30 years of large river restoration: How long do restored floodplain channels remain suitable for targeted rheophilic fishes in the lower river Rhine?

The ecological efficacy of river restoration projects may change over time, resulting in the loss of their ecological function for targeted species. The goal of this study was to evaluate the rheophilic nursery function of restored floodplain channels over time, by analysing 30 years of monitoring data from 12 restoration projects in the lower river Rhine. We hypothesised that the nursery function would change over time, caused by the combined effects of decreasing flow conditions and succession processes affecting habitat heterogeneity. We found that nursery area suitability for rheophilic fish was almost 4 times higher in two-sided connected channels than in one-sided connected channels, although the response trends of rheophilic fish were similar for both water body types. These response curves showed clear optima with channel age, for rheophilic fish abundance at 13 to 14 years post-restoration, indicating optimal nursery conditions. On the other hand, rheophilic species richness showed a steadily decreasing trend with channel age, suggesting aging channels became less suitable as nursery areas for most rheophilic fish species. The presence of permanent flow was found to be the main driving factor in explaining both rheophilic fish community trends and habitat succession in individual restored channels. We did not observe an effect of habitat heterogeneity on nursery function for rheophilic fish. To create and maintain optimal nursery conditions in restored floodplain channels of strongly anthropogenically influenced rivers such as the river Rhine, we propose a management strategy involving cyclic rejuvenation through human intervention, focusing on restoring permanent flow, with a frequency of on average every 15 years, depending on the rate of aggradation and targeted rheophilic species. We also propose a thorough investigation of the relationship between habitat heterogeneity and nursery success in floodplain channels, as a next step in the identification of suitable nursery areas for rheophilic fishes.

Longitudinal training dams mitigate effects of shipping on environmental conditions and fish density in the littoral zones of the river Rhine.

The stability of habitat conditions in littoral zones of navigated rivers is strongly affected by shipping induced waves and water displacements. In particular, the increase of variability in flow conditions diminishes the suitability of these habitats for juvenile fishes. Recently, a novel ecosystem based river management strategy has resulted in the replacement of traditional river training structures (i.e., groynes) by longitudinal training dams (LTDs), and the creation of shore channels in the river Waal, the main, free-flowing and intensively navigated distributary of the river Rhine in the Netherlands. It was hypothesized that these innovative LTDs mitigated the effects of shipping on fishes by maintaining the natural variability of habitat conditions in the littoral zones during ship passages whereby shore channels served as refugia for juvenile fishes. Measurements of abiotic conditions showed a significantly lower water level fluctuation and significantly higher flow stability in shore channels compared to groyne fields. Flow velocity did not differ, nor did the variation in flow velocity fluctuation during ship passage between these habitats. Densities of fish were found to be significantly higher in the littoral zones of shore channels compared to nearby groyne fields. Moreover, electrofishing along the inner side of the newly constructed LTD showed a significant linear relationship between fish density and distance from highly dynamic in- and outflow sections and to lowered inflow sections in the LTD. Results of our field sampling clearly indicate successful ecological rehabilitation of littoral zones that coincides with a facilitation of navigation in the main river channel and increased flood safety.