The South Yuba River (SYR), located on the western portion of the Sierra Nevada in California, is highly contaminated with mercury (Hg) as a result of historical gold (Au) mining that took place throughout this region starting in the mid 1800s and continuing into the early 1900s. During this period, the hydraulic mining of alluvial Au deposits formed during the Tertiary period (65.5 to 2.6 million years before present) was responsible for mobilizing hundreds of millions of tons of hydraulic mining debris (HMD), which was and continues to be redeposited in the SYR, its tributaries, and the San Francisco Bay Delta. Hydraulic mining was used in combination with the mercurygold (Hg-Au) amalgamation process. Elemental mercury (Hg(0)) was introduced into Au recovery sluices to trap Au flakes, which were mixed with the sediment-water slurry produced from the hydraulic mobilization of sediment. As a result of inefficient trapping, some amount of both Hg(0) and Hg-Au amalgam was lost in this process. And along with the HMD, both can still be found throughout the SYR watershed, downstream of the major historic mining areas.