# Spatial Variations in Microfiber Transport in a Transnational River Basin
## 2 Materials and Methods  
### Study Area  
#### 2.1.1 Geographical Setting  
The study was performed along the Tisza River from its spring in Ukraine to its confluence with the Danube River in Serbia (Figure 1). Our research is unique from the point of view, that (1) along a quite long (946 km) river high number (26) of water samples were collected for SSC and MP; and (2) the measurement was repeated in two subsequent years. The Tisza River drains the eastern part (157,000 km\({}^{2}\)) of the Carpathian Basin, Central Europe. The catchment area has a mountainous character in Ukraine, Romania, and in east Slovakia, whereas the lowland parts are in Hungary and Serbia [49]. The Tisza and its tributaries usually flood in early spring (March-April) and early summer (June-July); meanwhile, the river has long-lasting low stages from early summer to late winter (August-February). The Tisza River has three reaches (Upper, Middle and Lower Tisza); however, we divided them further into sections (S1-S5) based on the hydrological and morphological characteristics of the channel and the catchment (Figure 1). The Upper Tisza (946-688 km) along its upstream section (S1) has a steep-sided, deep valley with a high slope (20-50 m/km), and thus the water velocity (2-3 m/s) is the greatest in this section. In the downstream section of the Upper Tisza (S2) the channel gradually widens, and the slope decreases (from 110 to 13 cm/km), thus the flow velocity decreases to 1 m/s [50]. Due to the great slope of the Upper Tisza, floods here usually last just for a few days. The maximum discharge of the reach is 3360 m\({}^{3}\)/s (at Tiszabecs), whereas the mean is 197 m\({}^{3}\)/s, and the minimum is 29 m\({}^{3}\)/s [50]. The height difference between the highest and lowest water stages is 10.0 m [38]. The high bedload transport (22.6 thousand m\({}^{3}\)/y) is attributed to low suspended sediment load (0.9 million m\({}^{3}\)/y) [51]. Similarly, the Middle Tisza (688-177 km) was divided into upstream (S3) and downstream (S4) sections (Figure 1). The slope of the meandering Middle Tisza decreases significantly, as at the upstream section it is \(\geq\)3 cm/km, but just further downstream it drops to 1-3 cm/km. Therefore, the flow velocity decreases by approximately 70% (S3: 0.1-0.5 m/s; S4: 0.1-0.2 m/s). Due to the significant decrease in flow velocity, seasonal floods last for weeks, and the bedload transport declines (by 56%) compared to the Upper Tisza (S3: 8.8 thousand m\({}^{3}\)/y; S4: 11 thousand m\({}^{3}\)/y). Meanwhile, the suspended sediment load increases five-fold (5 million m\({}^{3}\)/year) in the upstream section and three-fold (3.3 million m\({}^{3}\)/year) in the downstream section. Compared to the Upper Tisza, all characteristic discharge values increased (at Szolnok Q\({}_{\text{max}}\): 4336 m\({}^{3}\)/s; Q\({}_{\text{mean}}\): 738 m\({}^{3}\)/s; Q\({}_{\text{min}}\): 58 m\({}^{3}\)/s), and the elevation difference between the highest and lowest stages also became greater (11.95 m). The water and sediment budget of the reach is influenced by  
Figure 1: **(A)** The study area is located in Central Europe. **(B)** The research was performed along the Tisza River (Sites a–z) and its tributaries (Sites E–L). The flow of the Tisza is controlled by three dams at Tiszalök (TD), Kisköre (KD) and Novi Becej (ND). The grey colors indicate the sub-catchments of the sections (S1–S5). several, large tributaries (e.g., Szamos, Kraszna, Bodrog, Sajo, Zagyva and Koros Rivers) and two dams at Tissalok and Kiskore [38]. The hydrological and morphological characteristics of the Lower Tisza (177-0 km) are similar along the entire reach; thus, the whole reach was considered as one section (S5). At the very beginning of the reach, the largest tributary of the Tisza (Marosover) joins (Figure 1); however, there are no more tributaries downstream. This reach is highly affected by the Novi Becej Dam; thus, the slope drops to 0 cm/km during low stages.