Streamflow in the Winnipeg River basin, Canada: Trends, extremes and climate linkages

SummaryThis study uses a network of long-term discharge gauges to examine how river flow in the Winnipeg River basin, Canada has behaved during the last one hundred years. The Winnipeg River influences the production of over 4600 MW of hydroelectricity, and is the most important component of the hydrological system used to generate power in Manitoba. Extreme low annual flows are caused by severe reductions in runoff from spring snowmelt, and follow dry weather during the previous summer and autumn over much of the basin. These conditions are associated with enhanced meridional flow across western Canada, and geopotential height anomalies during the previous autumn and winter that are very similar to the positive phase of the Pacific/North American (PNA) pattern. The winter PNA index appears to be an important control on streamflow in the Winnipeg River at both interannual and decadal time-scales, but may be modulated by conditions in the North Atlantic sector. Mean annual flows have increased by 58% since 1924, primarily because of large increases in winter discharge. Because similar trends are observed for both regulated and unregulated rivers, these increases are not artefacts caused by direct anthropogenic interference in the hydrological system. Increasing summer and autumn precipitation is the most probable cause of the changes in streamflow. The observed trends toward higher flows, combined with recent model projections, suggest that the potential threats to water supply faced by the Canadian Prairie provinces over the next few decades will not include decreasing streamflow in the Winnipeg River basin.