Evolution of an Alpine proglacial river during 7 decades of deglaciation

Alpine rivers have experienced considerable changes in channel morphology over the last century. Natural factors and human disturbance are the main drivers of changes in channel morphology thatmodify natural sediment and flow regimes at local, catchment, and regionalscales. In glaciated catchments, river sediment loads are likely to increase due to increasing snowmelt and glacier melt runoff, facilitated by climate change. Additionally, channel erosion and depositional dynamics andpatterns are influenced by sediment delivery from hillslopes and sedimentin the forefields of retreating glaciers. In order to reliably assess themagnitudes of the channel-changing processes and their frequencies due torecent climate change, the investigation period needs to be extended to thelast century, ideally back to the end of the Little Ice Age. Moreover, ahigh temporal resolution is required to account for the history of changesin channel morphology and for better detection and interpretation of relatedprocesses. The increasing availability of digitised historical aerial imagesand advancements in digital photogrammetry provide the basis forreconstructing and assessing the long-term evolution of the surface, interms of both planimetric mapping and the generation of historical digitalelevation models (DEMs). The main issue of current studies is the lack of information over a longerperiod. Therefore, this study contributes to research on fluvial sedimentchanges by estimating the sediment balance of a main Alpine river (Fagge) in a glaciated catchment (Kaunertal, Austria) over 19 surveyperiods from 1953 to 2019. Exploiting the potential of historicalmulti-temporal DEMs combined with recent topographic data, we quantify 66 years of geomorphic change within the active floodplain, including erosion,deposition, and the amounts of mobilised sediment. Our study focuses on aproglacial river that is undergoing a transition phase, resulting from anextensive glacier retreat of approximately 1.8 km. This has led to the formation of new channel networks and an overall negative cumulativesediment balance for the entire study area. We found that high-magnitudemeteorological and hydrological events associated with local glacierretreats have a significant impact on the sediment balance. The gauge recordindicates an increase in such events, as well as in runoff and probably insediment transport capacity. Despite this, the sediment supply has declinedin the last decade, which can be attributed to a lower contribution of thelateral moraines coupled to the channel network and less sediment sourcedfrom the melting Gepatsch Glacier as evidenced by roches moutonneesexposed in the current/most recent forefield. Nonetheless, we observedsignificant erosion in the tributary, leading to the transport of sedimentdownstream. Overall, this study enhances our understanding of the complexity of sediment dynamics in proglacial rivers across various spatial and temporal scales and their relationship to climate change factors.