We use the 10Be concentration in alluvial sand and gravel to document hillslope exposure history and trace the reduction in grain size of river alluvium in the Clearwater River basin, western Washington State. Clearwater tributary basins Wilson Creek (WC, 6 km2) and East Fork Miller Creek (EFMC, 12 km2) were each sampled in headwater and downstream sites for 10Be in alluvial sand (0.25–0.50 mm) and gravel (22.6–90 mm). Grain size distributions were determined at each sampling site as well as at several locations along the main stem of the Clearwater River...