This study aimed to adapt the Water Quality Index of the Canadian Council of Ministers of the Environment (CCME WQI) for its application to water quality assessment of drainage water and watercourses downstream of peat harvesting operations. It integrates different parameters that potentially reflect the overall water quality condition of a stream. Thus, it is calculated using multivariate water quality data and accounts for their conformity with respect to water quality guidelines. Adaptation of the index proceeded to identify, through a literature review, the physico-chemical parameters that may change due to peat harvesting. The CCME WQI was used to compare water quality of receiving watercourses to that of streams located within a 200 km radius from the study sites in three regions of Quebec. The availability of water quality data guided the selection of parameters among those identified. They are ammonia, conductivity, pH and suspended sediment concentrations. Results indicated a significant difference between WQI values of water from harvested peatlands and those of streams in two of the three regions studied. Results have also shown that it is the pH guideline that is not respected in most cases for harvested peatlands.