Dispersal is an essential component of plant life, especially under the current threats of anthropogenic habitat fragmentation and climate change. For many wetland species, water is a key dispersal vector, as it can presumably disperse seeds long distances and towards suitable sites for establishment. Seed dispersal distance is affected by stream characteristics and seed traits. Yet, the effect of relevant seed traits, such as size, remains largely unknown. Here, we report on an experimental field study examining the effect of seed size on dispersal distance in lowland streams. We released cork seed mimics of different sizes in four Dutch lowland streams in restored and channelized sections. After 24 hours, we recorded their entrapment location, entrapment mechanism, and the vegetation type in which they were caught. Large seeds generally dispersed over longer distances than smaller seeds. This effect of seed size is likely caused by the different entrapment mechanisms – net trapping, surface tension and wake trapping – which were highly correlated with seed size. Especially net trapping was responsible for the capture of a large proportion of small seed mimics in vegetation such as aquatic and riparian grasses, starwort, and reed. Due to the prevalent occurrence of these vegetation types in lowland streams, particularly during summer, smaller seeds are more likely to become entrapped and, hence, disperse less far. Our analysis on existing seed data reveals that water-dispersed riparian plants have relatively large seeds and are thereby evolutionarily adapted to long-distance dispersal. Furthermore, our results indicate that median dispersal distances are 0.02-1.8 km (99-percentile <8.5 km) in lowland streams in summer. In winter, less vegetation is present in and surrounding the streams, which leads to median dispersal distances of 0.12-14.2 km (99-percentile <65 km). Synthesis: This study demonstrates that (1) large seeds generally disperse further than smaller seeds in lowland steams and (2) distances depend strongly on stream vegetation. This information should inform future restoration by, for instance, planning efforts to coincide with times or conditions of open water which are more favourable for the dispersal of target plant species – especially those with small seeds (<10 mm).