Animal, vegetable or mineral? The relative contributions of hydraulic and animal-mediated transport to bedload in a gravel-bed stream

Measuring and predicting sediment transport is a fundamental objective of geomorphology, yet research in rivers focusses almost exclusively on the transport of mineral sediment by water. In reality, material transported over river beds also includes organic sediment and is transported both hydraulically and through the actions of animals. Caddisfly (Trichoptera) are a widespread aquatic insect group which transport sediment over river beds in the form of case bioconstructions. While mineral sediment typically requires high discharge to mobilise, organic material and animal transported sediment may occur under any flow, potentially accounting for a significant proportion of total river bedload. Here, we measure bedload transport for a UK river and partition the contributions of mineral grains, organic materials and case-building caddisfly. Sampling was conducted every two months for a year and captured low-medium discharge conditions.

For particles >2 mm, organic material contributed approximately 50% of total bedload transport, with strong seasonal variation, peaking in autumn. Finer size fractions were dominated by mineral sediment. Caddisfly-transported sediment consisted primarily of medium to coarse sand (D₅₀ = 0.91 mm) and accounted, on average, for 30% of bedload transport of this size fraction, equivalent to 1.27 g m⁻¹ day⁻¹ (465 g m⁻¹ yr⁻¹). Caddisfly contributions were especially important during very low flows when hydraulic sediment transport was minimal. These results demonstrate that invertebrate activity can play a significant role in bedload transport, particularly under low-flow conditions, by altering both the magnitude and grain-size distribution of transported sediment. Our findings highlight the importance of explicitly incorporating biological processes into our understanding of sediment dynamics in rivers.