49-6 Episodic Erosion and Deposition in Mountain Rivers: Outburst Floods as a Mediator of Climate-Tectonic Interactions

Session: Exploring Feedbacks Between Tectonics and Climate on Lithospheric Evolution Using Multidisciplinary Approaches

Presenting Author:

Katharine Huntington

Authors:

Huntington, Katharine W.¹, Lang, Karl², Shobe, Charles M.³, Goncalves Santana, Gabriel⁴, Nath, Srinanda⁵, Morey, Susannah M.⁶, Montgomery, David R.⁷, Duvall, Alison⁸

(1) U Washington, Seattle, WA, USA, (2) Georgia Tech, Atlanta, GA, USA, (3) USFS Rocky Mountain Research Station, Fort Collins, CO, USA, (4) University of Washington - Seattle, WA, Seattle, WA, USA, (5) Georgia Tech, Atlanta, GA, USA, (6) Vanderbilty University, Nashville, TN, USA, (7) University of Washington, Dept Earth & Space Sciences, Seattle, WA, USA, (8) University of Washington, Seattle, WA, USA,

Abstract:

Bedrock river incision shapes the topographic evolution of actively uplifting mountain landscapes and mediates climate-tectonic interactions. The Eastern Himalaya, an iconic region of rapid rock uplift, extreme relief, and proposed surface–lithosphere coupling, has experienced episodic, high-magnitude outburst floods throughout the Quaternary with potentially profound impacts on river-hillslope dynamics. These extreme discharge events—from modern glacial and landslide-dam outburst floods (10⁴–10⁵ m³/s) to prehistoric glacial lake outburst megafloods (10⁶ m³/s)—have been proposed as a key mechanism for evacuating hillslope-derived sediment and enabling bedrock river incision to keep pace with rapid uplift. Yet these floods also deposit vast volumes of sediment, raising fundamental questions about how episodic deposition influences bedrock river incision across the timescales relevant to landscape evolution. 

We integrate observations of flood deposits, hydraulic simulations, geomorphic process models, and topographic analysis to evaluate the impacts of extreme floods and their aftermath in steep, actively uplifting mountain river systems. The Siang River drainage preserves a rich record of Quaternary megafloods. Sedimentologic, stratigraphic, provenance, and geochronologic analyses reveal distinctive flood facies—including megaflood boulder (>5 m) bars, comminution gravels, slackwater silts and sands, and peat horizons—reflecting flood hydraulics and paleo-flood sources in southeastern Tibet. Geomorphic models of channel profile evolution suggest that megaflood boulder deposition creates steeper, stepped mainstem channels—a prediction supported by comparisons of channel steepness and knickpoint distribution between the megaflood-affected Siang River and an adjacent drainage with no known megaflood history. Ongoing work extends fluid flow and sediment transport modeling to explore post-flood river dynamics, including variable channel width, transport of both boulders and finer sediment, and potential feedbacks between megaflood fine sediment deposition and terrace formation. Together, our observations and modeling highlight not only the erosive power of megafloods but also the enduring influence of flood deposits on river form and process. These deposits can drive persistent channel disequilibrium that compounds over multiple flood events, challenging assumptions in stream power-based models of river incision. In active mountain landscapes, uplift promotes the formation of dammed lakes and outburst floods—making these events not anomalies, but fundamental agents of erosion and deposition. Their geomorphic legacy highlights the need to consider episodic sedimentation alongside discharge variability in landscape evolution models that explore climate-tectonic-surface process interactions.

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

doi: 10.1130/abs/2025AM-10922

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