Evolution of an Alpine proglacial river during seven decades of deglaciation quantified from photogrammetric and LiDAR digital elevation models

Abstract. 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 that modify natural sediment and flow regimes at local, catchment, and regional scales. In glaciated catchments, river sediment loads are likely to increase due to increasing snow and glacier melt runoff, facilitated by climate changes. Additionally, channel erosion and depositional dynamics and patterns are influenced by sediment delivery from hillslopes, and sediment in the forefields of retreating glaciers. In order to reliably assess the magnitudes of the channel-changing processes and their frequencies due to recent climate change, the investigation period needs to be extended to the last century, ideally back to the end of the Little Ice Age. Moreover, a high temporal resolution is required to account for the history of changes in channel morphology and for better detection and interpretation of related processes. The increasing availability of digitized historical aerial images and advancements in digital photogrammetry provides the basis for reconstructing and assessing the long-term evolution of the surface, both in terms of planimetric mapping and the generation of historical digital elevation models (DEMs).

The main issue of current studies is the lack of information over a longer period. Therefore, this study makes a major contribution to research on fluvial sediment changes by estimating the sediment balance of a main Alpine river (Fagge River) in a glaciated catchment (Kaunertal, Austria) over nineteen survey periods from 1953 to 2019. Exploiting the potential of historical multi-temporal DEMs, combined with recent topographic data, we quantify 66 years of fluvial changes (i.e. the active floodplain) in terms of geomorphic changes, erosion, and deposition, and the amounts of mobilized sediment. We show that geomorphic changes and the cumulative sediment balance are mainly driven by glacier retreat as well as a short advance phase in the 1980s, sediment delivery from recently deglaciated steep lateral moraines, an increasing runoff trend and extreme runoff events (such as subglacial water pocket outburst, and heavy rainfall). Overall, this work has contributed to improving our understanding of the complexity of sediment dynamics and river changes across various spatial and temporal scales and their relationship to climate change factors.