Restoration using flow deflectors: a question of fine‐scale spatial layout in headwater stream beds

This is the authors' abstract. We don't add key points for this paper.

Current European Union objectives for 2030 aim to restore ecosystems to halt biodiversity loss. Here, we studied the consequences of headwater stream restoration by placing large wood in the streambed to act as flow deflectors. We hypothesized that these flow deflectors would create a fine‐scale spatial arrangement of habitats, fostering a diverse and patchy mosaic riverscape that supports the life cycles of a wide range of benthic macroinvertebrates. Our analysis of 300 correlations between biological and aquatic habitat metrics across 11 stations in the Northern Vosges Regional Natural Park, northeastern France, revealed 22 statistically significant relationships. The results demonstrated that habitat composition, patch complexity, and diversity of flow velocities were strongly correlated with biological outcomes and were largely influenced by flow deflectors in anthropic environments disturbed by stream rectification and the presence of invasive crayfish. Equally important is that these habitats form a coherent ecological pathway that maximizes species presence and movement. Despite positive outcomes, flow deflector restoration partially failed to meet expectations, as the resulting habitats deviated from the anticipated increase in coarse‐grained sediments, instead favoring organic substrates and leading to new mosaic configurations with larger patches and fewer edges. However, while the configuration was less optimal, the shift in composition introduced new functionalities that could support natural ecological processes.