4.3. Climate influence
The influence of climate change on the formation of sediment load in the rivers of the North Caucasus is quite complex, which is due to differences in microclimatic conditions both in individual altitudinal zones and even between individual river basins. At the same time, the limited number of meteorological stations does not allow assessing the specificity of the influence of microclimatic characteristics on individual river basins (Toropov et al. , 2019). In this regard, it is possible to estimate most reliably the contribution of changes in mountain glaciation to the suspended sediment flux. Paleoreconstruction and direct observations demonstrated that the retreat of glaciers in the Caucasus continues throughout the middle 19-20th century with a slight slowdown in the period 1980-1990s and the sharp increase during the last two decades (Solomina et al. , 2016; Tielidze and Wheate, 2018; Verhaegen et al. , 2020). That is, it does not correlate with global warming in the last quarter of the 20th century. With a glacier area reduction, denudation processes are activated in the proglacial zone, which is reflected, among other things, in an increase in the frequency of debris flows. By the first decades of the 21st century, the area of glaciation in the Caucasus decreased the most sharply (Shahgedanova et al. , 2007). The frequency of debris flows has also decreased due to a reduction in the glacial component of water and sediment fluxes associated with a diminishing in the glacier area (Retset al. , 2020).
Nonetheless, also scale dependencies in sediment sources can play a role. For example, hillslope erosion processes (landslides, rockfalls) are often considerable sediment sources in small mountainous catchments and highly sensitive to climate change. Apart from debris flow, the frequency of rockfalls and rock avalanches is reported to increase in the 21st century in the Caucasus (Dokukin et al. , 2020) and worldwide (e.g., Valderrama Murillo and Vilca, 2012; Byers et al. , 2019).
Overall, our findings are in line with the studies mentioned above. In addition, we found that the share of the glacierized area has a strong positive impact on SSL trends (Spearman r = 0.73, p < 0.0001). This indicates that SSL of more glacierized catchments is likely to decrease slower (Fig. 7b ) or even increase in some cases (cf. Table 2 ). Surprisingly, the impact of glacier cover is significant only for gauging stations located in lowlands (Fig. 8a ).
An indicator of the influence of local microclimatic conditions on the formation of water and sediment runoff in the proglacial zone is the different rate of glacier retreat, depending on differences in exposure, topography, and other landscape characteristics (Tielidze and Wheate, 2018). However, only the availability of water discharge measurements in combination with meteorological observations in the proglacial zone makes it possible to quantitatively assess the contribution of various microclimatic factors to the formation of river sediment flux. The opposite situation is observed on the Tibet plateau, where an increase in air temperatures provoked a sharp increase in the flow of water and sediment fluxes (Li et al. , 2020). This is due to the higher altitude of this region, which led to a time lag in the effect of global warming on the temperature regime.