Evolution of an Alpine proglacial river during 7 decades of deglaciation
Authors: Piermattei Livia , Heckmann Tobias , Betz-Nutz Sarah , Altmann Moritz , Rom Jakob , Fleischer Fabian , Stark Manuel , Haas Florian , Ressl Camillo , Wimmer Michael H. , Pfeifer Norbert , Becht Michael 
Published: May 2023

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 snowmelt and glacier melt runoff, facilitated by climate change. 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 digitised historical aerial images
and advancements in digital photogrammetry provide the basis for
reconstructing and assessing the long-term evolution of the surface, in
terms of both 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 contributes to research on fluvial sediment
changes by estimating the sediment balance of a main Alpine river (Fagge) in a glaciated catchment (Kaunertal, Austria) over 19 survey
periods from 1953 to 2019. Exploiting the potential of historical
multi-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 a
proglacial river that is undergoing a transition phase, resulting from an
extensive glacier retreat of approximately 1.8 km. This has led to the formation of new channel networks and an overall negative cumulative
sediment balance for the entire study area. We found that high-magnitude
meteorological and hydrological events associated with local glacier
retreats have a significant impact on the sediment balance. The gauge record
indicates an increase in such events, as well as in runoff and probably in
sediment transport capacity. Despite this, the sediment supply has declined
in the last decade, which can be attributed to a lower contribution of the
lateral moraines coupled to the channel network and less sediment sourced
from the melting Gepatsch Glacier as evidenced by roches moutonnées
exposed in the current/most recent forefield. Nonetheless, we observed
significant erosion in the tributary, leading to the transport of sediment
downstream. 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.