We know little about the routes travelled by adult caddisflies when flying between streams, nor what proportion of the local species pool will disperse. For movement between major catchments (e.g. rivers with separate outflows into the marine environment), the shortest route may be to fly up one stream valley to the catchment boundary, and then down a valley into the adjacent catchment. If caddisflies do fly along these topographic features created by stream valleys, then we expect to find adults on the ridge between catchments and, if this dispersal is successful, high community similarity between streams on opposite sides of the boundary. In five pairs of streams that each share a catchment boundary, we sampled adult caddisflies at the streams margins (“resident spp.”) and on the boundary ridge (“boundary spp.”). From a species pool of >120 spp., approximately 50% were boundary species, and this group was taxonomically and morphologically diverse. Species composition did not differ between resident assemblages within stream pairs. Boundary species dominated resident assemblages in terms of species numbers and overall abundance of individuals. However, boundary species were not just those that were the most common within assemblages. Dispersal was sex-biased for only a few species, but not of a direction or magnitude likely to constrain demographic connectivity. Overall, this evidence suggests that there is extensive movement of diverse caddisflies over catchment boundaries, and to our knowledge has not been shown before for such a wide range of different species. This dispersal could ensure connectivity between communities and facilitate recovery of populations after local extinctions.