This dataset includes geochemical, isotopic, and particle-size data for soil and sediment samples collected at approximately 500 sites in the Clear Lake drainage basin, Lake County, California, between November 2021 and December 2023. Data are also reported for ten soil samples collected during October 2018 soon after the Ranch Fire in the upper Scotts Creek watershed. Samples of lakebed sediment (top four centimeters (cm)) were collected at 26 locations in Clear Lake in September 2023 using an Eckman dredge. Samples collected in tributary watersheds included soils, roadside ditches, streambed sediment, and stream-side sediment (including streambanks, floodplain deposits, colluvium, or alluvium). Some of the streambed sites were sampled on multiple occasions, including a monthly time series during July to December 2023. For 589 samples, the size fraction sieved to less than 0.063 millimeter (mm) was analyzed for the following parameters: 61 elements determined by inductively coupled plasma methods, low-level mercury by cold-vapor atomic absorption spectroscopy, total sulfur and carbon by combustion methods, carbonate carbon, stable isotopes of carbon and nitrogen, total organic carbon, total organic nitrogen, and particle-size distribution. A suite of 101 samples (preserved frozen in the field) was analyzed for nutrient species (forms of phosphorus and nitrogen) as well as particle-size distribution. Version History Summary: First Published: March 2025 Version 2.0: August 2025