Suitability and sustainability of spawning gravel placement in degraded river reaches, Belgium

Abstract Restoring in-stream spawning habitats in degraded rivers has received increasing attention as a tool for mitigating local wild fish population declines, notably in response to the European Water Framework Directive (WFD). However, spawning gravel placements are far too often designed without accurate knowledge of the morphodynamic river processes, resulting in a limited efficacy and longevity of the artificial spawning ground. To address the combined effects of bedload transport and fine sediment clogging processes on the sustainability of artificial spawning gravel habitats, we examined the effects of such rehabilitation actions on six degraded river reaches in Wallonia, Belgium. The monitoring scheme was based on the evolution of the thickness and clogging of the spawning gravel (using wooden stakes driven into the gravel layer to measure the depth of the anoxia) and on the PIT-tagged tracking of gravel dispersion over a period of 3.6–8.5 yrs. On the one hand, the results highlighted that several artificial spawning grounds were quickly clogged because of improper sizing of the spawning material. Gravel that was too coarse to be mobilized by the river and that had a narrow grain size range favoured fine sediment accumulation within the interstices of the gravel layer. On the other hand, one spawning gravel placement was rapidly scoured (after 2.2 yrs) because of an undersizing of the gravel with respect to flow competence. In the end, one gravel placement presented adequate gravel sizing, allowing periodic gravel transport over short distances (the mean annual travel distance was ~3 m). The longevity of a gravel placement and the ability of the displaced gravel to form new spawning grounds downstream were strongly dependent on the distance that the placed gravel was likely to travel, which in turn depended on several hydromorphological parameters, such as unit stream power, channel morphology and bed texture. The key parameters highlighted in this study need to be acknowledged when designing spawning gravel placement projects.