Long-term sediment storage and flux in a small, catastrophically aggraded mountain catchment

A discrete period of hydraulic gold mining in the northwestern Sierra Nevada, California (1853-1884) and extreme channel aggradation provide an opportunity to examine long-term catchment-scale sediment transport. This study used high-resolution LiDAR topographic data to reconstruct historical surfaces and compute spatially distributed sediment budgets in Greenhorn Creek, a small (42-km2) unregulated mountainous catchment. Digital elevation models (DEMs) were constructed for three periods: pre-mining (ca.1853), maximum aggradation (ca.1884), and modern (2013-14). Differencing DEMs enabled computation of changes in sediment volume between dates, and computation of sediment production, storage, and flux. Hydraulic mines in upper Greenhorn Creek produced 41×106m3 of sediment, of which 16×106m3 (38%) and 4.8×106m3 (12%) were stored by 1884 and 2014, respectively. Mining-sediment fluxes were high compared to global sediment-yield databases averaging 33,042 t km-2yr-1 (1853-1884) and 3,845 t km-2yr-1 (1884-2014). Catchment denudation rates computed from mine-pit volumes ranged from one to four orders of magnitude greater than background. Valley-bottom mining-sediment storage decreased through time but remained substantial 130 years after mining ceased. Storage in 2014 occurred mostly in the lower catchment except for substantial storage in two cut-off valley bends. Many studies compute or model sediment delivery ratios as an inherent property of basin size or physiography that can be used to route sediment, but how do sediment delivery ratios change through time? A hypothesis that post-mining sediment delivery ratios were at least 10% greater during the mining period was not rejected. Results support a dynamic sediment-delivery ratio concept in which the proportion of sediment delivered increases through time following a discrete sediment pulse. Large sediment-production events can leave lasting legacies that should be recognized by river managers.