Laboratory optical measurements for selected streams in the Clear Lake basin, Lake County, California

Clear Lake, situated in the Coast Range west of California's Central Valley, is the largest natural freshwater lake entirely within the state. It consists of three distinct arms: the Upper Arm-a large, open-water area to the northwest; the Lower Arm-a smaller, narrower section to the southeast; and the Oaks Arm-the smallest and narrowest region, located to the east. Water quality in Clear Lake is impaired by both mercury contamination and harmful algal blooms (HABs). The primary source of mercury is the Sulphur Bank Mercury Mine (SBMM), a Superfund site managed by the U.S. Environmental Protection Agency (USEPA) since 1992, located at the eastern end of the Oaks Arm. The U.S. Geological Survey (USGS), in collaboration with USEPA, is conducting research focused on mercury contamination from SBMM and the lake's biogeochemical mercury cycling. Ongoing monitoring and modeling efforts aim to better understand how nutrient dynamics influence mercury cycling, transport, and bioaccumulation in the lake. As part of its Total Maximum Daily Load (TMDL) program, the California Central Valley Regional Water Quality Control Board has established goals to reduce nutrient and mercury inputs to Clear Lake. This dataset includes optical measurements for samples collected at sixteen sites in the Clear Lake Drainage Basin, Lake County, California, between October 2021 and March 2024. Additional site information can be found in Sushch and others (2025).