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The frequency and intensity of riverbed mobility are of paramount importance to the inhabitants of river ecosystems as well as to the evolution of bed surface structure. Because sediment supply varies by orders of magnitude across North America, the intensity of bedload transport varies by over an order of magnitude. Climate also varies widely across the continent, yielding a range of flood timing, duration, and intermittency. Together, the differences in sediment supply and hydroclimate result in diverse regimes of bed surface stability. To quantitatively characterize this regional variation, we calculate multidecadal time series of estimated bed surface mobility for 29 rivers using sediment transport equations. We use these data to compare predicted bed mobility between rivers and regions. There are statistically significant regional differences in the (a) exceedance probability of bed-mobilizing flows (W* > 0.002), (b) maximum bed mobility, and (c) number of discrete bed-mobilizing events in a year.

Plain Language Summary How often does the gravel on a riverbed move? Do the timing and intensity of gravel riverbed motion vary between regions? The answers (a) shape habitat for macroinvertebrates that live on the riverbed surface and (b) drive "history effects" that render river gravel more stable through time between storms. To answer these questions, we calculate gravel mobility using decade-long river gage records and channel measurements from 29 rivers across the United States. We show that there are strong regional trends in the timing and intensity of riverbed mobility between the West Coast, Rocky Mountains, and central Appalachians. These previously unrecognized regional differences are explained by trends in flood timing and trends in the amount of sediment that flows into the rivers. We show that the Appalachians, a region with few published river gravel transport studies, may be especially susceptible to history effects.