5. Conclusion
For the Northern Caucasus region, where increased suspended sediment concentration is one of the environmental concerns, regional-scale studies on the temporal variability of sediment flux are lacking. This study explored temporal trends of suspended sediment load in the Terek River basin during 1925-2018 using observed suspended sediment discharge at 33 gauging stations. Our results provide one of the first robust assessments of the SSL change and clearly indicate its decreasing trend over the last decades. In general, the suspended sediment flux over the last hundred years changes relatively synchronously for most rivers in the Terek basin, draining the Northern mega-slope of the Greater Caucasus. This indicates a vital role of natural factors since the influence of anthropogenic factors differs significantly for different river basins.
The CUSUM charts analysis revealed a change point roughly within the same range of years at various gauging stations. As a result, several transition years are expected in the Northern Caucasus: increasing trends from the 1950s and decreasing trends from 1988-1994. The latter is most likely due to a decrease in glacier areas and a decrease in the area of arable land. It is critical for catchments with a high cropland fraction located in the foothill belt (< 500 m). At the same time, an absence of a pronounced trend or even an insignificant increase of suspended sediment flux was established for some river basins.
Our results were less clear for high-altitude catchments. Future research based on a more significant number and range of gauging stations located higher than 1000 m is required. Nonetheless, there are several reasons to expect that high-altitude gauging stations are less exposed to a considerable reduction in suspended sediment load (cf. Figure 6 ).
We found that summer precipitation controls up to 22% of mean annual suspended sediment discharge for the upper Terek basin rivers. Additionally, the reduction of croplands and glaciers has impacted the suspended sediment load. While it is impossible to measure their effect on the sediment yield quantitatively in this research, we suggest a qualitative assessment of their impact. The observed SSD changes in Malka, Kambileevka, Cheget, Ardon, and Sunzha are most likely connected to the collapse of the Soviet Union in 1991 and the subsequent land reforms and armed conflicts during the 1990s.