Centennial-Scale Anthropogeomorphic Change: Legacies of Hydraulic Mining in Greenhorn Creek, California

Authors: Allan James*, University Of South Carolina, Allison M. Pfeiffer, Western Washington University, Chen-Ling J. Hung, Tamkang University, Taiwan
Topics: Geomorphology, Environmental Science, Human-Environment Geography
Keywords: sediment budgets; fluvial geomorphology; human impacts; mining
Session Type: Virtual Paper
Day: 4/8/2021
Start / End Time: 8:00 AM / 9:15 AM
Room: Virtual 3
Presentation File: No File Uploaded

Hydraulic mining in upper Greenhorn Creek, a small mountainous watershed (41.2 km^2) in northwestern Sierra Nevada, was practiced extensively from 1853 until 1884. This is an ideal site to study long-term geomorphic responses to a catchment-scale sediment pulse due to the discrete 31-year period of production, large volume of distinctive clastic sediment produced, and lack of subsequent engineering. Geomorphic responses to mining include mine pits and extreme valley-bottom aggradation below the mines. High-resolution (1m) airborne LiDAR topographic data were used to map geomorphic features, compute volumetric changes by DEM differencing, and develop spatially distributed sediment budgets for the catchment. The 25 hydraulic mine pits produced 41.3×10^6m^3 of hydraulic mining sediment (HMS) by 1884, of which 15.8×10^6m^3 (35% of production) was stored at the time of penultimate aggradation (ca.1884) and 4.8×10^6m^3 (11%) was stored when LiDAR data were acquired (2014). Differences between production and storage volumes indicate that sediment delivery ratios (SDR) were 62% in 1884 and 88% in 2014, documenting persistent storage in the mountains. Bulk densities of mined material (1.78 kg m^-3) and HMS (1.92 kg m^-3) were measured and used to convert volumes to mass and compute long-term HMS flux rates. Specific sediment yields (SSY) were 33,039 t km^-2yr^-1 during mining (1853-1884) and 3,845 t km^-2yr^-1 thereafter (1884-2014). During mining, SSYs were two orders of magnitude higher than globally predicted values for catchments this size. Volumes of production, storage, and flux were much greater than in neighboring Steephollow Creek, a comparable-sized watershed.

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