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.