## 2 Materials and Methods  
### Study Area  
The research was carried out along the Tisza River and its tributaries, from its source in Ukraine to the Danubian confluence in Serbia (length: 946 km; catchment area: 157 thousand km\({}^{2}\)[31]). The river drains water from the eastern part of the Carpathian Basin, from Ukraine, Romania, Slovakia, Hungary and Serbia. The Tisza was divided into three reaches and five sections (S1-5 based on their hydrological and morphological characteristics) (Figure 1). The _Upper Tisza_ (946-688 r km) originates from two springs merging at Rahiv (Ukraine), and it eroded a steep-sided deep valley with a high slope (20-50 m/km). The rock-bedded valley gradually widens, and the slope of the channel decreases (5 m/km). In the slightly sinuous channel, side-bars were formed of blocks and gravel (\(d_{50}\) = 10-20 cm [31]). The Upper Tisza was divided into two sections. The sub-catchment of the _upstream section of the Upper Tisza_ (S1. ; sampling points No. 1-6) is located in Ukraine (Figure 1). It has the smallest area and population (Table 1). Although the sub-catchment represents only a small part (1.3%) of the entire catchment, it contributes significantly to the plastic pollution of the entire Tisza River [29]. The _downstream section of the Upper Tisza_ (S2. ; No. 7-19) formed a wide valley with a gradually decreasing channel slope (from 110 to 13 cm/km). As the channel-bed material is loose gravel, the bank erosion is fast, and in the braided-anastomosing channel, islands and bars have formed of sand gravel [32]. As the slope and flow velocity decrease, the medium grain size (\(d_{50}\)) of the bed load also decreases (from 50-100 to 2.5 mm [33]). The braided-anastomosing channel becomes meandering, as the suspended-sediment transport is 918 thousand t/y [31]. Along the downstream section of the Upper Tisza, several tributaries join the Tisza. The sampled tributaries (A: Shopurka, B: Tarac, C: Talabor and D: Nagyag rivers) represent the Ukrainian sub-catchment (Table 1). In the entire sub-catchment of the Upper Tisza, the low rate of waste and wastewater management is a critical environmental issue, especially in the mountainous and hilly sub-catchments. Here, waste management and wastewater treatment practically do not exist, as it is not profitable for companies to collect communal waste from the small settlements [34]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
Therefore, the communal waste is deposited in rivers or on their floodplains, and the wastewater is mainly drained directly to the river system. In the lowland Ukrainian sub-catchments, wastewater drainage systems are built in 68% of the large towns, in 58% of the small towns and only in 1.5% in rural areas [34]. If the wastewater treatment is considered, the situation is worse, as most treatment plants need reconstruction and further capacity building [34]. In the downstream section of the Upper Tisza (Hungary), the large-scale cross-border transport of macroplastic pollution has been a problem since the beginning of the 21 st century. The amount of drifting debris is 20-60 tons during a month with a flood wave, and it is collected by the local hydrological institute (FETIVZIG) at Vasarosnameny, Hungary [35]. Most of it is organic material (e.g., woody debris), but up to 300-500 bottles/minute could be drifted during floods [36]. The plastics drifting in the river and trapped along the shores are collected by a civil organization (PET Cup), so in 10 years, 600,000 plastic bottles and 56 tons of garbage were collected on the Tisza [37]. The _Middle Tisza_ (688-177 r km) was also divided into two sections. In the _upstream section of the Middle Tisza_ (S3; No. 20-31), the slope of the river drops to 3 cm/km.