Sediment-discharge events recorded at the outlet of high-mountain catchments provide an integrated signal of upstream geomorphic activity. At the same time, a key limitation of DEMs of Difference (DoDs) is that sediment source areas and geomorphic responses cannot be directly linked to individual events, especially when DoDs span extended time periods. To address this, we apply the method proposed by Skålevåg et al. (2024) to detect, cluster and characterise sediment-discharge events, which can subsequently be related to observed sediment mobilization signals in the DoD.15-min time series of water discharge and suspended sediment concentration from Gepatschalm, Kaunertal (Austria), covering the period 2008-2025, were used to detect the sediment-discharge events and derive 16 metrics, which were used to cluster the events with a Gaussian mixture model. Gridded meteorological data were used to characterise the clusters with respect to antecedent and intra-event forcing conditions. The resulting event catalogue was evaluated using DEMs of difference (DoDs) covering the entire catchment and investigation period.Over the 16-year period we identified a total of 850 sediment-discharge events. Clustering results reveal three patterns: (i) melt-dominated events (average contribution ~30% to annual suspended sediment yield), (ii) early- and late-season freeze–thaw-modulated events (~15%), and (iii) compound rainfall–melt events (~33%). A marked increase in event frequency was observed in 2022, which also recorded the highest annual suspended sediment yield in the dataset. The majority of 2022 events were assigned to clusters 2 and 3. Combining multiple DoDs from summer 2022 with gridded precipitation data allowed the identification of a distinct sediment-discharge event on 28 June 2022, which triggered fluvial erosion in a specific sub-catchment of Kaunertal. For this event and other events, the main sediment source areas can clearly be delineated with the DoD analysis. Skålevåg, A.; Korup, O.; Bronstert, A. (2024): Inferring sediment-discharge event types in an alpine catchment from sub-daily time series. In: Hydrology and Earth System Sciences Discussions.

Sediment connectivity is an important property of geomorphic systems reflecting the potential to route material through themselves and hence modulating the propagation of geomorphic changes. While the relevance of the concept is clear, connectivity cannot be measured directly and the discussion on the best methods to quantify connectivity is still ongoing. Probably the most frequently used approach is based on the index of connectivity (IC) as it was developed by Borselli et al. (2008) and later adapted by Cavalli et al. (2013) for alpine catchments. This index aims at quantifying the structural connectivity that is governed by the spatial configuration and properties of system components. Nevertheless, the predictive capabilities of this index for functional connectivity, i.e. the actual transfer of sediment between the system components, have not been conclusively validated with field data. Most importantly, previous studies have, to our knowledge, not taken into account the spatial variability of the hydrometeorological forcing that leads to different functional connectivity in locations with similar structural connectivity.In this study, we use a unique dataset to test the predictive capability of the IC for hillslope-channel coupling of debris flows in the Horlachtal, Austria (described by Rom et al., 2023). The dataset consists of aerial imagery and two airborne LiDAR digital elevation models from which n=156 debris flows were mapped and quantified that were triggered by intense rainstorms on July 20th and 23rd, 2022. For this event, adjusted radar data (INCA data from the Austrian meteorological survey, ZAMG, and measurements from weather stations within the study area) give a high-resolution account of the spatial distribution of rainfall intensities and sums. Using these data, each debris flow was characterised with respect to (i) the meteorological forcing that affected its contributing area, (ii) morphometric properties of the latter, (iii) its sediment volume, and (iv) its runout length indicating functional connectivity, i.e. the degree of coupling to the main channel. Then we assessed the influence of structural connectivity (indicated by the IC) and hydrometeorological forcing on the observed functional connectivity. To our knowledge, this is the first study investigating the predictive capacity of the IC taking into account the spatial variability of the forcing. Among others, our results show that the IC is significantly higher for those debris flows that reached the main channel, compared to those that did not.

The availability of comprehensive aerial photography is limited to the mid-20th century, posing a challenge for quantitatively analyzing long-term surface changes in proglacial areas. This creates a gap of approximately 100 years, spanning the end of the Little Ice Age (LIA). Employing digital monoplotting and historical terrestrial images, our study reveals quantitative surface changes in a LIA lateral moraine section dating back to the second half of the 19th century, encompassing a total study period of 130 years (1890 to 2020). With the long-term analysis at the steep lateral moraines of Gepatschferner (Kauner Valley, Tyrol, Austria) we aimed to identify changes in vegetation development in context with morphodynamic processes and the changing climate. In 1953, there was an expansion in the area covered by vegetation, notably encompassing scree communities, alpine grassland, and dwarf shrubs. However, the destabilization of the system after 1980, triggered by rising temperatures and the resulting thawing of permafrost, led to a decline in vegetation cover by 2020. Notably, our observations indicated that, in addition to morphodynamic processes, the overarching trends in temperature and precipitation exerted a substantial influence on vegetation development. Furthermore, areas with robust vegetation cover, once stabilised, were reactivated and subjected to erosion, possibly attributed to rising temperatures post-1980. This study demonstrates the capability of historical terrestrial images to enhance the reconstruction of vegetation development in context with morphodynamics in high alpine environments within the context of climate change. However, it is important to note that long-term mapping of vegetation development through digital monoplotting has limitations, contingent on the accessibility and quality of historical terrestrial images, as well as the challenges posed by shadows in high alpine regions. Despite these limitations, this long-term approach offers fundamental data on vegetation development for future modelling efforts.

We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the European Central–Eastern Alps, covering a total time period from 1953 to 2019 and including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM (digital elevation model) of Differences (DoDs) 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 in the spatial distribution of erosion values within the sites. Second, we use volume calculations to determine the total and the mean sediment yield (as well as erosion rates) of the entire sites. Subsequently, both the sites and the different time periods of both approaches are compared. Based on the slopes of the calculated regression lines, it can be shown that the highest variability in the sediment yield at the sites occurs in the first time period (mainly 1950s to 1970s). This can be attributed to the fact that within some sites the sediment yield per square metre increases clearly more strongly (regression lines with slopes up to 1.5). In contrast, in the later time periods (1970s to mid-2000s and mid-2000s to 2017/2019), there is generally a decrease in 10 out of 12 cases (regression lines with slopes around 1). However, even at sites with an increase in the variability in the 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 time periods in 10 out of 12 sites, while 2 sites 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 sites, such as size, slope length, and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with influence of dead ice over decades 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, time period (mid-2000s to 2017/2019) and may have led to an increase in erosion at the sites.

<p>Since the end of the Little Ice Age around 1850, global warming has led to rapid landscape changes, especially in high mountain areas. The ongoing glacier melt leads to an expansion of the LIA glacier forefields, so-called proglacial areas. The exposed lateral moraines often show increased sediment activity over decades and centuries, which is generally described as the paraglacial adjustment process. Slope instabilities are caused, for example, by the loss of the support from the melting glaciers, which can lead to large landslides and thus heavy deformations. In order to understand corresponding geomorphological processes, it is important that surface changes can be reconstructed and analysed in high spatial and temporal resolution. However, aerial photographs of the European Alps, which are well suited for observing proglacial areas, only extend to the middle of the 20th century, thus resulting in a temporal limitation.</p><p>Therefore, in this work we show a nearly 100-year quantitative monitoring of a large-scale deformation of a LIA lateral moraine in the glacier forefield of the Gepatschferner in the upper Kaunertal (Tyrol, Austria). We achieve this long-term (1922 to 2021) observation by combining different topographic data sets based on different remote sensing methods and techniques. The reconstructed earth surfaces are based on airborne LiDAR data (2006 to 2021) and photogrammetric DEMs (1953 to 2003) as well as a historical stereophotogrammetric map from 1922, which was also generated into a DEM. In total, eight DEMs were generated and corresponding DoDs calculated.</p><p>Different landslides within the first three epochs (1922 to 1953, 1953 to 1971 and 1971 to 1983) could be determined on the slope, which can be directly linked to the corresponding glacier melt. Even after the landslide processes (from 1983 onwards), continuous geomorphological activity could be observed until today (2021), whereby the total volume of net erosion of all epochs (from 1922 to 2021) added up to approx. 486,000 m³.</p>

We present a 99-year monitoring (1922 to 2021) of a large-scale deformation of the terminal part of a Little Ice Age lateral moraine in the glacier forefield of the Gepatschferner in the Upper Kaunertal in Tyrol, Austria.The reconstruction of this large-scale landslide was carried out in high temporal and spatial resolution using 12 DEMs of differences based on 13 different Digital elevation models from several epochs and remote sensing techniques.These were generated from a stereo photogrammetric map (1922), historical aerial photographs (between 1953 and 2003) using structure-from-motion photogrammetry with multi-view-stereo and airborne Light Detection and Ranging data (between 2006 and 2021).Based on the analysis of the different epochs, the period of the main landslide could be dated between 1953 and 1971.However, landslides were detected in all the first three epochs (1922 to 1953, 1953 to 1971 and 1971 to 1983), covering a period of 61 years.In the epoch of the main landslide, there was a vertical and horizontal displacement of about 70 and 100 m, respectively, while the landslide was about 190 m wide, which results in a calculated volume of 287,537 (±2,026) m 3 .We assume that the instability of the slope was caused by glacial debuttressing due to the glacier retreat since the end of the Little Ice Age.During the entire period, the slope was subject to strong variations in geomorphological activity, both spatially and temporally.The landslide area shows continuous geomorphological activity over the entire study period (1922 to 2021), summing up the total volume of net erosion to about 486,000 m 3 .

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.

Deglaciation in high mountain areas signifies the transition from glacial to periglacial conditioned landscapes. Due to the reduced melt rate of debris-covered glacier ice, these areas of the glacier may persist long after the surrounding glacier has melted, resulting in the formation of distinct post-glacial landforms. In this study, we examine the geomorphological evolution and potential future development of a 19,267 m3 ± 204 m3 rockfall from the permafrost-affected headwall on the low-elevated Zwieselbachferner in the Horlachtal, Stubai Alps, Austria. The analysis uses multi-epochal remote sensing data, including photogrammetrically and airborne laser scanning-derived digital elevation models, orthophotos, and satellite data, covering a period from the initial rockfall in 2003/2004 to 2022. The data reveals that the rockfall event resulted in the formation of a supraglacial debris layer of varying thickness, spanning an area of 15,920 m2. Subsequently, 13 further rockfalls ranging from 67 m3 ± 6 m3 to 4250 m3 ± 121 m3 were detected. The mean ice thickness of the debris-covered area only slightly decreased between 2006 and 2022, in contrast to the surrounding glacier, whose thickness and length have strongly decreased. This results in the formation of a steep front and flanks that become increasingly covered by debris redistribution. The study suggests that the glacier ice covered by rockfall-derived debris will remain as a periglacial landform of glacial origin after the complete melting of the surrounding glacier.

Aerial photographs of the European Alps usually only reach back to the middle of the 20th century, which limits the time span of corresponding studies that quantitatively analyse long-term surface changes of proglacial areas using georeferenced orthophotos. To the end of the Little Ice Age, this leads to a gap of about 100 years. Using digital monoplotting and several historical terrestrial photographs, we show the quantification of surface changes of a Little Ice Age lateral moraine section until the late second half of the 19th century, reaching a total study period of 130 years (1890–2020). The (initial) gully system expands (almost) continuously over the entire study period from 1890 to 2020. Until 1953, the vegetation-covered areas also expanded (mainly scree communities, alpine grasslands and dwarf shrub communities), before decreasing again, especially between 1990 and 2003, due to large-scale erosion within the gully system. Furthermore, our results show that the land-cover development was impacted by temperature and precipitation changes. With the 130-year study period, we contribute to a substantial improvement in the understanding of the processes in the proglacial by analysing the early phase and thus the immediate response of the lateral moraine to the ice exposure.

Establishing a record of large debris flow events in high Alpine areas prior to the availability of high resolution remote sensing data can be very challenging. In this study, we investigate the debris flow activity in two tributary valleys of the Horlachtal catchment in Tyrol, Austria between the end of the Little Ice Age at about 1850 and the first available area wide aerial images from 1947. To accomplish this, we calculated a local lichenometric calibration curve using the long axis diameters of the five largest Rhizocarpon lichen thalli at 51 different reference locations. Because of the interval-censored dating of most of the reference sites, we established a bootstrapping approach within the calibration curve calculation process. With the help of the lichenometric calibration data, we were able to date 47 old debris flow deposits in the study area. The results indicate no increasing or decreasing trends in frequencies of extreme debris flow events. In addition, the results point to a very local character of debris flow triggering precipitation events, as we can detect major differences in neighbouring valleys. Lichenometric derived datings also provide temporal informations about the end of debris flow activity at some sites in the study area and thus can contribute to a better understanding of debris flow systems.

<p>Sediment connectivity is defined as the potential of a catchment to route material through itself. It is a system property that regulates the propagation of geomorphic changes through a catchment and is therefore a factor of its sensitivity to climatic change. In well-connected catchments, changes are effectively propagated; where the coupling of hillslopes to channels, or between channel reaches is poor, changes may remain localised. Structural connectivity itself is not a static property; it can be affected by process-response feedbacks, gradual or rapid changes, for example as a consequence of extreme events. In this study, we use a multi-method approach to investigate changes in structural sediment connectivity over time periods of up to 70 years in three alpine catchments.</p> <p>First, we calculate the Index of Connectivity (IC) and corresponding change maps to identify areas and time periods with major changes in structural connectivity.  The required multitemporal digital elevation models (DEMs) are computed with historical aerial images and Structure-from-Motion Photogrammetry, more recent DEMs are obtained from ALS surveys. The channel networks as targets are manually mapped using the DEMs and orthomosaics.</p> <p>The second approach for selected areas makes use of multitemporal geomorphological maps, digital elevation models and graph theory. The geomorphological maps were produced based on historical orthomosaics, DEM derivatives and DEMs of Difference. The landforms in the geomorphological maps form the nodes of a graph, and edges connecting the nodes along the direction of flow represent potential or actual sediment transfer between them. The graphs reflect the system structure for a certain point in time; graph metrics can be used to assess the structural connectivity including spatial differences and temporal changes.  </p>

In order to calibrate and validate debris flow models, high precision in situ measurements are essential. However, it is quite difficult to acquire detailed information about debris flows, as they only rarely occur during exceptional high precipitation intensities. In July 2022, a series of such high-intensity short-duration precipitation events triggered several debris flows within the area of the Stubai Alps/Austria, which caused severe damage. On the 20th and 23rd of July 2022, two of these convective events initiated multiple debris flows on the slopes of the Horlachtal, a side valley of the Oetztal.These events have been registered by measurements of three different meteorological stations and four different discharge gauges distributed over the study area. In addition, INCA (Integrated Nowcasting through Comprehensive Analysis) rainfall data provided by ZAMG (Austrian Central Institute for Meteorology and Geodynamics) allow insights in the spatial and temporal characteristics of the rainfall patterns. Furthermore, two airborne LiDAR (Light Detection and Ranging) campaigns of the Chair of Physical Geography at the University of Eichstätt-Ingolstadt covering the whole Horlachtal (about 55 km²) provide detailed pre and post event topographical data.A combined evaluation of the different data sets allows us to characterise the debris flow events in the study area in great detail. Topographical analyses show that a total number of 156 debris flows were triggered with accumulation volumes up to 40.000 m³. These volumes can be related to the individual catchment areas in combination with precipitation intensities. Furthermore, the spatial distribution of the triggered debris flows show a concentration to a certain region within the study area, which relates to the spatial patterns of the precipitation events.

Rock glaciers are cryo‐conditioned downslope‐creeping landforms in high mountains. Their dynamics are changing due to external factors influenced by climate change. Although there has been a growing scientific interest in mountain permafrost and thus in rock glaciers in recent years, their historical development, especially before the first alpine‐wide aerial image flights in the 1950s, has hardly been researched. Therefore, we utilize a historical stereophotogrammetric map from 1922 and historical flow velocity profiles (1938–1953) and relate them to data derived from historical aerial photographs and airborne laser scanning data in several time slices between 1953 and 2021. By doing so, the development of flow velocity, surface elevation changes, and frontal advance of the two lobes of the composite rock glacier Inner Ölgrube, Kaunertal, Austria, is analyzed and compared over almost a century. Results indicate an increased frontal advance in the laterally confined area of one lobe and a severe subsidence in the upper area of both lobes between 1922 and 1953. Whereas the former could be explained by a combination of the short warm phase in the 1940s and 1950s and the (subsurface) topography, the latter might be attributed to the strong melting of superimposed debris‐covered dead ice bodies, a relict of the Little Ice Age (LIA) glaciation. Both factors might also contribute to the increased flow velocities between 1938 and 1953, which are still recognizable in the 1953–1970 time step. Although both lobes follow a general similar trend, which is in line with the alpine‐wide trend of flow velocity acceleration in the 1990s, differences in the geomorphic development of the two lobes were identified. In addition to a slightly varying evolution of the flow velocities, the timing and magnitude of the volume changes are different. Furthermore, both lobes display a dissimilar mechanism of frontal advance over the entire study period. Because the external forcing is identical, the varying development might be attributed to variations in internal structure, bedrock topography, or upslope connection of the lobes. Due to the lateral constriction, the subsurface topography, and the LIA maximum extent of the glacier, it is assumed that the geomorphic development of the Innere Ölgruben rock glacier, particularly before 1953, represents a special case, and the results are not simply transferable to other rock glaciers.

High-quality in situ measurements are essential for hazard assessment of debris flow events. However, precise data on debris flow triggering thresholds, accumulation volumes and spatial characteristics of large-scale events on catchment scale are scarce due to the rare occurrence of debris flows and the challenges of acquiring accurate data for a larger area. In this study, we present quantitative analyses of a single extreme debris flow event in the Horlachtal, Austria, triggered by local high-intensity short-duration precipitation events on 20 and 23 July 2022. Pre- and post-event airborne LiDAR (light detection and ranging) data with a high spatial resolution reveal that 156 different debris flow processes were initiated during these events, with accumulation volumes of up to approximately 40,000 m³. The calculated debris flow deposition volumes also show a power-law relationship with the total amount of rainfall in the respective debris flow catchments. The spatial appearance of the debris flows shows a concentration of processes in a particular area rather than a uniform distribution, suggesting a local nature of the triggering event. This is further supported by the measurements from three meteorological stations and four discharge gauges within the study area. The gridded area-wide INCA (Integrated Nowcasting through Comprehensive Analysis) rainfall data further point to a local convective event on 20 July 2022, with a maximum rainfall intensity of 44 mm/h.

<p>Deglaciation in high mountain areas signifies the transition from glacial to periglacial conditioned landscapes. Due to the reduced melt rate of debris coved glacier ice, these parts of the glacier might persist long after the surrounding glacier has melted, forming periglacial landforms in the post-glacial landscape. Therefore, in this case study, we examine the geomorphological development of a recent 19267 m³ ± 204 m³ rockfall from the glacier headwall on the small, low elevated Zwieselbachferner in the Horlachtal, Stubai Alps, Austria. The multi-epochal analysis is based on different remote sensing data (photogrammetrically and airborne laserscanning derived digital elevation models, orthophotos and satellite data) and covers the period from the occurrence of the initial rockfall in 2003/2004 until 2022. Results show that the headwall in this area is still very active, supplying 13 further rockfalls of varying magnitude to the debris covered glacier part during the study period. The debris cover created by rockfall, estimated to be several meters to a few decimeters thick, causes the surface elevation change of the glacier to decrease by a factor of 5 to 6 compared to the surrounding glacier. This results in the formation of a steep front and flanks, which become progressively covered and thus isolated by debris redistribution. In contrast to the surrounding glacier, whose thickness and length has strongly decreased during the study period, the mean ice thickness of the debris-covered area only decreases from 23.5 m to 21.8 m between 2006 and 2022. The extrapolation of ice thickness development shows that this part of the glacier will remain as a debris covered, ice-cored landform after the complete melting of the surrounding glacier. As glaciers melt rapidly, ELA rises and glacier headwalls become more unstable due to glacier melt and permafrost warming, we expect this process to occur more frequently in the future and in some cases to shape the appearance of formerly glaciated landscapes.</p>

In order to get a better understanding of the future development of alpine slope-type debris flows in the frame of climate change, complete and gapless records of the last century for this type of geomorphologic process are necessary. However, up to now such records have been scarce. Here, the slope-type debris flow activity in Horlachtal, Austria, has been investigated since 1947 with the help of historic and recent area-wide remote sensing data. Using geomorphological mapping, both spatial and temporal variabilities in debris flow dynamics can be shown. The results indicate short-term variations rather than consistent increasing or decreasing trends of slope-type debris flow activity in Horlachtal. Specifically, three active periods between 1954 and 1973, 1990 and 2009, as well as 2015 and 2018, can be registered. Analyses of the deposited debris flow volumes show that for parts of the study area the largest volumes appeared in the early 1990s, which might have even influenced the dynamics in the following years. Studies on the spatial variabilities revealed differences of slope-type debris flow activity within the study area and point to local rainfall events as triggers. However, long-term precipitation data of high temporal resolution of two alpine meteorological stations do not reveal increasing or decreasing trends in the occurrence of such events.

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.

<p>The system of proglacial streams in the Alps has experienced significant changes since the end of the Little Ice Age. Previous studies showed different patterns of aggradation and degradation in proglacial channels over time. This leads to the question of which factors determine the sediment dynamics in the channels and on their floodplains in the long term with ongoing glacier melting. Possible influencing variables are the distance of a channel section to the recent glacier tongue and the percentage of glaciation in the catchment area. Moreover, we suppose an influence of local topographic characteristics such as the slope gradient and the width or confinement of the channel. In addition to these factors, there is also the question of whether large individual events overlay a trend of aggradation or degradation.</p> <p>In order to analyse the long-term sediment dynamics in channels and the factors influencing it, we used numerous digital elevation models (DEMs) covering several decades and different streams within three main catchments (Kaunertal and Horlachtal in Tyrol and Martelltal in South Tyrol). The DEMs were generated from aerial images dating back until 1953. From the 2000s on, airborne LiDAR datasets and DEMs based on drone images were available. This data basis enables a comparative investigation and the identification of local topographic influences.</p>

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.