The ubiquitous pollution of various environments with microplastics (MP) is a global issue of growing concern. Freshwater environments, especially rivers, have gathered increasing attention as recent findings point out their role as relevant pathways of MP distribution. To see whether rivers with different characteristics carry different MP loads and polymer compositions, we investigated three rivers (city influenced, agriculture influenced and pristine) in South Eastern Norway regarding their MP concentration, loads, polymer composition and size class distribution. Water samples of 1 m3 volume were taken with a well-established, custom-build filtration device (UFO-system), allowing for sampling MP down to 10 µm. Samples were prepared for analysis applying a multistep enzymatic-oxidative treatment and separated into two size fractions (10–300 µm and 300–5000 µm) which were analysed via state-of-the-art spectroscopic methods and the smaller size fraction additionally applying a thermoanalytical method. MP concentrations and loads in the small size fraction (10–300 µm) were highest in the city influenced river (1067 MP/m3 and 2740 µg/m3) followed by the two rivers in the rural area (138 MP/m3 and 50 µg/m3 and 140 MP/m3 and 450 µg/m3, respectively). Furthermore, the thermoanalytical analysis revealed the presence of tire wear particles in the city influenced river. More than 70% of the MP were between 10 to 100 µm in size, and hardly any MP were larger than 300 µm. MP concentrations of the large size fraction (300–5000 µm) were 150–530 times lower than in the small size fraction. This highlights the importance of analysing the small size fraction (MP 10–300 µm) when considering the impact of land-based discharge of MP to the oceans, as the particle number, weight, and polymer composition of the larger size fraction (MP ¿300 µm) is not sufficient to gain the full picture.