Quantification of Flood Induced Fish Habitat Changes in an Alpine River

Abstract:

The dynamics of river morphology play an important role in habitat use of fish and other lotic species. Due to climate change, flood occurrence is expected to increase in Alpine regions. Understanding the habitat changes due to morphological change is therefore of great interest for river management. On June 2019 a secular flood event occurred in the Moesa River, Canton Grissons, Switzerland, which completely reworked the river topography in several reaches. We performed a high-resolution topographical survey before and after the flood on an 800m long and 100m wide floodplain of the Moesa. We setup and calibrated a 2D numerical hydrodynamic model (BASEMENT), to simulate multiple discharges for both the pre- and post-flood conditions. Using a novel unsupervised mesohabitat identification method, we identified mesoscale patches in the river based on the depth and velocity distributions obtained from the numerical modelling. These patches were then grouped based on their characteristics. Comparing the groups to general habitat preferences of brown trout (salmo trutta) and bullhead (cottus gobio) at different lifestages allowed us to quantify habitats for these species. Overall, the flood caused a deepening of the floodplain channels, consequently the group associated with high depths increased in prevalence under all discharge conditions. The other groups increased or decreased in prevalence seemingly independent from the discharge. Instead, whether certain side channels were open determined the abundance of these groups. Adult brown trout habitat was the least prevalent of the investigated habitat types in both the pre- and post-flood conditions, but showed a significant and consistent increase after the flood. The other investigated habitat types (bullhead, brown trout spawning and brown trout juveniles) showed increases under high discharge conditions, but decreases under low and medium discharge conditions. Improving our knowledge of the effects of large disturbance events like secular floods is paramount to identify possible measures that may be taken to manage our rivers effectively in the future. This ecohydraulic research introduces a method to identify habitat changes due to a large disturbance and aids in the understanding of the effects of a secular flood in an Alpine environment.