## 1 Introduction  
Simultaneously with the increasing amount of plastic used all over the world, more and more plastic waste is being released into the natural environment [1]. While larger plastics (e.g., macroplastics) spectacularly pollute the environment and degrade the aesthetic appearance of seas and riverbanks [2], microplastics (\(\leq\)5 mm) are virtually invisible contaminants of the environment [3, 4]. The rivers are important transport routes and sinks of the MP pollution, as [PERSON] et al. [5] estimated that by 2016 approximately 19-23 million tonnes of plastic waste were released into the aquatic ecosystem worldwide. Microplastics (MPs/MiPs) entering rivers can originate from two main sources: from inadequately or not-at-all-treated wastewater and from landfills, from where pollution (including plastics) is transported into rivers by the wind or surface run-off [6, 7]. Further sources of the MPs could be agricultural areas, especially if they were treated by biosolids and dehydrated sewage sludge [8, 9], industrial areas [10], roads (e.g., weathering of tires and road paints) [11, 12], construction sites or tourism [4]. Most of the MPs that enter the rivers are secondary, so they were formed by the fragmentation of larger plastics [13], but some primary MPs that are consciously manufacturedto this size, e.g., pellets in detergents [1], can also be released into rivers. In the vicinity of cultivated areas, the presence of fragments is common as the result of the weathering of green-house foils [14; 15], while in the vicinity of settlements, mostly fibers pollute the rivers [16; 17]. The reason for the appearance of fibers is that the current wastewater treatment technologies cannot completely filter out MPs [12; 15; 18]; therefore, river sediment samples collected at sewage outlets usually contain a high amount of MP particles [17; 19]. In treated wastewater, the most common MP form is fiber that has originated from the washing of synthetic clothes, but MP pellets are also released from cleaning agents [6; 15; 18], and in river sediments, they might be abundant [13; 20]. Various environmental factors can influence the transport of MP particles entering rivers. We hypothesized that MP particles are transported similarly to natural grains. Thus, MPs could be transported by the river in the water column as a suspended load or on the bottom of the channel as a bed load [17; 21]. Therefore, their transport is probably affected by similar factors as the natural grains. The concentration of natural suspended sediment is regulated by the amount of the fine-grained sediment eroded from the catchment [22]. Usually, the high suspended-sediment discharge is connected to high water discharge or floods, though plots of concentration against discharge often show a very wide scatter of points [22; 23]. There are ambiguous results on the connection between water discharge and suspended MPs. A negative correlation between the transported MPs and discharge was found by [PERSON] et al. [24] and [PERSON] et al. [25]. They concluded that stormwater-generated increased run-off is not a source of MP pollution, but it diluted the MP pollution from other sources. A positive correlation between the MP pollution, water discharge and catchment size was found by [PERSON] et al. [26] and [PERSON] et al. [27], though they could not reveal a clear temporal pattern in the MP transport between seasons. [PERSON] et al. [13] had similar results, as during low discharges, the MP concentration was just one-tenth of the concentration during high stages. There are uncertain statements on the spatial pattern of the MP transport as well. According to [PERSON] et al. [23], the MP concentration in the water decreased toward downstream; however, [PERSON] et al. [24] did not find a longitudinal downstream trend in the MP pollution in the water samples. On the contrary, [PERSON] et al.