River Discharge can change from day-to-day, month-to-month, and year-to-year. Day-to-day changes are usually due to a river’s response to rainfall or rapid snowmelt events. It is not uncommon for ricer discharge to increase by as much as ten times in a single day only to return to previous levels a few days later.
Most rivers in Alberta follow an annual flow pattern that is typical of a snow-fed river system. River flows typically reach a seasonal low in late winter or early spring, followed by a rapid rise as spring snow melt releases large volumes of water into river systems. Summer rainfall events then bring the highest flow rates in most years, followed by a slow decline in flow from late summer through fall and early winter. While most other river systems in Alberta include large dams and reservoirs that significantly affect the natural pattern of river flow, this is not the case in the Athabasca River basin (Oil Sands Regions).
Alberta rivers also experience large changes in flow from year-to-year. Most of the water that falls as rain or snow in Alberta originates as evaporation from the Pacific Ocean. The amount of rain and snow that falls in Alberta each year therefore depends heavily on temperature and wind patterns over the Pacific Ocean and Western Canada. Because Ocean temperatures change very slowly, Alberta can experience several years of relatively wet or dry conditions.
River flows can also change slowly in response to the steady increase in air temperature over the last century in response to increases in the atmospheric concentration of greenhouse gasses. These temperature increases are expected to also bring increases in total annual precipitation. At the same time, the increased temperatures will result in increased evaporation and a shortening of the winter snowfall season. Whether or not the expected precipitation increases will be sufficient to overcome the expected increases in evaporation and reduction in snow remains an area of active research.