Using LiDAR, ArcGIS, and on-the-ground data collection to describe geomorphometric characteristics of hydraulic mining features in the Yuba River watershed

Hydraulic mining, a method of mining placer gold in alluvial deposits invented in California in 1853 and practiced for approximately a century afterwards, had and continues to have effects on sediment processes and water resources in northern California. This study focuses on three subwatersheds (Willow Creek, Oregon Creek, and Scotchman Creek) within the Yuba River basin, the most heavily hydraulically-mined watershed in the Sierra Nevada. Prior work related to hydraulic mining in the study area was reviewed, resulting in a total of 35 previously identified hydraulic mining features (HMFs) being included in this study. These incorporate both locations where hydraulic mining occurred and locations where sediment produced by hydraulic mining is currently held, either behind a debris control dam (DCD) or in a natural drainage channel along a waterway. For each of these HMFs, the initial objective of this study was to conduct (1) a geomorphometric characterization and assessment of current conditions of the HMFs, for which a total of 40 categories were determined. As part of the geomorphometric characterization, (2) methodologies for estimating sediment volume results using LiDAR and ArcGIS were developed. Using the inventory results, sediment volumes were determined for (3.a) the amount produced by hydraulic mining sites and (3.b) the amount still held in the study area in sediment deposits, and (3.c) the two amounts were compared to inform what proportion of sediment produced remained within the study areas in the Upper Yuba River basin. Finally, (3.d) additional inventory results are compared and contrasted to better understand these HMFs and their characteristics. Although the number of hydraulic mines in each subwatershed was similar, the sediment volumes produced by hydraulic mining varied greatly (~20 million m3 for Willow Creek, ~2 million m3 for Oregon Creek, and ~42 million m3 for Scotchman Creek). Known sediment deposit volumes comprised only a fraction of the volume produced by hydraulic mining (~0.6 million m3 for Willow Creek, ~0.4 million m3 for Oregon Creek, and ~0.4 million m3 for Scotchman Creek). A comparison of these volumes resulted in sediment delivery ratios of approximately 0.97 for Willow Creek, 0.83 for Oregon Creek, and 0.98 for Scotchman Creek. This shows that a majority of sediment produced has either been transported out of the subwatershed areas or is stored in locations other than the large deposits (primarily held behind DCDs) assessed in this study, or likely some combination of both. Though they only contain a fraction of the sediment produced from hydraulic mining sites, the sediment deposits need to be further studied and better understood, and additional sediment storage within these subwatersheds should be investigated. The numbers presented above should be considered an initial conservative estimate. There are likely additional HMFs (both mines and deposits) requiring identification and assessment. As a first step towards this, Potential HMF (P-HMF) Mines were identified from the LiDAR dataset based on similarities in appearance to previously identified HMFs. Four of these were visited in person and evidence was found indicating that they may represent additional locations where mining occurred within the study areas, although further investigation would be required to confirm this. There are also additional P-HMF Deposits that were identified, and there is likely HMS scattered throughout the study area in streambanks, high terraces, and other locations apart from the concentrated deposits focused on in this study. Finally, on-the-ground field work determined that sediment transport from HMFs is continuing, and no HMFs were visited that were hydrologically isolated from nearby waterways. Quantitative data about water quality and sediment transport would be valuable. In summary, using airborne LiDAR dataset with a geospatial software program such as ArcGIS has proven to be an effective method of both evaluating HMFs and their impacts on watersheds, and identifying P-HMFs for future study and evaluation. However, on-the-ground research is needed as well to both verify and expand on results obtained from the LiDAR dataset