## 1 Introduction
[13] reported that in the upper section of the studied river, where the slope and flow velocity were higher, the MP concentration in the water was lower than in the downstream sections with lower flow velocity, so the MP concentration in the water increased proportionally downstream. The deposition of the MP particles and its influencing factors are also not yet fully understood. The deposition of naturally transported suspended or bottom sediments is determined by flow velocity [17; 22]. It is probably also applicable for MP particles because, in general, the higher the flow rate, the more particles are transported and the less that are accumulated [17; 19]. However, the effect of the water discharge on the MP deposition is not clear, as [PERSON] et al. [23] found that within one year, during high stages, only slightly more MPs (100-629 item/dry kg) were deposited in the sediment than at low stages (118-514 item/dry kg). [PERSON] et al. [20] investigated the rearrangement of MPs in a fluvial system by a flood. While the MP content (300-4800 item/dry kg) of the sediments of even small streams was high before the studied flood, the flood rearranged the MP pollution. Therefore, the MP concentration decreased by one order of magnitude after the flood in almost half of the study sites. They estimated that 70% (approx. 43 \(\pm\) 14 billion) of the previously accumulated MP particles were mobilized and transported into the sea. [PERSON] et al. [20] highlighted that a flood wave could significantly rearrange the hot-spots of the MP pollutions; in addition, during the rearrangement, the MP concentration in the water can vary. The mobilization and deposition of natural sediments are influenced by the particle size, morphological properties of the grains and their density [22]. Microplastic particles tend to form aggregates with organic and inorganic particles, which increase their particle size, density and sedimentation rate, so the concentration of the MP particles in sediments may be higher than in the water [14]. In addition, the MP particles tend to accumulate in the sediment due to their hydrophobic properties [14; 28]. Lower-density MPs have greater mobility, allowing them to travel greater distances [16], while higher-density plastic particles can only travel shorter distances [28]. Along longer sections, the flow velocity changes, which can also influence the amount of deposited MPs. In general, the MP pollution in sediments is distance-dominated and caused by flow velocity changes [13; 26]. However, if the flow conditions change, for example, the temporal impoundment terminates, the deposited MP and the aggraded sediment could be mobilized. Thus, the MP concentration in the sediment might decrease, terminating the previous downstream trend in the MP pollution [13]. Dams and reservoirs can also slow down the transport of the MPs as they support the accumulation of the sediment and MP particles [15; 29]. The effect of the geomorphological setting on the MP deposition was studied just at confluences and not in other geomorphological forms. In confluences, the flow conditions can be fundamentally influenced by the impoundment of the two joining rivers; thus, simultaneously with a dropping flow velocity, the amount of deposited MPs increases [29; 30]. However, within a river system, various sedimentary bodies occur (e.g., various bars), formed under different flow conditions and built of various grain sizes. As the sediment samples for the MP analysis are collected from these very different forms, it is necessary to understand their geomorphological differences, as the comparison of the MP data from various forms without considering their sedimentary conditions might be misleading. The fluvial system of the Tisza River (Central Europe) is highly polluted by MPs based on our previous results [29], as in 2019, the amount of the MPs in the freshly deposited sediment varied between 515 and 8067 item/dry kg.