199-6 Racing to Round(er): Relative Rates of Down Valley Clast Rounding in Fluvial and Lahar Deposits from Glacier Peak, WA

Session: Advances in Fluvial Processes and Sediment Transport, Part II

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Abstract:

In rivers, grain size governs channel morphology, sediment flux, and aquatic habitat. In debris flows, grain size influences runout distance and therefore downstream hazards. As these grains transport downstream, they abrade. While complexities of bed material abrasion in fluvial settings have been widely studied, the controls on abrasion in debris flows remain poorly constrained. There exists a tradeoff in which debris flows are both more energetic than fluvial systems, promoting grain wear, while also containing higher fines content, which has the potential to dampen grain-to-grain contact and thus suppress the frictional and collisional grain interactions that drive abrasion.  

In this study, we address a simple question: how does the relative rate of downstream rounding of dacitic clasts differ between those transported by lahars (volcanic debris flows) and those transported by fluvial processes? 

Lahar and fluvial deposits are preserved in the Suiattle-Sauk-Skagit drainage, a basin in the North Cascades of Washington State with well-documented non-cohesive lahars from the Glacier Peak stratovolcano. This system provides an ideal natural laboratory, with a well-constrained dacitic source isolated to the headwaters. We compare lithology and roundness across deposits of known transport mechanism, calculating 2D normalized isoperimetric ratios (IRn) for each sampled clast. 

Our findings demonstrate clear evidence of abrasion during debris flow transport, even in these fines-rich lahars. In debris flow deposits, we observe a general trend of increasing roundness and decreasing dacite content with increasing distance downstream. The rate of downstream rounding is greatest upstream and becomes more gradual in the lower river. In fluvial deposits, clasts round more efficiently in the upper drainage, however, dacite content becomes rapidly sparse with increasing distance downstream. These results suggest abrasion plays a significant role in the downstream evolution of grain size in debris flows, even in the presence of high fines content, though the rates of abrasion appear to be more gradual than those in fluvial systems.  

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

doi: 10.1130/abs/2025AM-10619

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