Restoring fluvial forms and processes by gravel augmentation or bank erosion below dams: A systematic review of ecological responses

Aquatic biological communities have directly undergone human-induced changes. Altered hydrological and morphological processes in running waters have caused the degradation of main habitats for biotas and have disturbed ecosystem functionality. The latest advances in river restoration concerned the rise in far-reaching hydromorphological restoration actions that have been implemented below dams to reverse well-known negative impacts of anthropogenic pressures. Some authors emphasized the enhancement of sediment supply and habitat diversity using gravel augmentation or bank erosion to restore morphodynamics, and thus improve biodiversity. We explored the Web of Science database for empirical research papers that specifically addressed such hydromorphological river restoration actions. Articles were examined using a text-content analysis tool to determine the major concepts or ideas they deal with. It has also been proved as useful in defining interrelationships and degree of interdisciplinary. Results showed that a low number of published scientific articles exist about such projects, mainly condensed in the North hemisphere. Divergent ecological issues were highlighted by the word co-occurrence networks: (i) gravel augmentation was used to improve spawning habitats for fish of economic interest whereas (ii) erodible corridor was designed to safeguard natural riparian systems, approaching morphological goals of channel widening. Overall, ecological responses were consistent with those expected, leading however rather to functional shifts than richness increase. Gravel augmentation or bank erosion were not usually combined with in-channel structure management. However, this might be an option to consider since the biological communities seem to be sensitive during first restorations with such combination. This review demonstrates the value of word co-occurrence networks in exploring a high number of previous publications, keys for formulating guidance to manage gravel augmentation or bank erosion along ecological purposes.