Concurrent Low Flows in the Athabasca River Basin

A hydrologic parameter that has become synonymous with water quality evaluations of rivers is the term “7Q10”. This term represents the annual minimum 7-day discharge at a particular location along a river, below which flows would be expected to occur in only 10% of the years. The complement to this definition is that there is a 90% chance in any year that the average 7-day flow would never be less than the 7Q10 value. Areal variability in climatic and physiographic parameters throughout the Athabasca River basin produces a number of possible low flow scenarios. The question is then: what is the likely flow at one location if the flow at another is known to be the 7Q10 flow? The statistical analysis that answers this question is called conditional probability. It is an approach whereby the distribution of flow at one location is mathematically related to flow at another. Scenarios of expected concurrent flow along the Athabasca River are developed on the assumption that a 7Q10 event occurs at either Hinton, Whitecourt, Athabasca, or Fort McMurray. While expected flows represent the most probable situations, it is possible that a 7Q10 event can occur from a totally different flow pattern than expected. The likelihood of these other scenarios is outlined where appropriate. Both annual and open water scenarios are provided. A number of interesting flow patterns are evident. The premise that low flow events throughout the Athabasca River basin cannot be treated as independent events is confirmed. In the annual flow case, concurrent 7Q10 flows can span the reach from the Lesser Slave River to Lake Athabasca. In the open water case, there is a reasonable chance that concurrent low flows could extend from Whitecourt to Athabasca or from Athabasca to Fort McMurray. All scenarios point to the value in basin-wide assessments of low flow.