Microplastic particles are increasingly found in freshwater environments in a range of sizes, shapes, amounts and compositions. While recent studies have focused on the detection and quantification of microplastics in various river and estuarine environments, researchers have just begun to unravel the underlying transport and retention processes for the different microplastics particles. Of special interest in perennial rivers is microplastic transport and retention in the hyporheic zone, i.e. the biogeochemically active upper part of the riverbed and banks. To better understand the fate of polyamide fragments and fibers in riverine sediments, we conducted mesocosm studies at the Environmental Change Outdoor Laboratory (ECOLAB) facility of the University of Birmingham, UK. Twelve recirculating flumes (dimensions are 200 by 42 by 15 cm) were filled with pre-characterized gravel or sand sediment and 45L of water and flow velocities were recorded in each flume with a flowmeter. During one-day-flow experiments, the flumes were either exposed to polyamide fragments (sizes: 150-250 um and 400-600 um, pre-stained with Nile Red), red nylon fibers (500 um, 1.7 dtex) or a mix of both. Samples (20 mL) were taken over a course of up to 24 hours at three locations per flume at predefined intervals. After filtration on 0.45 um GF/D filters they were analyzed using a Zeiss Stemi 2000 stereomicroscope. Results showed that both fibers and fragments settled out of the water column over the course of the experiments and formed distinct depositional pattern on the sediment. While larger fragments mostly settled out within the first 30 minutes, smaller particles of 150 um or less as well as fibers remained in the water column significantly longer. Results will serve as input to further studies of these specific microplastics on resuspension and long-term behaviour under various environmental conditions.