According to the bacteriological and physicochemical water quality index IQBP5, the water quality of the St. Lawrence River in Québec (Canada) is good upstream of Montreal, but deteriorates downstream, both in the northern water mass and the navigation channel. This is due to fecal contamination from the Montreal, Longueuil and Repentigny waste water treatment plants. South of the navigation channel, water quality is good throughout the river corridor. In the Québec region, water quality at all sampling stations was either good or at the limit of this class. Between 2000 and 2014, the number of sites with bad or very bad water quality decreased, but the frequency of water quality exceedances for phosphorus and fecal coliforms showed no significant trend. The various tributaries of the St. Lawrence that drain agricultural land contained between 4 and 33 pesticides, and 20 of these were also detected in the Lac Saint-Pierre (Lake Saint Pierre). In some of the lake samples, the concentrations of clothianidin, thiamethoxam and atrazine exceeded water quality criteria for the protection of aquatic life. Emerging contaminants, such as nonylphenol ethoxylates, polybrominated diphenyl ethers (PBDEs), perfluorinated compounds and pharmaceutical and personal care products are also present in the St. Lawrence River. The concentrations of PBDEs found in fish exceed criteria for the protection of fish and fish-eating terrestrial wildlife. Concentrations of nonylphenol ethoxylates decreased as a result of government controls introduced during the period between 2000 and 2010. Following 2 decades of improvement, the water quality of the St. Lawrence River remained relatively stable between 2000 and 2014. However, new pesticides and contaminants of emerging concern are now being detected and, in many cases, the environmental significance of these new findings remains unclear.