MEASURING THE LEGACY OF 19TH- AND 20TH- CENTURY GOLD MINING USING LIDAR, GEOMORPHIC AND GEOCHEMICAL EVIDENCE, FOURMILE CANYON, COLORADO

Legacy deposits from mineral exploration, mining, and milling record anthropogenic production and redistribution of sediment that may affect hillslopes and channels for many generations. We used 1- and 2-m lidar digital elevation models (DEMs) to map waste piles and other land surface modifications produced by mine-related activity in the 63-km² Fourmile catchment, Colorado Front Range (USA), during the gold rush that began in 1859. We coupled lidar analysis with study of historic photos, extensive sampling of floodplain sediment deposited in 2011-13 and reconnaissance sampling of historic mine waste rock and mill tailings. Anthropocene events, particularly placer operations, control floodplain stratigraphy, and waste rock piles, particularly from vein mining and road-building before 1942, have volumes exceeding 10⁶ m³ and cover >5% of steep Fourmile hillslopes and gullies. Sediment exposed by mining activities that was subsequently mobilized by flood events in the Fourmile catchment exceeds steady-state measures of erosion by >50X. Anthropocene deposits largely obscure the record of Holocene floodplain evolution, which was characterized by lateral erosion and minor downcutting. High concentrations of As and Au in the <150 micron fraction of hillslope soil, mining-related deposits, and tributary and floodplain sediment record the pervasive signature of mining activity. Future large floods similar to that in 2013 will rework sediment and produce local erosion along Fourmile Creek channels, but transport of mining legacy sediment from hillslopes by smaller events is likely to continue supplying sediment rich in As and Au to the floodplain. Lowering concentrations of As and other contaminants in floodplain sediment will probably take hundreds of years to millennia.