276-7 From rock strength to river response: Bed material abrasion controls downstream cascading hazards from crumbling volcanoes

Session: Natural and Urban Channel Responses to Disturbance, Management, and Restoration

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Abstract:

Large sediment pulses from stratovolcanoes are an increasingly frequent hazard, as climate change-induced thawing of rock permafrost accelerates catastrophic alpine mass wasting events. Beyond the initial impact, the vast quantities of sediment deposited in river channels can trigger cascading hazards, including downstream channel aggradation and increased flood risk. However, geomorphologists have yet to construct a comprehensive framework for predicting the magnitude and timing of these fluvial responses. We suggest that bed material abrasion of potentially weak volcanic sediment is a key control on these downstream impacts.

To investigate this, we first use the Landlab Network Sediment Transporter to model the downstream evolution of a volcanic sediment pulse in the Suiattle River, Washington. The model incorporates sediment heterogeneity in grain size, density, and abrasion potential. Results indicate that while higher sediment abrasion rates produce only subtle changes in bed elevation, they drive a distinct wave of bed material fining. From a hazard perspective, this reveals a critical tradeoff: rapid abrasion may reduce flood risk to downstream communities by breaking coarse sediment into easily transportable sand and wash load, but it concurrently degrades aquatic habitat through excessive bed fining proximally and silting estuaries distally. Second, we analyze field data from five catastrophic mass wasting events at stratovolcanoes across the Pacific Northwest. This regional assessment reveals high variability in the initial rock strength, and therefore abrasion potential, of the mass wasting deposits. The primary controls on rock strength are lithological, with rock vesicularity and the degree of hydrothermal alteration being the most significant factors.

By combining our modeling and field observations, we demonstrate that the downstream propagation of hazards from volcanic sediment pulses is governed in part by the lithology of the source material. This finding has broad implications, as highly abradable lithologies are not unique to volcanic settings; many common sedimentary rocks, for example, are equally or even more susceptible to abrasion. Ultimately, our work shows that forecasting large-scale landscape response to major disturbances will require a framework that integrates lithology and its fundamental control on bed material abrasion and grain size.

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

doi: 10.1130/abs/2025AM-9228

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