<p>Passive transport is an important way of spreading to new localities for slow-moving animals such as snails. Such modes of transport, often mentioned in the literature, include zoochory or various human means of transportation. Watercourses are neglected in this regard, although they have been previously discussed as river corridors and proven routes for spreading certain plant species. Land snails can use them to move across recently fragmented landscapes, which may have helped snails shift ranges during the Quaternary climate cycle. We tested how such dispersal can occur in the field in two streams of different sizes with snails of two size categories. Our analysis revealed no statistically significant differences between the distances travelled by the species of varying shell sizes in the small stream. However, in the river, smaller shells travelled further than larger ones. Our experiment suggests that land snails spread along watercourses in small steps, colonising new habitats step by step rather than over long distances.</p>