Morphological and fish mesohabitat dynamics following an experimental flood under different sediment availability

Abstract

Experimental floods have been increasingly used as a promising practice to rehabilitate river ecosystems downstream of dams; however, the morphological and habitat dynamics they determine under different sediment supply conditions still poses relevant research and management questions. This study investigates the morphological and fish mesohabitat dynamics following an experimental flood, in two river reaches subject to different sediment supply regimes. We chose the lower Spöl River (Switzerland) as a relevant case study, subject to an experimental flood program for several years. Downstream of the dam, a tributary supplies large amounts of sediment to the Spöl dividing the study area into two homogeneous reaches with different sediment availability but similar flow conditions during the experimental flood. We analyzed and quantified the changes in morphology and fish habitat suitability for the Brown Trout (Salmo trutta) at the mesoscale in these two reaches caused by the 2021 experimental flood, which lasted 11 h and had a peak magnitude corresponding to a 1-year return interval in the pre-dam flow regime. We found almost no correlation between changes in the channel morphology and in habitat suitability for this event. In the upstream reach, located immediately downstream of the dam, we observed a narrower channel with a regular longitudinal sequence featuring nearly immobile coarse rapids, interspersed with more dynamic, finer riffles. Here, reach-scale morphodynamics and the shifts of the mesohabitat mosaic and the suitable habitats were below 10%. Conversely, the downstream reach, characterised by a wider channel and much higher sediment supply of well-sorted, finer bed material, was dominated by alternate bar instability and migration at the reach scale, which caused a 45% shift in its pre-flood habitat mosaic. Nevertheless, in the same reach, the overall suitability of habitats remained relatively unchanged. We attributed these different dynamics to two main factors: (i) more prolonged bedload mobility conditions and (ii) the occurrence of bar migration in the downstream reach compared to the upstream one. This study (i) underscores the critical importance of considering sediment supply from downstream tributaries when designing and monitoring the effects of experimental floods, (ii) supports the use of morphodynamic models in the related planning and monitoring phases and (iii) shows the relevance of integrating morphodynamics and eco-hydraulic analysis to support the implementation of such flow restoration programs.