1. The delivery, entrainment and deposition of inert fine sediments are among the most significant contributors to stream and river impairment worldwide. Associated ecological effects have been observed frequently, but specific experiments to identify sensitivity and avoidance behaviour in stream organisms are few, particularly in headwaters.
2. In a field-experiment, we added fine sand at low levels (c. 4–5 kg m−2) to 10 m reaches of two replicate headwater streams in the Usk catchment (Wales, U.K.) over two periods (autumn and summer). Upstream reaches were used as control in a classic before-after-control-impact design. Invertebrate drift and benthic composition were measured for 2 days before and 1 day after sediment impact.
3. Sediment addition significantly increased overall drift density (by 45%) and propensity (by 200%), with effects largest on the night following addition rather than immediately (i.e. within 9 h). The mayflies Baetis rhodani, B. muticus and Ecdyonurus spp., simuliid and chironomid dipterans, and helodid beetles were the strongest contributors.
4. There were no marked effects on benthic composition, but density declined in treated reaches by 30–60%, particularly in B. rhodani, Ecdyonurus spp. and Leuctra hippopus + L. moselyi.
5. All effects were consistent between both seasons and streams.
6. These data show how even low-level, short-term, increases in fine sediment loading to upland, stony streams can reduce overall benthic density through increased drift. We suggest that the likely cause of the delayed drift response was a change in habitat quality which prompted avoidance behaviour. Longer-term experiments are required to assess whether these effects reduce fitness or explain the losses of some types of organisms observed recently in sediment-impaired reaches of this and other catchments.