Alluvial scour into shallow marine sediments may be caused by the incision of a river adjusting to a new base level(1-4) following a fall in sea level. The identification of such erosion surfaces(1-3) has therefore been pivotal in the reconstruction of past sea-level changes from ancient sedimentary sequences(1-14). Here we report data from a study of the Tamuna river, Bangladesh, one of the world’s largest modern braided rivers(15), which illustrate that bed scour associated with channel confluences and bends alone can be substantial-as much as five times greater than the mean channel depth. Indeed, the basal erosion surfaces produced by such deep scours have characteristics similar to those of boundaries in some ancient sedimentary sequences that have been assumed to result from sea-level fall(1-14), potentially leading to radically different interpretations of past variation in base level and climate. We suggest that, to discount unambiguously the influence of fluvial scour in ancient sediments, the erosive boundary should be greater than five times the mean channel depth and extend for distances greater than the floodplain width. Ideally, it should be traceable between different basins.