This study systematically analyzes long-term (1973-2011) daily flow data collected near the Mackenzie basin outlet. It clearly defines the variability, extreme events, and changes in daily flow records over the past 4 decades. The results of this study accurately determine the seasonal cycle of river discharge, including the range of highest and lowest daily flows. The interannual variation of daily flow is generally small in the cold season, highest in the spring melt period, and large over the summer months mainly due to rainfall storm activities and associated floods. This study also shows that Mackenzie River flow regime has changed over the past 4 decades due to climate variation, with the advance of snowmelt peak timing by several days, decrease in maximum spring flows by about 3000 m³/s, and weak rise of cold season base flows. These results are the consequence of hydrological response to regional climate warming, and they provide new knowledge to improve our understanding of large-scale environmental changes over the broader northern regions.