Diverse malacofauna communities have been found in flood sediments accumulated in the Ponikiewka stream valley in the Beskid Mały range (Polish West Carpathians). The analyzed material included 60 taxa of molluscs represented by over 15,000 specimens. The composition and structure of malacofauna refer to the features of the environment in which sedimentation took place. The research indicates a limited scale of shell material transport within the valley. Longitudinal transport takes place over short distances, particularly in high-energy flows leading to mechanical destruction of shells. Transportation across the valley is inhibited by vegetation growing on the flat floodplain and plays only a marginal role in valleys over 30 m wide. Research on the deposition mechanisms in thanatocenosis that are currently emerging is very important for the methodology of interpreting the environment based on fauna communities found in Quaternary fluvial sediments.