Lake St. Charles is the primary drinking water reservoir of Quebec City, Canada, providing water to about 300,000 people. Over the past decade, several occurrences of blue-green algae (cyanobacteria) bloom have been recorded, affecting the quality of water in this strategic reservoir. Evaluation of observed data collected over the past decade indicates that the water quality of the lake is undergoing fast degradation due to anthropogenic activities around the lake and in the watershed.