Lead
More floods - more sediment transport - less fish? The frequency and possibly the extent of flooding will change as the climate changes. An increase or a seasonal shift in the amount of gravel and sand that is transported by rivers may be expected. Existing measures against floods need to be adapted. What effects do flooding and sediment transport have on fish environments?

Lay summary

Background
Climate change will considerably affect flooding and sediment transport in mountain streams over the coming 40 to 100 years. It is to be expected that the number, intensity and temporal distribution of rainfall events will change. This could lead to more frequent and larger floods. Also, the melting of glaciers and permafrost uncovers sediment that can be washed away. For this reason, in some areas mountain streams will carry more gravel and sand. If considerably larger sediment volumes are transported, existing protection measures will only partially fulfil their functions. The modified distribution of material in the rivers will also affect the living conditions of fish. Flood occurrence in winter and spring is likely to change and can interfere with the success of natural reproduction. However, the exact repercussions are hardly known yet.

Objectives and methods

In this project, the consequences of climate change on sediment transport and the quality of fish habitat are examined. Using a model, we calculate sediment budgets for three river catchments, i.e. the Kleine Emme, Brenno, and Hasliaare rivers. We compare the model predictions with data from past flood events in these rivers. Habitat conditions for brown trout are investigated with field campaigns. Climate models predict a future shift in the flow regime during winter periods. We examine how the resulting change of sediment transport in the study rivers influences the survival of brown trout populations. We also analyze how changes in flow discharge and increased sediment delivery for example due to permafrost retreat influence bedload transport and flood safety in mountain streams.

Significance
This study will help to quantify the time evolution of sediment transfer in mountain streams in response to climate change, i.e. to changed runoff and sediment input conditions. The results will support the evaluation of future flood hazards with bed load displacement in mountain streams on the one hand, and the assessment of living conditions for brown trout on the other. The developed methods will help authorities and decision makers to evaluate potential flood-related sediment hazards in Alpine mountain river catchments and to further develop sustainable flood protection and river management concepts.