Samples are collected at 29 sites from major tributary systems in 15 subwatersheds across the Lake Simcoe watershed, carried-out in collaboration with Lake Simcoe Region Conservation Authority (LSRCA) and provincial and municipal government partners. LSRCA has two tributary sampling programs, the Lake Simcoe Protection Plan program (LSPP; formerly known as LSEMS), which was initiated in the 1980s, and the Provincial Water Quality Monitoring Network (PWQMN), starting at some sites in the 1960s. Samples are collected year-round and include both non-event flow and targeted runoff event sampling, especially during the spring freshet. Both programs include laboratory analysis of nitrogen and phosphorus species, total suspended solids, and chloride, and the PWQMN program includes a suite of metals as well. Physical parameters including pH, temperature, dissolved oxygen, conductivity, and turbidity are measured instantaneously on-site using a hand-held sonde. Water quality is closely tied to changes in the watershed including land use and natural heritage areas, climate change, and road salting, and impacts the water quality of Lake Simcoe. Data are used to assess the health of the streams through comparison to water quality objectives and with long-term trend analysis, and as part of a Key Performance Indicator (KPI) program. Estimates of total phosphorus (TP) loads to the lake are used to evaluate the progress towards achieving water quality-related objectives of the Lake Simcoe Protection Plan (MECP, 2009) and other various research initiatives.