High-resolution, terrestrial laser scanning, also known as ground-based lidar (light detection and ranging), was used to quantify the volume of mercury-contaminated sediment eroded from an outcrop of historical placer-mining debris at the confluence of Humbug Creek and the South Yuba River in the Sierra Nevada foothills, about 17 kilometers northeast of Grass Valley, California, and delivered to a zone below an observed flood stage of the South Yuba River. Substantial quantities of mercury were used and lost to the environment from historical placer gold mining activities on the western slope of the Sierra Nevada, California, and recent studies have documented continued persistence of mercury and methylmercury concentrations in water, sediment, fish, and predatory invertebrates in the Yuba River drainage basin in relation to suspected mercury sources. To identify areas that have high levels of mercury contamination as possible remediation targets in the Yuba River drainage basin and other areas in the Sierra Nevada, the U.S. Geological Survey worked in cooperation with the Bureau of Land Management on this and other detailed studies. Malakoff Diggings, one of the largest hydraulic gold mines in the Sierra Nevada, is 3.5 kilometers north of the study site in the Humbug Creek subbasin.

Terrestrial laser scanning was used to produce centimeter-scale, three-dimensional maps of the complex outcrop surface, which was composed of an upper erosional area (cliff and over-steepened slope) and a lower depositional area (colluvial slope). The outcrop could not be mapped non-destructively or in sufficient detail by traditional surveying techniques. The study site, which was approximately 70 meters long, 30 meters wide and 20 meters high, was surveyed four times in 2 years (December 15, 2011; October 25, 2012; January 4, 2013; and November 22, 2013) to determine volumetric differences in the upper erosional and lower depositional areas between surveys. Measured changes in volume for the upper erosional area and lower depositional area were multiplied by the corresponding sediment density so that a mass-balance relationship, between the eroded and deposited sediment during each period, could be used to estimate the amount of mercury-contaminated sediment that was transported to below the base of the colluvial slope, where it could be mobilized by the South Yuba River during a flood having a 5-to-10-year recurrence interval. On December 2, 2012, a flood of this estimated magnitude reached the base of the colluvial slope.

Between the first and second surveys (December 15, 2011–October 25, 2012), an estimated mass of 18±9.2 kilograms of sediment was transported from steeper slopes to the gently sloping river bank below the base of the colluvial slope. Between the second and third surveys (October 25, 2012–January 4, 2013), an atmospheric river caused heavy precipitation at the study site during late November and early December 2012. This short-duration, high-intensity rain resulted in a large amount of erosion and deposition at the study site and also caused high streamflow (flood stage) in the South Yuba River. From October 2012 to January 2013, 51±31 kilograms of sediment was transported to below the base of the colluvial slope, that is, below the high-water mark of December 2, 2012. Between the third and fourth surveys (January 4, 2013–November 22, 2013), an additional 10±26 kilograms of sediment was transported to below the base of the colluvial slope. During the 24 months of the study, the total mass of sediment delivered below the base of the colluvial slope and the high-water mark of December 2, 2012, was 79±66 kilograms.

In any given year there is a 10–20-percent chance (5-to-10-year recurrence interval) of a flood equal to or greater than that of the December 2, 2012, flood, which could transport mercury-contaminated sediment at the study site into the South Yuba River. Hydraulically modeled estimates of the South Yuba River stage during floods having a 50- and 100-year recurrence interval (2- and 1-percent annual exceedance probability, respectively) indicated that resulting river stages could be 2.2–3.0 meters above the base of the colluvial slope, or 2.2–3.0 meters above the high-water mark of December 2, 2012. Such high river stages would be likely to inundate the lower half of the colluvial slope and mobilize a substantial volume of mercury-contaminated sediment to downstream areas.