Abstract Acid mine drainage (AMD) causes different responses in riverine benthic macroinvertebrate communities than that caused by organic pollution. The response is similar to that for metal toxicity and acidity where the impact is severe, or for inert solids where the impact is moderate to mild. Biotic indices are based on saprobity and so do not accurately reflect community disturbance for either toxicity or inert solids and thus cannot be considered as reliable indicators for AMD. The expected community response to both toxicity and inert solids is best described simply in terms of suppression of both taxon richness ( S) and abundance ( n) regardless of saprobity. A simple model (AMD′) is proposed that provides a precise and reliable metric of the effects of AMD in rivers.