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.

Abstract. Since the end of the Little Ice Age (LIA) in the middle of the 19th century, Alpine glaciers have been subject to severe recession that is enhanced by the recent global warming. The melting glaciers expose large areas with loose sediments, amongst others in the form of lateral moraines. Due to their instability and high slope angle, the lateral moraines are reworked by geomorphological processes such as debris flows, slides or fluvial erosion. In this study, the development of the morphodynamics and changes of geomorphological processes on lateral moraines were observed over decades, based on a selection of 10 glacier forefields in the eastern Alps. To identify geomorphological changes over time, several datasets of archival aerial images reaching back to the 1950s were utilized in order to generate digital elevation models (DEMs) and DEMs of difference. The aerial images were complemented by recent drone images for selected moraine sections, enabling a high-resolution analysis of the processes currently occurring. The results concerning the development of morphodynamics on lateral moraine sections are diverse: some slopes display a stagnation of the erosion rates, the rates on one section increase significantly and the majority of the slopes show a decline of the morphodynamics over decades, which, however, stay on a high level in many cases. In particular, moraine sections with high morphodynamics in the beginning of the observation period mostly show high erosion rates up until now with values up to 11 cm per year. These moraine sections also feature heavy gullying on their upper slopes. A correlation between the development of morphodynamics and the time since deglaciation could scarcely be established. In fact, the results rather indicate that characteristics of the lateral moraines such as the initial slope angle at the time of deglaciation have a significant influence on the later morphodynamics. These observations raise concerns whether often conducted analyses based on the comparison of lateral moraine sections with different distances to the glacier terminus, assumed to represent varying time spans since deglaciation, can provide sound evidence concerning their process of stabilization.

Since the end of the Little Ice Age (LIA), formerly glaciated areas have undergone considerable changes in their morphodynamics due to external forces and system-internal dynamics. Using multi-temporal high-resolution digital elevation models (DEMs) from different remote sensing techniques such as historical digital aerial images and light detection and ranging (LiDAR), and the resulting DEMs of difference (DoD), spatial erosion and accumulation patterns can be analyzed in proglacial areas over several decades. In this study, several morphological sediment budgets of different test sites on lateral moraines and different long-term periods were determined, covering a total period of 49 years. The test sites show high ongoing morphodynamics, and therefore low vegetation development. A decrease as well as an increase of the mean annual erosion volume could be demonstrated at the different test sites. All test sites show a slope–channel coupling and a decrease in the efficiency of sediment transport from slopes to channels. These developments are generally subject to conditions of increasing temperature, decreasing short-term precipitation patterns and increasing runoff from adjacent mountain streams. Finally, the study shows that sediment is still available on the investigated test sites and the paraglacial adjustment process is still in progress even after several decades of deglaciation (~133 years).

Abstract. We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the central Eastern Alps covering a total time period from 1953 to 2019 including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM of Differences were calculated based on multitemporal high-resolution digital elevation models from historical aerial images (generated by structure-from-motion photogrammetry with multi-view-stereo) and light detection and ranging from airborne platforms. Two approaches were implemented to achieve the corresponding objectives. First, by calculating linear regression models using the accumulated sediment yield and the corresponding catchment area (on a log-log scale), the range of the variability of the spatial distribution of erosion values within the areas of interest is shown. Secondly, we use volume calculations to determine the total/mean sediment output (and erosion rates) of the entire areas of interest. Subsequently, a comparison is made between the areas of interest and the epochs of both approaches. Based on the slopes of the calculated regression lines, it could be shown that the highest range of the variability of sediment yield within all areas of interest is in the first epoch (mainly 1950s to 1970s), as in some areas of interest sediment yield per square metre increases clearly more (regression lines with slopes up to 1.5), which in the later epochs (1970s to mid-2000s and mid-2000s to 2017/2019) generally decreases in 10 out of 12 cases (regression lines with slopes around 1). However, even in the areas of interest with an increase in the variability of sediment yield over time, the earlier high variabilities are no longer reached. This means that the spatial pattern of erosion in the gully heads changes over time as it becomes more uniform. Furthermore, using sediment volume calculations and corresponding erosion rates, we show a generally decreasing trend in geomorphic activity (amount of sediment yield) between the different epochs in 10 out of 12 areas of interest, while 2 areas of interest show an opposite trend where morphodynamics increase and remain at the same level. Finally, we summarise the results of long-term changes in the morphodynamics of geomorphologically active areas on lateral moraines by presenting the "sediment activity concept", which, in contrast to theoretical models, is based on actually calculated erosion. The level of geomorphic activity depends strongly on the characteristics of the areas of interest, such as size, slope length and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with decades of dead ice influence in the lower slope area show high geomorphic activity. Furthermore, we show that system-internal factors as well as the general paraglacial adjustment process have a greater influence on long-term morphodynamics than changing external weather and climate conditions, which, however, had a slight impact mainly in the last, i.e. most recent epoch (mid-2000s to 2017/2019) and may have led to an increase in erosion at the areas of interest.

Abstract. Since the end of the Little Ice Age (LIA) in the middle of the 19th century, Alpine glaciers have been subject to severe recession that is enhanced by the recent global warming. The melting glaciers expose large areas with loose sediments in the form of lateral moraines, amongst other forms. Due to their instability and high slope angle, the lateral moraines are reworked by geomorphological processes such as debris flows, slides, or fluvial erosion. In this study, the development of the morphodynamics and changes in geomorphological processes on lateral moraines were observed over decades, based on a selection of 10 glacier forefields in the Eastern Alps. To identify geomorphological changes over time, several datasets of archival aerial images reaching back to the 1950s were utilized in order to generate digital elevation models (DEMs) and DEMs of difference. The aerial images were complemented by recent drone images for selected moraine sections, enabling a high-resolution analysis of the processes currently occurring. The results concerning the development of morphodynamics on lateral moraine sections are diverse: some slopes display a stagnation of the erosion rates, whereas the rates of one section increase significantly; however, the majority of the slopes show a decline in morphodynamics over decades but stay on a high level in many cases. In particular, moraine sections with high morphodynamics at the beginning of the observation period mostly show high erosion rates up until present-day measurements, with values up to 11 cm yr−1. These moraine sections also feature heavy gullying on their upper slopes. A correlation between the development of morphodynamics and the time since deglaciation could scarcely be established. In fact, the results instead indicate that characteristics of the lateral moraines such as the initial slope angle at the time of deglaciation have a significant influence on the later morphodynamics. These observations raise concerns as to whether the until now often conducted analyses based on the comparison of lateral moraine sections with different distances to the glacier terminus, assumed to represent varying time spans since deglaciation, can provide sound evidence concerning the process of stabilization.