CO, USA, (4) Georgia Tech, Atlanta, GA, USA, (5) University of Washington, Seattle, WA, USA, (6) University of Washington, Dept Earth & Space Sciences, Seattle, WA, USA,

Abstract:

Catastrophic failures of landslide or glacial dams can release extreme discharge floods (10³-10⁶ m³/s) through steep mountain valleys. Although most well known for their erosive power, field observations and modeling show that these outburst floods also transport and deposit vast volumes of coarse and fine sediment. Despite their significance, the lasting impacts of these floods on valley and channel morphology remain largely unexplored, and are often overlooked in predictive models of intermontane bedrock river evolution. Here, we use river morphometric analysis to assess the geomorphic legacy of glacial lake outburst megaflood deposition along the Yarlung-Siang River (Eastern Himalaya), focusing on 1) how post-megaflood deposits modify bedrock channel and valley processes, and 2) what signatures of megaflood-related processes are preserved in tectonically active, rapidly eroding terrain. 

We analyzed topographic differences between the Yarlung-Siang River–which experienced numerous Quaternary megafloods–and the neighboring Subansiri River, which lacks evidence of such events. We mapped knickpoints and channel steepness and compared their distributions to lithologic boundaries and previously mapped megaflood boulder bars to test two hypotheses. First, process-based models of channel evolution predict that megaflood boulder deposits produce steepened, stepped channel profiles. In the lower Siang River, knickpoints align with mapped megaflood boulder bars, consistent with model predictions. In contrast, the Subansiri River–unaffected by megafloods–shows fewer stepped reaches, with knickpoints primarily associated with lithologic boundaries. These preliminary findings support the hypothesis that boulder armoring of the channel bed reduces incision efficiency, disrupting the balance with rock uplift and leading to stationary knickpoints and localized channel steepening. Second, we test the hypothesis that in megaflood-affected basins, flushing of hillslope-derived sediment triggers widespread baselevel fall along the mainstem channel, initiating tributary knickpoints that propagate upstream at consistent elevations above tributary junctions. In contrast, in the megaflood-free basin, we expect knickpoints to reflect lithologic boundaries or baselevel fall driven by differential rock uplift across rangefront faults. Ongoing work including valley width and cross-valley topographic analysis, terrace mapping, and field observations aims to evaluate the role of flood-derived sand and gravel deposition in shaping post-flood channel dynamics and further test proposed  impacts of megaflood-derived sand and gravel deposition. Accounting for both megaflood erosion and sedimentation may better link channel form and process in the Eastern Himalaya and improve landscape evolution models for intermontane rivers.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7947

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