How often does a gravel riverbed mobilize? The answer depends on sediment supply and hydrology, which vary enormously between regions

The frequency and intensity of riverbed mobility exert a fundamental control on river morphodynamics and thus on the life cycle of the inhabitants of river ecosystems. Across North America, the magnitude of sediment supplied to river channels varies by orders of magnitude. At the same time, climate also varies widely across the continent, yielding a range of flood timing, duration, and intermittency. Together, these differences in sediment supply and hydroclimate result in diverse regimes of river bed surface stability. Building on the work presented by Pfeiffer and Finnegan (2017, AGU Fall Meeting), here we explore the broader geomorphic implications of these region-to-region differences in the timing and intensity of riverbed mobility. We calculate multi-decadal time series of estimated bed surface mobility for 29 rivers across the United States using sediment transport equations and long-term USGS discharge records. We use these data to compare predicted gravel river bed mobility between rivers and regions. We find statistically significant regional differences in (a) the exceedance probability of bed-mobilizing flows, (b) the maximum bed mobility, and (c) the number of discrete bed-mobilizing events in a year. Summing the total sediment transport and total water discharge across the multi-decadal time series, we find that the ratio of sediment to water (Q_s/Q_w) is higher in high sediment supply landscapes. These differences have broad implications for our understanding of fluvial geomorphology and sediment transport. First, our results suggest that high sediment supply river channels have adjusted their channel dimensions and river bed surface grain size to accomplish more sediment transport for a given discharge. Second, 'history effects' may have greater implications for sediment transport prediction in rainfall-dominated regions (where the length of hiatuses in transport are randomly distributed) when compared to snowmelt-dominated regions (where the length of hiatuses in transport are extremely consistent year-to-year). Third, the ecological disturbance caused by sediment transport is most predictable in snowmelt-dominated regions and least predictable in low-sediment supply, rainfall dominated regions such as the Appalachians.