128-3 Grain shape trends of quartz sand from the Salmon River, British Columbia and Alaska

Abstract:

The Salmon River is a short (26.7 km) pro-glacial stream located along the Alaska–British Columbia border that experiences seasonal jökulhlaups capable of generating abrupt increases in discharge. These high-magnitude flow events may accelerate grain rounding and surface chipping over relatively short transport distances compared to sustained moderate-flow conditions. Although downstream changes in gravel roundness have been extensively documented, comparable studies of quartz sand are limited because it is generally thought that water viscosity limits grain-to-grain contact, reducing abrasion making quantitative research challenging. Automated grain-shape analysis provide an opportunity to quantify these morphological changes with greater precision.

To investigate downstream evolution of sand-grain morphology, quartz sand samples were collected along the length of the Salmon River and analyzed using the Malvern Morphologi G3 automated imaging system. Using 0.5-1mm and 0.25-0.5mm sized grains, hundreds of grains from each sample were measured to quantify circularity, convexity, aspect ratio, and solidity. Trends of these parameters indicate that the greatest increase in grain shape change occurs within the first several kilometers of transport, after which the rate of morphological change decreases substantially.

The observed downstream trends suggest that the largest grain shape modifications occur rapidly following entrainment from glacial sources, with progressively smaller changes occurring as transport continues. This stretch of river is also the steepest, which is likely to increase turbulence and thus grain abrasion. These results are consistent with other investigations of quartz grain modification in fluvial systems, including rivers in central Spain, indicating that similar controls on sand grain rounding may operate despite substantial differences in fluvial geomorphic setting. Together, these findings demonstrate that automated grain shape metrics provide a useful quantitative proxy for sediment transport history while highlighting the need for additional studies across a wider range of fluvial environmental settings.

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