2.1. Data
This research used sediment transport data from 33 gauging stations of
the Russian Federal Service for Hydrometeorology and Environmental
Monitoring (Roshydromet) located in the Terek River basin in the
Northern Caucasus (cf. Table 1 and Fig. 1 for
location). This dataset covers a period from 1925 to 2018 (seeTable 1 for details). It consists of average annual suspended
sediment discharges (SSD , kg·s−1) and annual
suspended sediment load (SSL , t·yr−1). In this
study we sometimes opt to an SSD as a proxy of SSL .
We have split our database into three groups according to the gauging
station altitude, i.e., low mountains (< 500 m), middle
mountains (500-1000 m), and high mountains (> 1000 m). It
should be noticed that bedload data is not available for rivers of the
Terek River basin.
### TABLE 1 ###
### FIGURE 1 ###
According to Handbook for hydrological measurements at these gauging
stations (РД 52.08.104-2002), suspended sediment data is collected using
depth-integrated sediment samplers. The samples should be mainly
collected once (at 8:00 AM) or twice a day (at 8:00 AM/PM), depending on
the river and its flow regime. Then water samples are filtered using
paper filters with a pore diameter of 2-3 µm. This is crucial, as, in
sediment transport studies, membrane filters with a pore diameter of
0.45 µm are more common (Kennedy et al. , 1974). Therefore,
uncertainties related to this issue are discussed in an appropriate
section.
Suspended sediment discharge in the upper Terek basin is characterized
by a seasonal pattern typical for an Alpine environment (Fig.
2 ). During winter (November–March), sediment sources are limited
because a significant fraction of the catchments is covered by snow, and
precipitation occurs in solid form. Streamflow is mainly determined by
baseflow (Rets et al. , 2018; Kireeva et al. , 2020), andSSD reaches the lowest values. In spring, SSD increases
when snowmelt-driven runoff mobilizes sediments along hillslopes and in
channels. Simultaneously, snow cover decreases, and rainfall events over
gradually increasing snow-free surfaces erode and transport sediment
downstream, resulting in SSD peaks in June-July.
### FIGURE 2 ###
Several Caucasian rivers are regulated with dams and diversions
constructed and operated for hydropower production (seeSupplementary 1 ). Most of them are small run-of-river
hydropower plants (<10 megawatts [MW]), but there are also
run-of-river medium-sized ones (10-100 MW). Such diversion projects aim
to channel a portion of the river through a canal or penstock to produce
electrical energy. However, the role and importance of flow diversion
for small hydroelectric power plants on suspended sediment transport are
currently unclear. Indeed, this type of hydropower plant in steep
mountain rivers has the lowest impact on sediment flow than other
hydropower plant types (Kondolf, 1997). A recent study by Csiki and
Rhoads (2010) shows that run-of-river dams have almost no impact on fine
suspended sediment flux if the dam is fully submerged. Moreover, the
transport of fine and coarse sediments should be unimpeded during high
flow periods (Angelaki and Harbor, 1995; Csiki and Rhoads, 2010). We
checked literature, photos, and satellite images for every of the
18th hydropower plants. We assume that only 8 of them
have altered the suspended sediment flux (see Supplementary 1 ).
These are mostly run-of-river dams with in-channel reservoirs, e.g.,
Zaramagskaia, Kashkhatau, Gizeldonskaia, etc. The others either have a
low-height dam (< 2 m), which leads to minimal upstream
sediment deposition (e.g., Kokadoiskaia, Fasnalskaia, Mukholskaia,
etc.), or they are located on irrigation channels (e.g., Psykhurey,
Akbashskaya) or lakes (e.g., Bekanskaya).