CENTENNIAL-SCALE STORAGE AND REWORKING OF A DISCRETE SLUG OF CLASTIC MINING SEDIMENT

Abstract

Hydraulic gold mining in California between 1853 and 1884 generated an extreme, discrete pulse of highly distinctive hydraulic mining sediment (HMS). The sudden cessation of mining in 1884 provides an opportunity to study centennial-scale clastic sediment storage and reworking at the catchment scale. Initial channel aggradation in the mountains was followed by channel incision, but large volumes of HMS remain in Greenhorn Creek, a small (42 km2) unregulated mountain catchment. Two DEMs were developed to reconstruct historical surfaces (1853 pre-mining and ca.1884 maximum aggradation). Spatially distributed sediment budgets were computed for the aggradation (1853-1884) and degradation (1884-2014) periods by differencing the DEMs with a high-resolution (1-m) LiDAR DEM (2014). Sediment production, storage, and flux were extracted from the budgets. Mines in upper Greenhorn Creek produced 41×106m3 of HMS, of which 16×106m3 (38%) and 4.8×106m3 (12%) were stored in 1884 and 2014, respectively. Hydraulic mining was a geomorphically catastrophic event that generated denudation rates of 3160 cm/kyr for the catchment, i.e., one to four orders of magnitude greater than geologic background rates. Mean annual sediment flux rates out of the catchment were 820 x 103 m3/yr (33,039 t km-2/yr) from 1853 to 1884 and 84 x 103 m3/yr (3,845 t km-2/yr) from 1884 to 2014. Values of sediment delivery ratios and residence times are compared for the two periods. Storage and flux rates of mining sediment decreased through time but remain high. Most storage in 2014 was in the lower catchment except for substantial storage in two cutoff valley meanders that have removed large volumes of sediment from the ‘jerky conveyor belt’ for centuries.