Although regional climates throughout much of the world appear to have become more arid in late Cenozoic time, sedimentation rates, and therefore presumably erosion rates, have increased. For sustained erosion of elevated terrain, at least where glaciers are not the major erosive agent, rivers must incise. Therefore bed-load transport by rivers should be a rate-limiting process in erosion. Theories of bed-load transport call for a threshold in either stream power per unit area or shear stress before the bed load can be moved, and most transport should be accomplished during high discharge. The frequency- magnitude distribution of floods shows that the ratio of magnitudes of, for example, 100 yr floods and annual floods is greater in arid than in humid environments. Thus, a shift toward more arid conditions may have increased relative magnitudes of rare floods or, conversely, increased the frequency of large floods. Such a shift, despite a decrease in precipitation and discharge, could have doubled incision rates, particularly in regions already quite arid.