QUANTIFYING 18TH TO EARLY 20TH CENTURY HUMAN IMPACTS ON HILLSLOPES AND SEDIMENT MOBILIZATION IN SOUTHERN NEW ENGLAND AND COLORADO FRONT RANGE

In this study, we examine 17^th to early 20^th century human impacts on hillslopes and sediment mobilization in southern New England and the Colorado Front Range. Southern New England exhibits widespread evidence of historic land clearing for agriculture and pasture (stonewalls) and timber harvesting for charcoal production (relict kilns). The Colorado Front Range, meanwhile, displays widespread evidence of historic mining with large mine shafts, exploration pits and waste piles on hillslopes and tailings pond sediment at mill sites along valley bottoms. LiDAR based mapping and field studies in both regions highlight the extensive nature of these periods of human impact. In forested terrain throughout northwestern and eastern Connecticut, we find that towns 100-150 km² in area have an average of 408 km of abandoned stonewalls per town representing 300,000 to 400,000 m³ of moved stone. A regional analysis of a 1,170 km² study area in northwestern Connecticut reveals at least 20,434 relict kiln sites 10 to 16 m in diameter, an intensity of about 17 platforms per km². In Colorado, analysis within our 65 km² Fourmile watershed study area revealed 18,600 prospect pits and small mines (all bigger than ~10 m²), 455 km of smaller roads and trails near mine sites, 33 km of historic railroad grades, and ~60,000 m³ of tailings pond sediment. Severe flooding along Fourmile Canyon in September 2013 caused direct erosion and mobilization of mine waste piles, placer deposits, historic railroad and modern road fill, and tailings pond sediment, highlighting the ongoing legacy of historic soil and sediment mobilization. Overall, we find that the human impacts outlined above characterize the Anthropocene for these regions and quantifying them serves as a benchmark for understanding human activities in the context of Holocene landscape evolution. Furthermore, these case studies highlight the utility of LiDAR for fine scale mapping of human induced hillslope sediment transport and historic land modification preserved under sparse and dense forest canopy.