Dismantling artificial levees and channel revetments promotes channel widening and regeneration of riparian vegetation over long river segments

Abstract Longitudinal structures manipulation can re-activate channel migration and thus restore flood-dependent riparian plant communities in human-constrained floodplains. However, it has been rarely implemented over long restored river segments and has been infrequently assessed while taking into account river conditions prior to restoration. This study describes the morphological and vegetation response to this type of restoration in a project completed in 2012 along a 21.6 km river segment in the Orbigo River (NW Spain). Land cover changes and channel planform evolution in the restored segment were compared with a downstream non-restored (control) segment and to an upstream unregulated (reference) segment before (2011) and shortly after (2014) the restoration implementation. Riparian vegetation was surveyed in 18 gravel bars of the three river segments four years after restoration completion (2016). The restored segment presented the largest increase of active channel area. Rejuvenation of landforms predominated over transition toward mature stages (succession) in the restored and the reference segment, while succession predominated in the control segment. The sinuosity and braiding indexes in the restored segment increased much more than in the reference and, especially, than in the control segment. Riparian plant communities that colonized gravel bars in the restored segment resembled those found in the unregulated segment and slightly differed from that found in the non-restored segment. Within-segment variability was much higher, indicating the dependence of riparian plant communities on local processes. Although positive, our results showed that the high stability of floodplain areas in the human-constrained rivers of industrialized societies limits the short-term effectiveness of longitudinal structures manipulation as a restoration strategy. We also showed that assessments using relatively simple aerial photointerpretation and vegetation surveys in pioneer habitats can illustrate trajectories in river restoration projects shortly after their completion. Long-term monitoring of the geomorphic trajectory and associated plant communities, however, will help define the timing of future additional interventions to assure the natural resilience of riparian habitats.