The riverine barrier hypothesis is a central concept in Amazonian
biogeography. It states that large rivers limit species distributions
and trigger vicariant speciation. Although the hypothesis has
explanatory power, many recent biogeographical observations addressing
it have produced conflicting results. We propose that the controversies
arise because tributary arrangements in the Amazon river system have
changed in geologically recent times, such that large tracts of forest
that were on the same side of a river at one time got separated to
different sides at another. Based on topographical data and sediment
dating, we map about 20 major avulsion and river capture events that
have rearranged the river network in central Amazonia during the late
Pleistocene and Holocene. We identify areas where past riverine barrier
effects might still linger in the absence of a major river, as well as
areas where such effects may not yet have accumulated across an existing
river. These results call for a reinterpretation of previous
biogeographical studies and a reorientation of future works to take into
account the idiosyncratic histories of individual rivers.