This dataset provides water quality analyses of lake surface samples and river samples collected throughout the ice-free season (May/June to October) from 2016 to 2019 from the Lake St. Clair nearshore, with a focus on the Thames River discharge area. Data includes nutrients and related biological parameters, including: • Phosphorus • Nitrogen • Suspended solids • Chlorophyll • Bacteria • Algal toxins Samples were collected by Ontario’s Ministry of the Environment, Conservation and Parks (MECP) staff. Work was conducted in collaboration and partnership with Environment and Climate Change Canada (ECCC), the Great Lakes Institute of Ecosystem Research (GLIER, University of Windsor), Lower Thames Valley Conservation Authority (LVTCA) and the National Oceanic and Atmospheric Administration (NOAA). Keywords: Lake St. Clair, Thames River, Detroit River, Lake Erie, Water Quality, Nearshore, Cyanobacteria, Cyanotoxins, Phosphorus, Nutrients, Eutrophication, algal blooms