1. Introduction
Due to global warming and changes in precipitation pattern, high alpine geosystems have undergone considerable changes since the end of the Little Ice Age (LIA) around 1850 [
1,
2]. This includes rapid retreat of glaciers [
3,
4,
5,
6], permafrost-thawing [
3,
7], changes in alpine vegetation [
8,
9], and water discharge [
10,
11,
12].
As a result of this glacier loss, a large part of the formerly glaciated terrain has been exposed during the recent decades. On these lateral moraines, intense morphodynamics have taken place due to paraglacial reworking [
1,
2,
13,
14,
15,
16,
17,
18]. The high morphodynamics of these slopes and the spatial proximity to the channel system can lead in turn to an increased sediment transfer from the slopes to the channels [
15,
19,
20,
21,
22,
23,
24], which can also lead to sediment input into reservoirs used for hydropower [
25,
26]. The sediment regime of mountain slopes can be described by complex sediment cascades [
27,
28,
29,
30,
31], which include various geomorphic processes [
26,
32,
33]. These processes are in turn linked to external forces, e.g., precipitation events, and to system-internal dynamics. The latter represent, e.g., process interactions such as geomorphic (de)coupling, describing sediment transfer on a small spatial scale, and sediment connectivity, describing the integrated coupling state of a system at the meso and macro scales [
34,
35]. In recent decades, geomorphological research has focused on this in different concepts [
36,
37,
38,
39,
40,
41]. These coupling states determine the transport from the slopes to the channels (lateral sediment transfer) [
37,
38,
39,
42,
43,
44], as well as the sediment transport in the channels (longitudinal sediment transfer) [
36,
38,
39,
45,
46,
47,
48] and are subject to changes in different timescales [
42,
49]. In addition, a distinction is made between structural and functional connectivity [
50,
51,
52]. Structural connectivity describes the extent to which different landscape units are adjacent or physically connected to each other [
51], while functional connectivity describes the process dynamics, e.g., geomorphological processes through which sediment or water is transferred between different landscape elements [
51].
Proglacial areas are affected by strong geomorphological changes [
18], as these are vulnerable to numerous geomorphic processes, like fluvial erosion, slope wash, mass movements, debris flows and the sediment transport through ground avalanches due to the unconsolidated material, the missing or sparse vegetation, the high slope gradients and the occurrence of ground ice [
14,
15,
21,
24,
26,
33,
53]. Several studies showed a decrease in morphodynamics on LIA lateral moraines after a certain time of deglaciation, while other studies showed an ongoing intense morphodynamic even after several decades. Church and Ryder (1972) [
13] indicated an increase in geomorphological activity by fluvial erosion after deglaciation, which continues as long as sediment is available, while Ballantyne (2002) [
14] used a conceptual model (exhaustion model) to show that the reworking processes decrease exponentially over time in a system that is considered sensitive and susceptible to disturbances, which can cause delays. Curry et al. (2006) [
15] classified the geomorphological activity (of mainly debris flows) over time and demonstrated that the highest phase occurs after about 50 years and a levelling by filling of the gullies occurs after 80–140 years. Furthermore, Carrivick et al. (2013) [
54] indicate that geomorphological activity (areal extent and intensity) decreases with increasing distance from the glacier. This decrease in activity with increasing distance from the glacier is also described by Dusik (2019) [
23]. However, the latter also describes an ongoing intense morphodynamic and no stabilisation of these areas after several decades of deglaciation. Betz et al. (2019) [
55] showed in a comparison of several test sites of different lateral moraines both a decrease and an ongoing high morphodynamic after several decades, caused by thawing of dead ice, which can take decades [
56], high slope gradients and undercutting of the slopes by the adjacent rivers. In addition, Cossart and Fort (2008) [
17] also showed that the conceptual models, such as those of Ballantyne (2002) [
14] and Curry et al. (2006) [
15], are not always applicable, since reaching the maximum sediment yield of these slopes can be influenced by various parameters (e.g., the complexity of the depositional forms) and the paraglacial adjustment process can thus be delayed over several decades. After ice release, the developed vegetation can also lead to stabilisation [
57], while heavy rainfall events can lead to sediment transporting processes [
58,
59] and a higher slope channel coupling [
21,
23,
24].
The reconstruction of morphodynamic changes (surface change detection) in proglacial areas can be analyzed in high spatial and temporal resolution by processing accurate and precise digital elevation models (DEM) in different time intervals and deriving the height change from the resulting DEM of differences (DoD). Thus, sediment budget studies can be determined quickly and easily. The use of different remote sensing techniques for the acquisition of repeated topographic data also allows this approach to be applied to a time interval of several decades by analysing overlapping historical aerial images, using photogrammetry [
16,
55,
60,
61,
62] and current LiDAR data (light detection and ranging) from airborne or terrestrial platforms [
63,
64,
65,
66]. The sediment budget approach, widely used in geomorphological research with different methods, allows to detect surface changes of sediment sources and sinks, to interpret the geomorphic processes and to analyse and quantify the amount of erosion, accumulation, re-mobilisation, and finally the sediment volume balances of geomorphic systems, e.g., in rivers [
67,
68,
69,
70,
71,
72] or on mountain slopes [
73,
74,
75,
76,
77]. In addition to DoD analyses, geophysical measurement methods, such as ground-penetrating radar, can be used to investigate accumulation areas and their different types of sedimentary structures [
78,
79]. There are many short-term morphological (sediment) budget studies (several month or years) in proglacial areas [
21,
22,
23,
24,
26,
80], but quantitative long-term studies over several decades, involving multiple processes and different time periods on lateral moraines and the resulting sediment transfer from slopes to mountain streams are rare due to the difficulty of access to the terrain and the lack of long-term data [
26]. Therefore, there is an increasing need for research in these areas, which includes the monitoring of these systems through repetitive and high-resolution topographic data [
53].
Thus, this study aims to analyse and quantify two long-term periods (1970/1971–2006; 2006–2019) of sediment erosion and accumulation balances on three test sites on LIA lateral moraines in an alpine catchment area (Upper Kauner Valley, Central Alps, Tyrol, Austria), using the morphological sediment budgeting approach [
81]. Moreover, the resulting sediment transfer from the slopes into the proglacial river system was measured. The reconstruction of multitemporal digital elevation models (DEMs) is based on photogrammetry with historical aerial photos, airborne laser scanning (ALS) and terrestrial laser scanning (TLS) for the years 1970/1971, 2006, and 2019, respectively. The long-term morphodynamic analysis of selected LIA lateral moraines takes into account the different characteristics of the areas of interest (e.g., size, slope length, slope gradient, and elevation), the occurring vegetation and the changing meteorological and hydrological conditions of this high alpine geosystem.
2. Study Area and Locations of the Measurements
The test sites are located in Upper Kauner Valley in the Ötztaler Alps, which is part of the Central Austrian Alps (Tyrol/Austria) and belongs geologically to the Austroalpine crystalline complex [
82,
83]. The bedrock lithology is dominated by crystalline rocks, mainly ortho- and paragneisses [
84]. The valley is oriented in a north–south direction and borders the main Alpine divide and Italy in the southern part. The elevations range from 1810 m (Gepatsch reservoir) to 3535 m a.s.l. (Hochvernagtspitze) and 29.7% (2015) of the 62 km
2 catchment area is glaciated [
85]. The Gepatsch- and Weißsee glacier reached their maximum extent at the end of the LIA around 1855, which was investigated, e.g., for the Gepatsch glacier by Nicolussi and Patzelt (2001) [
86] and shown by Sonklar (1861) [
87]. In 1886 and 1887, the Gepatsch glacier and the adjacent areas were first measured trigonometrically by Finsterwalder et al. (1888) [
88]. This measurement was repeated in a survey in 1922, this time also including the Weißsee glacier [
89]. Since having reached the maximum extent, these glaciers have continuously lost ice mass and length, with two exceptions between 1920/1921 and 1977 to 1988, when minor re-advances occurred, as documented, e.g., for the Gepatschferner [
86]. Since the end of the LIA until 2015 the Gepatsch glacier has lost about 2.8 km and the Weißsee glacier about 1.8 km of length. Due to the good data basis (e.g., maps, terrestrial, and aerial images/orthophotos) of these glaciers since the end of LIA, the glacier extents could be continuously documented for the last decades (
Figure 1). The three test sites are located on the lateral moraines of the Gepatsch glacier (named G1 and G2), drained by the Fagge River, and the Weißsee glacier (named W1), drained by the Riffler River (a tributary to the Fagge River). The test sites are characterised by intense paraglacial morphodynamics and low vegetation cover. The material of the slopes can be described as typical moraine material, which is unsorted and contains grain sizes from silt to boulders. The valley belongs to the inner-alpine dry province and is described as a continental one with low mean annual precipitation values and is one of the driest regions of the Alps [
26,
90].
Figure 1 and
Table 1 give an overview.
5. Discussion
Several previous studies already calculated morphological sediment budgets of test sites in the proglacial area of the Upper Kauner Valley [
21,
22,
23,
24], or e.g., in a catchment in southern Switzerland in the proglacial area of the Bas Glacier d’Arolla, Val d’Hérens [
80]. However, these studies only used databases covering time ranges of months or years and not decades. The study by Heckmann and Vericat (2018) [
22] shows a sediment budget for the period of 2006–2012 from the G2 test site used in this study (a slightly different area of interest). Compared to the mean annual values of this study (1970/1971–2006 and 2006–2019), the decrease in the efficiency of sediment transport of the G2 test site can be confirmed. While 66% of the gross erosion of G2 was exported between 1970/1971 and 2006 (this study), 57% was transferred to the Fagge river between 2006 and 2012 [
22] and 47% from 2006 to 2019 (this study). The study by Heckmann and Vericat (2018) [
22] indicates a mean annual erosion of −1384 m
3 and a mean annual accumulation of +591 m
3 (values were calculated on the basis of the total morphological budget given in the study), while this study (2006–2019) shows a mean annual erosion of −1343 m
3 and a mean annual accumulation of +716 m
3. This shows that the erosion values have not significantly changed since the work of Heckmann and Vericat (2018) [
22]. Only a slight increase in accumulation can be stated. Dusik et al. (2019) [
24] also calculated a morphological sediment budget of the test site G2 from 2010 to 2015 (slightly different defined test site borders). For this period, a total erosion volume of −7089.6 m
3 (mean annual value: −1418 m
3) and a total accumulation volume of +4122.2 m
3 (mean annual value: +824 m
3) are reported, which corresponds to a net balance of −2967.4 m
3 (mean annual value: −593 m
3), meaning that 58% of the eroded material was exported from this test site. Thus, it can be shown, that there were slight changes in the morphological sediment budgets of this test site in the period from 2006 to 2019, but that clearly less sediment was eroded in comparison to the period from 1970/1971 to 2006 (this study).
The characteristics of the test sites (in particular, size of the test site, slope length, and the mean and maximum slope gradients) have a clear influence on the morphodynamics. The differences in these are related to the fact that the glacier forefield of the Gepatsch glacier was shaped much stronger than the forefield of the Weißsee glacier due to higher glacial erosional force. As a result, the test sites G1 and G2 are larger, have longer slope lengths and have higher mean and maximum slope gradients, in contrast to W1, which probably leads to higher morphodynamics due to the higher relief energy. In addition, the higher elevation of W1 (~500 m height differences) and the associated shorter spring and summer periods, as well as the resulting longer snow cover, can reduce the vulnerability to trigger geomorphological processes, for example by precipitation events [
32]. There are no differences in the deglaciation of the test sites (start of deglaciation ~1855/End of LIA and completely deglaciated between 1953 and 1969). Nevertheless, Dusik (2019) [
23] identified a decrease of the morphodynamics on the lateral moraine of the Gepatsch glacier with increasing distance from the glacier by using more test sites.
Dead ice areas could be identified in the first period due to the spacious lowering of the terrain without detectable geomorphological processes. However, it cannot be guaranteed that the dead ice has been fully mapped and completely excluded from the volume calculations. For example, the “real” deposition areas may be slightly higher than shown in the DoDs, as the thawing of the dead ice may have slightly lowered the terrain. No dead ice areas were detected in the DoDs of the second period. However, the analysis of the origin of the springs which emerges from the lateral moraines of the Gepatsch glacier (including the test sites G1 and G2 of this study), by relative age dating (I
129), shows that this water originates to a certain part from dead ice remaining in the lateral moraine [
56]. However, during the field work in 2019 and 2020 no exposed ice was seen on the test sites.
The mean annual erosion and accumulation volumes of these two determined long-term periods were strongly influenced by individual fluvial processes or gravitational mass movements, such as landslides. The described landslides at G1 in the first period and at W1 in the second period must be highlighted here. Due to the long periods that were compared, it is difficult to clearly define individual processes, except those which cause major surface changes like for example large landslides. Nevertheless, clear changes in the total morphodynamics and the corresponding slope channel coupling of the different test sites and in the different periods can be seen.
While at the steep test sites almost no vegetation occurred due to ongoing erosion processes, late successional vegetation was found in areas, which are more stable and have slightly lower slope gradients. This indicates that, in contrast to findings by Eichel et al. (2018) [
57], slope stabilization by vegetation and related soil formation is hindered when the morphodynamic processes are too strong, and that later successional stages only start to occur at steep slopes with fewer morphodynamic processes.
Extreme rainfall events in summer play a major role in the morphodynamics, and therefore represent the most active time of sediment transport processes in alpine catchment areas [
23,
24,
58,
59]. The study by Haas et al. (2012) [
21], which was also conducted on the slopes of this study (similar area of interest as G1 and G2), shows that 71% (2526.8 m
3) of the material transported by debris flows between August 2010 and September 2011, which were mainly triggered by one heavy rainfall event, accumulated at the foot of the slope, whereas 29% (1033.5 m
3) was transported into the Fagge River. The work of Dusik (2019) [
23] shows the triggering of sediment transport processes by precipitation events on the lateral moraines of the Gepatsch- and Weißsee glaciers, with a higher temporal resolution (several years as well as seasonally), and found that heavy rainfall events lead to sediment transport processes and thus to a higher slope channel coupling. Moreover, the analysis of Dusik (2019) [
23] revealed that even rainfall events of 10 mm/h are sufficient to trigger the transport of sediment on slopes and the transfer to the adjacent Fagge river. Sediment transport is highly dependent on moisture conditions, sediment availability and precipitation intensities. In addition, the spring months can create important preparatory conditions for sediment transport processes due to snowmelt on the slopes and the associated moisture [
24]. A positive correlation between the number of mass movements and the number of threshold exceedances of extreme daily precipitation sums and the number of extreme precipitation intensities was detected [
23]. By comparing the morphodynamics of this study within the two long-term periods with the precipitation events and with the runoff, it can be observed that the periods differ clearly regarding the morphodynamics and the meteorological and hydrological conditions. It can be assumed that there is a direct connection between the higher morphodynamics and the higher precipitation and the high runoff events especially in 1987 and 1988 and the high morphodynamics in the first period and the lower precipitation events and lower morphodynamics in the second period. Although the daily total precipitation (between 10 and 60 mm/d) of the weather station DF are increasing, the 15-min and 1-h events in particular are clearly decreasing.
Based on the results of the negative net balances on all test sites in all periods, a strong slope–channel coupling can be stated, but with a clear decrease from the first to the second period. For the test site G1, the export from the gross erosion decreased from 31% to 19%, for G2 from 65% to 46% and for W1 from 61% to 27%. This is consistent with the observations, e.g., of Harvey (2001) [
42], that coupling states can change over longer and also shorter time spans after extreme events [
21]. This shows that these coupling states can vary greatly in space and time [
39,
49]. Thus, the existence of functional connectivity, controlled by gravity and water [
40] between two sediment cascades [
30], the hillslope and the adjacent mountain streams is demonstrated.
Due to the higher runoff of the adjacent proglacial rivers and in combination with heavy precipitation events, the alluvial fans and debris cones were regularly and clearly undercut, especially in the second period. Thus, the formation of terraces and buffering floodplains is strongly influenced, as these are constantly being reworked. [
22,
47,
48]. It can be assumed, that the undercutting of the slope could have an impact on the morphodynamics of the slopes in the sense of a process response system by, e.g., preventing the filling of the gullies and thus levelling or flattening the hillslope channel and slope profiles, as sediments are transferred to the main river. This is in good agreement with the work of Haas (2008) [
31], who separated slope and adjacent river in a sediment cascade system, where both systems (slope and river) can influence each other and thus form a process response system.
The headcut of G1 and G2 still shows ongoing high morphodynamics on areas that have been deglaciated at least since 1886/1887 (~132/133 years ago,
Figure 1). This ongoing dynamic probably as a consequence of the undercutting of the slopes by the main river stays in contrast to the test sites of, e.g., Curry et al. (2006) [
15], where is showed a decreasing dynamic with time and a levelling of the slopes after about 80–140 years. The results of this study show that this seems only to be true for slopes, which are not affected by “external” processes like undercutting by rivers. For the test sites G1 and G2, it can be concluded that the paraglacial adjustment process is still in progress and sediment is still available to the Fagge river, but the dynamics seem to be decreasing. In contrast to G1 and G2, the test site W1 shows a clear increase in morphodynamics. If this is the consequence of the height difference between the G1/2 and W1 test sites (~500 m height difference) or the effect of the large landslide in the second period cannot be clearly stated and must be investigated in future work with a larger sample of test sites.
However, the study also shows that it is difficult to elucidate the nature of the occurring geomorphological processes only by DoD analyses, especially if longer investigation periods are used. To further clarify the nature of the dominant morphogenetic processes, additional investigations are necessary, such as using geophysics (e.g., ground-penetrating radar) to identify different types of sedimentary structures [
78,
79].