THE STABILITY OF BENTHIC HOMES VARIES BETWEEN RIVERS AND REGIONS: FLOW REGIME AND SEDIMENT SUPPLY AS CONTROLS ON GRAVEL BED MOBILITY AND ECOLOGICAL DISTURBANCE (Invited Presentation)

The frequency and intensity of riverbed mobility exert a fundamental control on 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, and potential feedbacks for both biotic and abiotic “history effects” in the threshold for sediment motion. We begin by considering the ecological implications of 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. These predicted region-to-region differences, shaped by sediment supply and hydroclimate, have broad implications for benthic ecosystems. The magnitude and timing of 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. This diversity in bed mobility regimes is not limited to region-to-region trends. Within a given region and given hydroclimate, the upstream supply of coarse sediment shapes bed surface grain size, which in turn shapes sediment transport dynamics. Below lakes and dams, channel beds tend to be starved of coarse sediment, while channel beds downstream of active landslides are consistently replenished with mobile gravel. In an ongoing study of sediment transport in lowland, rainfall-dominated channels of the Pacific Northwest, we quantify differences in the threshold for gravel entrainment across a spectrum of upstream sediment supply. These river-to-river differences in the frequency of habitat disturbance likely act as a key control on the spatial patterns in species abundance across landscapes.