Rivers are pivotal carriers of microplastic (MP) towards the oceans. Investigative data on MP pollution in rivers at specific timepoints is continuously compiled. However, such snapshot data can only roughly indicate the long-term extent of contamination and particle fluxes; modelling studies informed by this limited data are prone to large uncertainties. The present study sought to narrow this knowledge gap by examining the differences in MP concentrations, loads and compositions at three nival tributaries and the Rhine River in Basel, Switzerland, as well as two downstream pluvial Lower Rhine River locations in Germany over four seasons throughout 2016-2017. MP concentrations (>0.3 mm) correlated positively with average water discharge and catchment size of the evaluated stream locations and MP concentrations were significantly higher at the downstream pluvial than upstream nival sites. There was no coherent pattern in MP concentration fluctuations between seasons across the six sites investigated, and no correlation with recent precipitation. These findings suggest that temporal variations in MP fluxes towards the North Sea through the year are dominated by the different discharge regimes along the river course. This study also corroborates theoretical models that predict the highest MP loads move downstream the Rhine River during the European winter months.