A 161-Year Experiment in Sediment Production, Storage, and Flux in a Small, Episodically Aggraded Mountain Catchment

Abstract:

Production of hydraulic mining sediment (HMS) in the 19^th century generated aggradation-degradation episodes that provide an opportunity to study long-term behavior of clastic sediment in small mountain watersheds. High resolution topographic data (airborne LiDAR) were used to create a spatially distributed sediment budget for upper Greenhorn Creek, a 42.2 km² unregulated catchment in the northern Sierra Nevada, California. Digital elevation models (DEMs) were constructed for three points in time: pre-mining (ca. 1849), penultimate aggradation (ca. 1884), and modern (2014 LiDAR). Sediment budgets were constructed by measuring mine pit volume for HMS production (1853-1884), and simulating valley-bottom contours for 1849 and 1884 to measure valley-bottom HMS storage at two times (1884 and 2014). Differencing the DEMs gave changes in storage, HMS loads, HMS specific yields, and HMS delivery ratios for two periods after mining began. Bulk densities of mine-pit materials and HMS were measured and used to convert volume to mass.

Upper Greenhorn Creek had some of the most intense hydraulic mining in the state. Mapping mine pits identified 25 hydraulic mines that covered 2.61 km² or 6.18% of the watershed. The total volume of HMS produced in the basin was 41.3×10⁶m³ (73.5×10⁶ t), which represents denudation of 97.9 cm across the watershed for the 31-year period of mining or 3.16 cm yr^-1. Of the HMS produced, 38% was stored in the catchment in 1884, but this had dropped to 12% by 2014. Changes in storage indicate that annual HMS loads were 1.39 and 0.162×10⁶ t yr^-1 from 1853-1884 and 1884-2014, respectively and that specific yields were 33,039 and 3,845 t km^-2 yr^-1 from 1853-1884 and 1884-2014, respectively. These high values indicate an extreme HMS flux rate during mining and a dramatic reduction in mean rates over the post-mining period. Although the budget did not include post-2014 changes, field evidence of substantial erosion by a 2017 flood indicates that elevated HMS flux rates continue. At the time of maximum aggradation, elevations of high terraces indicate channels were graded except at tributary confluences that had tailings fans. These valley constrictions caused local steepening and coarsening and maintained fixed storage sites rather than sediment moving in a large translatory wave.