Most rivers in Canada show pronounced seasonal variation in flows. Minimum annual flow occurs in late summer when precipitation is low and evaporation is high, and in late winter when precipitation is frozen in ice and snow. Minimum flows can limit the availability of specific aquatic habitats and also influence water temperatures and dissolved oxygen levels. For example, a decrease in minimum flow can affect the quantity and temperature of water for late-spawning fish and increase thermal stress and exposure to predation for all fish.
In a study of 172 sites in naturally flowing rivers, the lowest annual flow increased between 1970 to 2005 at 13% of the sites. These sites were generally in the northern Montane Cordillera, Boreal Cordillera, Taiga Plains, Taiga Shield, and Arctic ecozones+. Twenty–eight percent of the sites had decreases in minimum flow, generally in the southern Pacific Maritime, southern Montane Cordillera, Boreal Shield, Mixedwood Plains, Atlantic Maritime, and Newfoundland Boreal ecozones+. Twelve percent of the sites, mostly in eastern Canada, Great Lakes, and the North, had later minimum flows, while 9%, mostly in the South and along the western coast, had earlier minimum flows.2
Maximum annual flow, or spring freshet, generally occurs in late spring and in early summer and is driven by snow melt and seasonal rainstorms. A change in maximum flow can affect species with life cycles synchronized to the spring freshet and the rich foods provided by flood plains.
Almost 20% of the sites showed a decrease in maximum flow. These sites were distributed across almost all ecozones+. About 5% showed an increase in maximum flow, mostly in the Atlantic Maritime Ecozone+. Maximum flow occurred earlier at about 10% of the sites and later at about 8% of the sites.2
The average flow of prairie rivers has been declining over the past 50 to 100 years, including:
Reduced flows like these can impact biodiversity in many ways, including reducing habitat availability, not meeting the minimum flow requirements for aquatic species, and increasing summer temperatures.5