Beavers, once abundant and widespread in the Northern Hemisphere, are now substantially reduced. Although beaver dams trap sediment, the relative importance of this sediment in Quaternary valley aggradation remains uncertain. We use ground penetrating radar (GPR) and near-surface seismic refraction to quantify the magnitude of beaver-induced Holocene sedimentation in Beaver Meadows, Rocky Mountain National Park, Colorado (United States). GPR was used to identify radar packages of genetically related strata of glacial and non-glacial origins. We demonstrate that GPR is a useful tool for identifying buried beaver-induced sedimentation with little to no surficial expression. Seismic refraction was used to determine the total volume of sediment above bedrock. Beaver-induced sedimentation constitutes 30%–50% of surficial post-glacial sediments, and post-glacial sediments constitute ∼13% of the total valley fill. Beaver damming in montane valleys was thus an important process trapping sediments within the Holocene at this site. If geoscientists ignore the contribution of beaver-ponded sediments to Quaternary stratigraphy in a wide variety of riverine environments, they neglect a potentially important biotic driver of valley sedimentation.