Imprint of climate and climate change in alluvial riverbeds: Continental United States, 1950-2011

Abstract

Alluvial riverbed elevation responds to the balance between sediment supply and transport capacity, which is largely dependent on climate and its translation into fl uvial discharge. We examine these relations using U.S. Geological Survey streamfl ow and channel measurements in conjunction with basin characteristics for 915 reference (“least disturbed”) measurement stations across the conterminous United States for the period A.D. 1950‐2011. We fi that (1) 68% of stations have bed elevation change (BEC) trends (p < 0.05) with median values of +0.5 cm/yr for aggradation and ‐0.6 cm/yr for degradation, with no obvious relation to drainage basin structure, physiography, or lithology; (2) BEC correlates with drainage basin area; (3) high-fl ow variability (Q 90 /Q 50 , where Q is discharge and 90 and 50 are annual fl ow percentiles) translates directly into the magnitude, though not the direction, of BEC, after accounting for the scale dependence; (4) Q 90 /Q 50 declines systematically from dry to wet climates, producing disproportionately high rates of BEC in drier regions; and (5) marked increases in precipitation and streamfl ow occurred disproportionately at dry sites, while streamfl ow declined disproportionately at wet sites. Climatic shifts in streamfl ow have the potential to increase/decrease sediment fl ux and thus affect riverbed elevation by altering fl ood frequency. These unforeseen responses of bed elevation to climate and climate change have important implications for sediment budgets, longitudinal profi les, ecology, and river management.