The taxonomic richness erosion and the role of tributaries in the maintenance of the taxonomic richness were considered in a Mediterranean catchment in southeastern France. Nine stations were chosen along the Arc stream (three stations downstream from an organic effluent and one station upstream from the pollution source) and on two groups of tributaries (three intermittent and two perennial). High biodiversity erosion was noticed in the main stem, revealing diffuse sources of pollution added to the expected effect of the localized organic pollution. Jackknife richness estimator and beta diversity indicated that the intermittent tributaries had the highest richness values and harboured 70% of the taxa recorded at the catchment scale. The intermittent flow tributaries seem to play a major role in maintaining the taxonomic richness in such catchments, highly impacted by anthropogenic activities. The detailed examination and the preservation of these ecosystems should be an important step in catchment management, and support the need for catchment-scale conservation of freshwater invertebrates.