## 5 Conclusions  
The transported MP pollution in rivers poses new challenges to the research community. While a very wide range of MP research is concerned with the chemical and physical properties of the MP particles, a relatively limited number of studies are investigating the geomorphological context of MP pollution. However, the hydrological and geomorphological aspects of the MP transport, deposition and mobilization would be very important, as these are complex processes, which fundamentally affect the MP concentration in sediment measured at a given point at a given time.  
Therefore, we analyzed the spatial and temporal characteristics of the MP accumulation and rearrangement, conducting two consecutive sampling campaigns. In 2019, we collected 62 samples from 59 sites, but in 2020, 120 samples were already collected from 70 sites from the source of the Tisza in Ukraine to its Danubian confluence (Serbia). Our results show that smaller, below-bankfull flood waves can effectively rearrange the MP in the water system. Therefore, to understand the spatio-temporal changes in MP pollution, it is worthwhile to repeat the surveys on a river at the same points, sampling as many sites as possible. A series of data from several years is likely needed to understand the MP transport and deposition processes in a river system; thus, the role of various hydrological situations (from low stages to peak floods) could be understood.  
While researchers are more or less familiar with the modes of natural sediment transport, the deposition and mobilization conditions of the MP particles are different from natural sediments due to their varied density, size and aggregate formation. The MP, which forms aggregates, can behave similarly to bed-load sediments, though suspended transport can also be significant. As sediments transported in different ways are deposited on different in-channel forms, it is important to compare the sampling sites in terms of geomorphology. Because different in-channel forms trap various sediments to different degrees, samplings should be performed from the same morphological forms, and during the evaluation of the MP contamination, the evolutionary characteristics of a given form should be considered. According to our results, the MP content of the point-bars is much larger than other in-channel forms, and they show greater downstream and temporal changes. We recommend collecting samples from side-bars, which are common forms in rivers, and they reflect the evolution of the MP content of river sections. The sediment-sheets are the least favorable forms for MP sampling, as their longitudinal variations are different than of the other forms or the section averages, though, if the sampling aims to evaluate the MP content (in suspended form) of a falling limb of a flood, the sediment sheets are a perfect choice.  
Conceptualization, T.K. and A.B.; methodology, A.B.; formal analysis, A.B.; investigation, S.G., T.N. and M.M.; resources, T.K. and A.B.; writing--original draft preparation, A.B.; writing--review and editing, T.K. and A.B.; visualization, S.G.; project administration, T.K., S.G. and M.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Hungarian Scientific Research (OTKA K:134306).  
Not applicable.  
The authors declare no conflict of interest.