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.