EXTREME ANTHROPOGENIC FLUX RATES FROM HYDRAULIC MINING IN NORTHERN CALIFORNIA, 1853-1884

Mining in upper Greenhorn Creek generated hydraulic mining sediment (HMS) between 1853 and 1884, which caused channels to aggrade severely. The discrete period of mining and its sudden cessation provide a large-scale, 160-year experiment in rates of clastic sediment transport from a small (42.2 km²), unregulated basin. Spatially distributed sediment budgets were calculated to specify sediment production, storage, and flux. Mapping mine pits indicates that 41.3×10⁶m³ of HMS were produced, indicating catchment denudation of 97.9 cm or 3.16 cm a^-1 over 31 years of mining. Three digital elevation models (DEMs) were used: pre-mining (ca. 1849) and penultimate aggradation (ca. 1884) from simulations of contours, and modern (2014) from airborne 1-m LiDAR data. DEM differencing between periods gave volumetric changes in storage, which were used to compute sediment fluxes out of the catchment for two periods: 1853-1884 and 1884-2014. HMS storage in the catchment was 15.7×10⁶m³ (38.2% of production) in 1884 and 4.80×10⁶m³ (11.6%) in 2014. Differences between HMS production and storage and changes in storage were used to compute volumetric sediment-delivery ratios (SDR) of 62% in 1884 and 88% by 2014. SDRs reveal the dynamics of longitudinal connectivity and storage potential. Seven dry-weight bulk-density samples averaged 1.78 for mine material (N=3) and 1.92 for HMS (N=4). Changes in sediment mass indicate that 43.2 and 21.1×10⁶ t of HMS left the basin between 1853-1884 and 1884-2014, respectively. This corresponds to HMS specific yields of 33,042 and 3,845×10⁶ t yr^-1 km^-2, respectively—including suspended and bed material transport. These values, which are for fluxes of HMS only and do not include other sources of sediment are high compared to global indices. Storage decreased greatly with time but remains much greater than would be predicted by the classic translational sediment-wave model. Spatial patterns of HMS storage in the catchment support a conceptual model of preferred long-term storage at tributary junctions and at valley constrictions where tailing fans met the main channel.