Given the importance of spatial heterogeneity in altering dispersal, interspecific interactions, and population persistence, high rates of habitat homogenisation across the globe are a concern. In river networks, confluences and water abstractions likely produce discontinuities in physical conditions, potentially creating hotspots of heterogeneity that influence fish assemblages, including interactions between native and invasive fish. However, mechanisms driving fish assemblage responses to such spatial heterogeneity are not well understood. I investigated how the spatial configuration of flow disturbance around confluences influenced spatial and temporal patterns in fish assemblages, and evaluated the impact of spatial heterogeneity associated with surface water abstractions on fish assemblage structure.

Electrofishing in mainstem and tributary branches of replicate confluences revealed highly context-dependent distributions, contingent upon the combination of flood disturbance history in branches, distance to the confluence and the direction of flow. Shifts in native–invasive species relative abundance were determined by preference of large predatory salmonids for more hydrologically stable conditions, resulting in higher native fish abundance in flood-prone conditions. Distance from confluence effects were stronger upstream than downstream, suggesting flow direction had influenced dispersal. Heterogeneity-related dynamics in fish assemblages meant confluences with homogenous flow conditions had lower fish abundance and higher temporal variability in the relative abundance of native and invasive species than confluences with heterogeneous flow conditions. Evenness scores differed downstream of confluences, with higher assemblage evenness downstream of heterogeneous confluences. These results demonstrate how flow- regime effects are spatially transferred at confluences, creating areas of influential riverscape heterogeneity.

Field surveys and stable isotope analysis revealed reduced flows downstream of abstraction points resulted in significantly lower fish abundances per metre of stream length, higher proportions of native species, shallower mass-abundance relationship slopes and shorter stable isotope-derived food-chain lengths due to the loss of larger fish sizes. Spatial heterogeneity in stream flow associated with water abstractions can therefore alter both the structure and composition of stream fish assemblages, and modify the outcomes of native–invasive species interactions. Overall, my research indicates understanding the role of heterogeneity will assist in identifying locations of ecological significance in river networks and improve the strategic management of fish assemblages, especially in invaded riverscapes.