Grow With the Flow? Impact of Experimental Floods on Riparian Vegetation in an Alpine River

Abstract

Experimental floods have been proposed as a measure to mitigate effects of river flow regulation, primarily vegetation encroachment, streambed colmation, and loss of bed and channel mobility. In this study, we investigated the short- and long-term biogeomorphic effects of an experimental flood programme conducted in the Spöl River (Swiss Alps) since 2000. Based on a GIS analysis of orthoimages between 1991 and 2023, we characterize the past and current state of fluvial biogeomorphic succession. From field surveys, we describe the current distribution of riparian vegetation in the lower Spöl. Lastly, we investigate the impact of experimental floods on riparian trees through dendrogeomorphological indicators. Over the study period, we identified a significant increase in active channel width and an associated reduction of vegetation encroachment. However, no significant colonization of gravel bars by pioneer species was observed. Tree-ring widths showed a general decrease in riparian tree growth since the beginning of the experimental flood programme. We show that this apparent control of hydrogeomorphic processes on biogeomorphology and the corresponding absence of vegetation succession patterns is due to high aggradation rates of sediment delivered by an unregulated tributary. We discuss the implications of these findings for future management of the river and experimental flood programme and raise caveats regarding the future developments of river morphology in the lower Spöl.