In the present research work, the Canadian Council of Ministers of the Environment water quality index 1.0 (CCME WQI 1.0) was applied to assess water quality of Tehri dam reservoir by using the drinking water standard prescribed by the WHO (1999) and BIS (IS:10500, 1991). The physico-chemical parameters, ions concentration and heavy metals concentration used in the index calculation were total dissolved solids, pH, alkalinity, dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, total hardness, calcium, chloride, phosphate, sulphate, nitrate, total coliform (MPN/100 ml), turbidity, zinc, manganese, lead, nickel, iron and chromium. It was observed during the course of study that at all the four sites BOD, phosphate and total coliform showed greater deviation from the objective values. Total coliform was found to be more deviated from the normal values. Few important parameters were observed beyond the permissible limit for many times. The values of water quality index have shown that most of the sites are not fit for drinking purpose. Finally it was concluded that reservoir water should not be consumed for drinking purposes frequently without proper treatment.