ABSTRACTGlaciers affect hydroelectric development by storing and rapidly releasing liquid water, by increasing the sediment load of a stream, by providing a constant source of stream-flow and by acting as naturally regulated storage reservoirs. A study was conducted in northern British Columbia to predict the probable maximum flood from a glacierized watershed in which three hydroelectric dams are planned. A streamflow model was developed to predict runoff based on weather data recorded at two stations that have been in operation since 1918. The model calculates daily glacier accumulation, ablation and the resulting annual net balance. An englacial storage and release of liquid water mechanism is proposed that is controlled primarily by season. Inflow to englacial storage is assumed to occur from late fall to mid-July, and release from storage for the remainder of the year. Implementation of this algorithm significantly reduced overall simulation error, particularly during the inflow period. The effect of the annual mass balance of these glaciers on watershed runoff was calculated for the 65 year period that these data were synthesized. It was found that increases or decreases in annual runoff of up to 40% occur in the sub-basin that is 67% glacierized.