THE IMPACT OF THE CREATION OF LAKE DIEFENBAKER ON GROUNDWATER LEVELS IN UNDERLYING EXTENSIVE SEDIMENTARY AQUIFERS; A 50-YEAR HISTORY

Lake Diefenbaker is an extensive reservoir, up to 50 m deep, that was created in the 1960’s within the 100 m deep valley of the South Saskatchewan River, by the construction of the Gardiner Dam on the South Saskatchewan River and the Qu’Appelle Dam on the Qu’Appelle River. Prior to filling of the reservoir, a series of nine monitor well nests were constructed along a line perpendicular to the reservoir and extending to about 8 km from the reservoir. At each of the nine sites, a monitor well was completed at the water table, in a glacial drift aquifer (if present), and in three Upper Cretaceous sedimentary rock aquifers at depths of approximately 130, 170 and 240 m beneath the adjoining uplands. The bedrock aquifers are regionally extensive and separated from each other and form the glacial drift and the river valley by regional aquitards largely consisting of shales. Water-level data for the monitor wells have been obtained on an on-going basis since 1963 before the reservoir was filled and together with the reservoir level data form a unique, long-term, data set that describes the on-going adjustment of the regional groundwater flow patterns to the filling of the reservoir .

Prior to reservoir filling the groundwater flow in all the aquifers converged towards the river valley and discharged to the river, likely via fractures in the overlying shale aquitards. From 1965 to 1968 the water level in the river/reservoir rose by about 40 m and has been fairly constant since then, but with about 6 m annual fluctuations. In contrast, the water levels in the bedrock aquifers rose more slowly and continue to rise at various rates, resulting in a complex pattern of flow directions near the reservoir. The initial response of the deeper aquifers to the filling of the reservoir may have been in part due to the loading effect of the added water mass in the reservoir. If the response in the aquifers had been purely due to loading the increased groundwater pressure would have dissipated in time. However the continued rise of the groundwater levels after the reservoir was filled clearly indicates a direct flow connection to the reservoir with continued flow into the aquifers. The data indicate that the flow systems in the aquifers will continue to be in a state of transient adjustment to the filling of the reservoir for many years to come and at distances of tens of kilometres.