The impact of macrophytes on winter flows along the Upper Qu’Appelle River

The Upper Qu’Appelle River conveys water from Lake Diefenbaker to Buffalo Pound Lake, which is a major source of water for domestic, industrial, agricultural and recreational use in southern Saskatchewan. Water demand continues to increase with time and additional conveyance capacity may be required during the winter months. Flows in the Upper Qu’Appelle are usually below 2 m3 s–1 from December to March; however, an increase in this flow rate will be needed to meet future water demands. A numerical modelling study was carried out to simulate the degree of ice cover thickening and backwater staging during freeze-up at various discharge scenarios (up to 6 m3 s–1). The Monte Carlo framework was used to capture the variability of possible parameter ranges within various freeze-up scenarios. A bottleneck to flow conveyance is a short 4-km stretch along the river (between the Prairie Farm Rehabilitation Administration [PFRA] and Tugaske Bridges) which is infested with macrophyte growth. Results show that the probability of overbank flow greatly increases with the presence of macrophytes in the channel, even at freeze-up conditions. Dredging was deemed the best mitigation option to reduce macrophyte density and remove nutrient-rich sediment to increase flow capacity.