Aim: Our aim was to investigate how the distance to water (flooding gradients) and riverine barriers (Tapajós and Jamanxim rivers) influence the distributional patterns of amphibian and squamate assemblages in the middle Tapajós River region, south-eastern Amazonia. We also considered the planned hydroelectric dams on both rivers in reviewing the possible impacts of these factors on the amphibian and squamate assemblages. Location: Middle Tapajós River system, south-eastern Amazonia. Methods: We conducted diurnal and nocturnal surveys combining pitfall traps and active searches along both banks of the Tapajós and Jamanxim rivers. We identified specimens using an integrative morphological, acoustic, ecological and molecular approach and evaluated the influence of riverine barriers and distance to water using uni- and multivariate ordinations, regressions and ANOVA. Results: We found changes in species composition for both groups along the flooding gradient and differential riverine barrier effects. The rivers restricted the distribution of 33% of the amphibian species and 8% of the squamates. For amphibians, the main distributional barrier was the Tapajós River, while for squamates both rivers were of similar importance. The assemblages most affected by riverine barriers were non-riparian amphibians and squamates, as well as riparian amphibians associated with small streams. The functional groups most affected were small, diurnal terrestrial amphibians and small–medium, diurnal partly tree-dwelling squamates. Main conclusions: The Tapajós River is a distributional boundary for lineages centred in western and eastern Amazonia. The fact that many taxa occur on both banks suggests that the Tapajós is a recent or semi-permeable barrier, while the Jamanxim is an even more recent or weaker barrier. Anthropogenic activities that affect water level, flooding cycles and river flow may influence these natural patterns and cause changes to the equilibrium of the riverine barrier effect. Studies seeking to identify these influences should focus on the most affected functional groups. © 2016 John Wiley & Sons Ltd
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