Archival aerial stereo-photographs are a source of information that can be used to create historical topography models for rapidly changing landscapes. Researchers completed a pilot study in Malakoff Diggins State Historic Park (MDSHP), which was once the largest hydraulic mine (1.6 square kilometers) in the Sierra Nevada of California. Legacy impacts in these mined landscapes include remnant steep exposures of highly erodible Eocene-aged auriferous sediments. Erosion of mined landscapes represents a chronic source of fine sediment and particle-bound mercury that impacts downstream water quality and water storage. USGS researchers compared reconstructed historical topography to modern topography to estimate upland erosion and deposition within the pit, which enabled them to quantify a long-term sediment budget over a 22-year period.
The USGS team used archived aerial photographs and stereo-photogrammetry methods to create a 1992 digital surface model (DSM) and orthophoto mosaic (https://doi.org/10.5066/F7T152J9). These datasets were co-referenced to a bare earth digital elevation model (DEM) constructed using aerial lidar data collected in 2014 (https://doi.org/10.5066/F7ST7NQZ). The 1992 DSM includes areas where vegetation creates a positive bias in elevations. USGS researchers corrected the bias using a ground cover classification and removed the biased elevations from the 1992 DSM. The 1992 DSM was differenced from the 2014 DEM to create a change map with a 0.76-meter minimum detection threshold. Erosion and depositional areas were deconvolved and volumetric change was summarized. The estimated annual erosion rate was 40,000 cubic meters per year (m3/yr), which equates to a denudation rate of 3.5 centimeters per year. The estimated annual deposition rate was 21,000 m3/yr. The difference between the erosion and deposition rates can be used to estimate sediment delivery, calculated to be 19,000 m3/yr. Results from this study were used to determine feasible methods for mitigating sediment-related problems at MDSHP.
Elevation change from 1992 to 2014 for Malakoff Diggins State Historic Park, Sierra Nevada, California, estimated by differencing a 1992 DSM and 2014 DEM.