128-2 Progressive Downstream Modification of Quartz Sand Along the Chitina River, Alaska, Using Automated Grain Shape Analysis

Abstract:

Rivers shape landscapes through erosion and transport of sediment, with grain morphology a partial record of that history. Abrasion during fluvial transport alters grain morphology of gravel, sand grain shape remains less understood because morphological changes are subtle and require large datasets for reliable analysis. This study investigates the progressive shape modification of glacially derived quartz sand along the 188 km Chitina River, Alaska, using automated image analysis. Grain shape trends are compared with the Salmon River and Spain's Arlanzón River to evaluate the consistency of fluvial abrasion patterns across contrasting river systems.

Ten samples were analyzed from the Chitina River. Organic matter and magnetic minerals were removed, and quartz grains from two sand-size fractions (1.0–0.5 mm and 0.5–0.25 mm) were isolated using heavy-liquid separation. Across all samples, approximately 8,000 quartz grains were analyzed with a Malvern Morphologi G3 system to measure HS Circularity, Convexity, Solidity, and Aspect Ratio. Regression analysis identified which of these grain shape parameters were most sensitive to fluvial abrasion. The 1.0–0.5 mm fraction showed systematic downstream modification, with HS Circularity (R2 = 0.95), Convexity (R2 = 0.88), and Solidity (R2 = 0.80) increasing with transport distance, while Aspect Ratio remained unchanged. In contrast, the finer 0.5–0.25 mm fraction exhibited greater scatter and weaker relationships with transport distance potentially suggested more suspension transport of this grain size fraction. Consequently, the coarse fraction preserves the downstream abrasion signal more clearly than the fine-grained quartz population.

The proglacial Salmon River of British Columbia and Alaska shows rapid grain rounding, whereas the Chitina River exhibits gradual downstream changes potentially because of continuous glacial sediment replenishment. The Arlanzón River displays the highest overall Circularity, Convexity, and Solidity values, potentially reflecting the non-glacial character of this river. Across all three rivers, Aspect Ratio remains nearly constant with distance, suggesting grain elongation is largely source-controlled. Overall, Circularity, Convexity, and Solidity are more sensitive indicators of transport induced abrasion than Aspect Ratio.

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