From habitat heterogeneity to flood refugia: linking flow intensity and morphology to inform river management

Floods bring pulse and rupture to river ecosystems. In near-natural rivers with high habitat heterogeneity, some habitats buffer harsh conditions during floods and serve as refugia for aquatic organisms, sustaining river resilience. Despite their crucial ecological role, refugia have been poorly studied and barely considered in the restoration of modified rivers with severely reduced habitat heterogeneity. This highly replicated study assessed how flow intensity influences habitat heterogeneity and refugia availability for macrophytes and macroinvertebrates in river reaches with varying levels of human modification. Hydraulic field data was combined with hydrodynamic modelling across three reach morphologies (heavily modified, slightly modified, restored) in ten Swiss rivers. For the 30 reaches investigated, habitat heterogeneity in flow velocity and bed shear stress as well as refugia availability were simulated at mean flow (Q_M) and six flood intensities (HQ₁-HQ₁₀₀). Habitat heterogeneity in flow velocity and bed shear stress was significantly higher in restored compared to heavily modified reaches across all flow intensities (+17% and +34%, respectively). Across the three reach morphologies, increasing flow intensity significantly reduced habitat heterogeneity in flow velocity (-45%) and in bed shear stress (-43%) averaged from Q_M to HQ₁₀₀. Refugia availability for macrophytes declined by 29 percentage points and for macroinvertebrates by 17 percentage points from HQ₁ to HQ₁₀₀. Even at high flood intensities (5-year floods and beyond), refugia were available in restored and slightly modified reaches, e.g., along riverbanks. Refugia availability was positively correlated with habitat heterogeneity. This study highlights the potential of river restoration to foster the resilience of modified river ecosystems by promoting refugia availability through increased habitat heterogeneity. The results suggest that floods of varying intensities should be explicitly considered in restoration planning based on the expectation that disturbances such as floods are likely to increase in the face of climate change.