1. We hypothesised that increased flow in the main stem of the River Rhine would influence the invertebrate communities of its tributaries and therefore investigated the invertebrate fauna of six tributaries over 2 years.
2. We collected quantitative invertebrate samples at three sites in each tributary: in the tributary mouth (influenced by Rhine water whenever flow in the Rhine exceeded mean annual level), in the zone reached by average floods (return period 1.5 years) in the Rhine (average flood level sites), and immediately upstream of the range of extreme Rhine floods (reference sites). Samples were taken in spring, summer and autumn of each year, at different flow levels of the Rhine. We also compared substratum composition at the three sites.
3. Tributary mouth sites had the finest substratum, the lowest total invertebrate density and the lowest taxon richness. At average flood level and reference sites, these three parameters were similar.
4. Taxa known to prefer larger rivers were mostly confined to the tributary mouth sites, and species preferring upland streams dominated at the average flood level and reference sites.
5. Multivariate analyses confirmed the influence of the Rhine on the tributary mouth sites. Invasive invertebrate species, which usually appear only in the Rhine itself, were found at the tributary mouth sites but not further up in the tributaries.
6. Our study shows that increased flow in the main stem of the Rhine influenced substratum composition and invertebrate communities at the tributary mouth sites. These results imply that the relationship between the main stem of a river and its tributaries is not one-way (from tributary to main stem), but rather a two-way interaction.