LAUSR

In this paper we examine how changes in suspended sediment transport influence invertebrate drift in a piedmont river (the Upper Cinca, South Central Pyrenees). In-channel gravel mining in the Cinca causes the release of large amounts of fine sediment from the river bed, resulting in high suspended sediment concentrations (SSC) downstream. This in-channel gravel mining provided the occasion to assess the influence of increased SSCs on invertebrate drift during periods of constant flow hydraulics. Water and drift sampling was undertaken on six days (two events concurrent with river bed mining and four events pre and post mining activity periods), at five sampling sections distributed along a 5-km reach (one site upstream of the impacted area and four downstream). The magnitude of the SSC increment associated with mining was similar to that observed in natural floods. Drift was highly variable, reflecting natural spatio-temporal heterogeneity. However, mean drift rates increased appreciably during mining and showed a significant positive relationship with SSC. Ongoing work will assess changes in the river’s morphosedimentary dynamics resulting from mining activity, and the implication of these changes for benthic assemblages.