Long-term hydraulic mining sediment budgets: Connectivity as a management tool

Abstract

Approximately 1.1 × 109 m3 of sediment were produced by late nineteenth century hydraulic gold mining in the Sierra Nevada of northern California. Modern geospatial methods combined with 2014 airborne LiDAR 1 × 1-m data are used to map and model a distributed sediment budget for upper Steephollow Creek, one of the most severely impacted catchments in the region. Digital elevation models (DEMs) were developed for three times using geomorphometry to construct sediment budgets. The 2014 surface is from the LiDAR bare-earth DEM, the pre-mining surface before 1853 was interpolated from contours extended along ridges and pre-mining valley bottoms, and the 1884 maximum aggradation surface was interpolated from contours extended across high terraces. Mine pit volumes indicate that ~23.5 × 106 m3 of hydraulic mining sediment (HMS) was produced in the 54.6 km2 study catchment. Volumes of HMS stored in the catchment were computed for 2014 (3.75 × 106 m3) and for ca. 1884 at the time of maximum aggradation (7.15 × 106 m3). The 2014 storage is 16% of the sediment produced in the catchment, indicating a sediment delivery ratio (SDR) of 84% of the HMS from the basin, which is higher than most agricultural basins and indicates a strong longitudinal sediment connectivity in this region. Storage in 1884 represents 30% of production indicating a SDR of 70% during the period of mining. Dynamics and strong scale dependencies of sediment connectivity are documented with regard to space and time. Over the past 130 years, 3.57 × 106 m3—approximately half of the storage in upper Steephollow Creek—was eroded and carried out of the catchment.