CENTENNIAL-SCALE FLUVIAL ADJUSTMENTS TO CATASTROPHIC SEDIMENTATION: A TALE OF TWO SMALL CATCHMENT SEDIMENT BUDGETS

A new sediment budget for upper Greenhorn Creek (GC) hydraulic mining sediment (HMS) is presented and compared with a previous HMS budget developed for upper Steephollow Creek (SC). Both small catchments are in the Bear River Basin in the northwestern Sierra Nevada, California and experienced extreme geomorphic perturbations from HMS deliveries in the 19^th century. Catchment extents were defined by the limits of 1-m airborne LiDAR topographic data that cover only the upper basins. Budgets were developed by geomorphometric methods to measure volumes of mine pits for sediment production and valley bottom burial for sediment storage. Sediment delivery ratios (SDR) were computed for two times: at penultimate aggradation (ca. 1884) and modern (2014 LiDAR acquisition). The GC catchment includes a longer extent of channels below the mines than the SC catchment, so a larger proportion of the HMS produced is stored within the catchment. Storage in both catchments has been governed by the emergence of bedrock constrictions that trap large tailings fans, but the constrictions in GC developed later in the valley bottom evolution, so the timing of storage and SDR changes differ between the two catchments. Both catchment budgets show that early estimates of storage were lower than the sediment stored in 1884 or now.

The Sierra Nevada HMS episode is a classic example of a neo-catastrophic change with rates and magnitudes of geomorphic responses that do not conform to uniformitarian principles. Unlike most neocatastrophic examples, however, this sediment cascade is an ongoing anthropogenic perturbation that was generated within a period of 31 years and is well documented by historical records. Thus, HMS provides an opportunity to study a semi-controlled experiment in large-scale fluvial perturbations over centennial time scales. Passive restoration acting on these channels over the past 135 years has removed most of the HMS from channel beds in the upper mining districts and exposed coarse lag cobbles and boulders that presumably represent pre-mining channel conditions. High terraces of HMS remain along most channel reaches and continue to produce large quantities of relatively fine sediment and downstream pre-mining channel bottoms remain buried by HMS.