Geomorphic river classifications based on different methods coincide in predicting fish assemblage structure

Abstract River classification is a necessary starting point for understanding river ecosystems and developing management guidelines. Using GIS to analyse an Andean river basin, we compare the application of two geomorphic classification methods at the segment scale: The Geomorphic Units Survey Classification System (GUS) and the Functional Process Zones (FPZ). Segment definition follows a manual procedure in GUS and a semi‐automated procedure in FPZ. Our objective was to assess the relationship between geomorphology and fish assemblages' structure. Fish sampling was carried out in collaboration with participants from local indigenous Mapuche‐Pewenche communities. Non‐parametric multivariate statistical analyses were conducted in order to quantify and describe geomorphic patterns and whether fish assemblages responded consistently with river classifications. Both classifications give insight into the physical characteristics of rivers, such as slope and floodplain width, that make habitat available for different fish assemblages. The two methods provide results that consistently coincide in their identification of geographic distribution and main geomorphic variables of different river types (geomorphic types) throughout the river network. The variation of different river geomorphologies was associated with variation in fish assemblage structure. Geomorphic variables that best characterize the distribution pattern of fish assemblages were elevation, confinement and valley floor width. Confined rivers accommodated highly similar fish assemblages dominated by invasive trout ( Salmo trutta and Oncorhynchus mykiss ). Unconfined rivers located at higher elevations had greater temporal variability and were mainly composed of trout and native catfish Trichomycterus areolatus . Semiconfined rivers presented the highest geomorphic variability and were associated with high spatial and temporal variability of fish assemblages characterized by both native and non‐native species. The association between fish assemblages and fluvial geomorphology could help in prioritizing rivers for exotic fish species control. These findings should aid in restoring highly intervened Andean rivers and improving management techniques in basins with a variety of human activities.