## 4 Discussion
### Parameters Affecting the Rearrangement of Deposited Microplastic Contamination  
The data reflect that the rearrangement of the MPs is not a straightforward process, as the amount of the MP did not vary to the same extent between the section and the in-channel forms. The _slope_ of the river mainly influenced the reduction in MPs from the sediments. The most significant decrease was observed in the sections with the highest and lowest slopes. In the uppermost section (S1) and the tributaries, the six subsequent flood waves with gradually increasing heights effectively mobilized the polluted sediments and transported them to the middle section of the Tisza. Simultaneously, in the section (S4) with an extremely low slope where the impoundment propagated the aggradation of MPs, the flood waves effectively mobilized the bottom sediment. In this case, the flood waves were relatively low; thus, neither the Danube nor the tributaries impounded them so the MP-contaminated sediments could be mobilized, and the sections (S4 and S5) became less polluted. This also suggests that the mobilized MPs probably reached the Danube. A similar spatial pattern of mobilization was described from the Mersey, UK [20]. However, [PERSON] et al. [23] pointed out that this rearrangement pattern may vary between hydrological situations, and the removing of MPs from the water system is gradual. The _tributaries_ play a key role in the replenishment of the MP load of the main river, as they transport polluted water from the sub-catchment in varying degrees (depending on the population and wastewater management). According to our results, the tributaries mainly determine the mean pollution of a section, especially in the close vicinity of the confluence. This is supported by the fact that 9 of the 11 tributaries had a higher (+20%) MP content in their sediments than the Tisza had near the confluence, and they increased the MP content of the Tisza by 52% on average. In addition, as larger tributaries join the Tisza with more and more transported sediments with more MPs, they effectively increase the general pollution (minimum and mean values) of the main river. Several authors also indicated the importance of tributaries as sources of MPs for the main river [29; 53; 59; 60]. The tributaries act as the conveyance belts in MP transport into the main river [19; 61], especially if the wastewater treatment plants are located on the tributaries instead of the main river [7; 49; 58]. On the catchments of tributaries, the high population density or dynamic economic growth can also effectively increase the MP pollution of a tributary, which finally increases the main river's contamination [23]. Three different _geomorphological forms_ were sampled along the Tisza River to see whether the MP content of the sediments was affected by the morphology of the sampling site. The point-bars developing in the convex side of the meanders proved to be the most polluted (mean: 2049 \(\pm\) 1218 item/dry kg), while the side-bars of the sinuous sections had less MP content by 18% (mean: 1692 \(\pm\) 1335 item/dry kg), and sediment sheets that formed along straight sections contained less MP than the point-bars by 23% (mean: 1584 \(\pm\) 1464 item/dry kg). Meanwhile, the point-bars had the most uniform MP distribution, without extremely high values. This can be explained by the fact that the material of the three forms was deposited under different hydrological conditions. While sedimentation on point-bars and side-bars is continuous and can take place at any stage, the deposition on sediment sheets can happen just during the falling limb of floods. In addition, the bars are built of mainly bed load, whereas the sediment sheets are built up by suspended sediments. Thus, it can be assumed that the different MP content of the forms is related to the fact that their sediments were deposited at different times when the MP flux was different.