Remotely sensed impact of flow alteration on Swiss floodplains

Abstract

Floodplains, though covering a very small fraction of the land, offer key ecosystem services to our society. Floodplains play an active role in the maintenance of river water quality, in the protection of floods, in supporting a unique biodiversity and in many other ecosystem services. In Switzerland, pristine floodplains have lost 90% of their occupied area since 1850. This work was undertaken within the framework of NRP-70, with the aim of reconciling the increase in hydropower production with support for natural biodiversity, which seems to lead to a difficult compromise in the current situation. Airborne and spaceborne remote sensing offer unique capabilities to observe and monitor river systems and floodplains through various technologies. The current dissertation presents the use of remote sensing for observations and monitoring of floodplains and their dynamics, specifically to observe the impact of water flow alteration on the riparian ecosystems. Three complementary aspects of Swiss floodplains are considered throughout the dissertation. The work carried out in the frame of this dissertation is focused mainly on a few sections of a restricted number of river reaches. A robust methodology for riverine habitat dynamics quantification is developed, based on land cover classification produced on high-resolution imagery. Standard and infrared cameras were used to acquire images over five consecutive seasons along three river reaches. The quantification method provides recommendations for monitoring floodplains in Switzerland using unmanned aerial systems. A second method was developed to observe the resource allocation strategies of riparian vegetation at the individual level based on imaging spectroscopy. This method allowed the comparison of the distribution of strategies located along different reaches. Four imaging spectroscopy datasets acquired during two consecutive years, as well as leaf samples from two field campaigns, were used to extract plant traits from alluvial willows. Individuals located along river reaches affected by water flow alteration show a shift towards more competitive strategies. A third method was developed to observe trends in riparian vegetation over the last decades using inter-annual satellite acquisitions. The channel extent was semi-automatically detected using spectral mixture analysis. Acquisitions from Landsat missions 4, 5, 7, and 8 covering the years 1988 to 2016 over four reaches were used to extract trends and correlations with discharge data from hydrometric stations. Correlations between floods and changes in vegetation indices were found only in the case of the natural, sub-mountainous river, while no correlation was found in the case of the two reaches affected by hydropower infrastructure and the mountainous reach. The description of vegetation development through resource allocation strate-gies offers a new point of view on the consequence of water flow alteration and fitswith usual observation, namely the oft-observed channel narrowing. The resultssuggest that the processes shaping the plant characteristics of riparian vegetationoperate over a relatively long period of time. In the different works undertaken, Ialso showed that the specificity of the riverine land cover allows for a robust description of the river landscape, such as the spatio-temporal relationships between water,gravel and vegetation cover. This thesis contributes to the discussion of impact ofwater flow alteration on riparian ecosystems and the possibility of management fora close future. The interactions between the flow regime, the sediment regime, riparian vegetation and other components of the floodplains are still lacking someunderstanding, but recent research is on the way to produce the required recommendations to implement environmental flows. Though river science is advancingfrom day to day, the most critical constraints to protect floodplains in Switzerlandremain on the political level.