Trends in Canadian Streamflow

The detection and attribution of past trends, changes, and variability in climate variables is essential towards the understanding of potential future changes resulting from anthropogenic activities. This is especially true for high latitude regions such as Canada where climate change signals are anticipated to be stronger. Analysis of variability in streamflow is very important, since streamflow represents the integrated basin response to the variations of several important climate variables such as precipitation and temperature. This study investigated trends in streamflow for 250 sites of the Reference Hydrometric Basin Network (RHBN) across Canada. It was found that the annual mean streamflow has generally decreased during the last 30-50 years, with significant decreases seen in the southern part of country, and especially in the southern British Columbia (B.C.). Significant increases were identified in lower percentiles of the daily discharge distribution over northern B.C and the Yukon Territory. In other regions of southern Canada, significant decreases were observed in all percentiles of the daily discharge distribution, indicating that the observed decrease in annual mean flow in the region resulted from decreases in high and low extreme daily flow as well as decreasesin the mean. Monthly mean streamflow for most months has also decreased, with the most significant decrease occurring in August and September. An exception is March and April when significant increases in river discharge were observed. Earlier snow melt caused by higher spring temperatures is associated with the observed increase in March and April streamflow. This is particularly pronounced in B.C. where the strongest increase in spring temperature was observed in the last 50 years.