NINETEENTH CENTURY HYDRAULIC MINING IN THE SIERRA NEVADA AS A CONTROLLED SEDIMENT TRANSPORT EXPERIMENT

Hydraulic mining was invented in the northwestern Sierra Nevada foothills in 1853. Sediment production accelerated in the first decade but the first deliveries to the Sacramento Valley did not begin until the 1862 flood of record. By 1884 when mining was suddenly halted by court injunction, a large volume of hydraulic mining sediment (HMS) had been produced in what represents a controlled experiment ranging in scale from small to large watersheds. A classic study by G. K. Gilbert demonstrated that more than 10⁹ m³ of HMS had been produced in the 31-year period of mining and that most of it had been transported to the Sacramento Valley and beyond. This implies that sediment delivery ratios (SDR) were much higher for this sediment event than in agricultural watersheds where most SDRs in North America have been calculated. Presumably, the high SDRs are due to the mountainous terrain of the mining districts. Until recently, no data were available on the HMS volumes at scales finer than large river basins so details about HMS transport were limited.

Geospatial modeling based on differencing digital elevation models (aka DEMs of difference or DoDs) was conducted to develop sediment budgets in selected watersheds for the HMS. Three DEMs were utilized: pre-mining (ca. 1853), the peak period of HMS production and channel aggradation (ca. 1884), and modern (airborne LiDAR flown in 2014). Volumetric analysis of HMS demonstrates several features of sediment transport from the steep mountain valleys in this experiment. First, extensive deposits of HMS remain in the mountains near the mines. Although this is to be expected from modern sedimentation studies that predict very low SDRs in small watersheds, it is more than what Gilbert described and contrary to what river engineers in the Sacramento Valley presumed throughout the late 20^th century. Second, decreases in HMS mountain deposits between ca. 1884 and 2014 indicate a progressive increase in SDR through time. Theoretically, the SDR for a single episodic event should increase through time as sediment continues to be eroded and transported out of the watershed. However, few studies in the scientific literature describe SDRs for a single sediment production event as a dynamic factor.