The Upper and Lower Campbell strandlines of glacial Lake Agassiz, were correlated from Dauphin, Manitoba northwestward toward the Clearwater River to determine if Lake Agassiz could have overflowed through a northwestern outlet at the Clearwater River into the Mackenzie River drainage basin. The results suggest that the Upper Campbell extended only as far as 350 km southeast of the Clearwater River Two lines of evidence suggest that waters of Upper Campbell-level Lake Agassiz were blocked from reaching the northwestern outlet. They are the termination of the Upper Campbell strandline on the northwestern side of the Wapawekka Hills, in north-central Saskatchewan, and a rapid rise in elevation of the Upper Campbell strandline from the southern side of the Hills to the northern side. The Lower Campbell beach, extends farther to the northwest, possibly reaching the northwestern outlet.
A high-resolution differential GPS survey along the correlated Campbell strandlines has allowed the construction of isobases across the western side of the Lake Agassiz basin. The new isobase information indicates that there was more post-glacial rebound in the northwestern region than previously estimated. This reconstruction indicates that, immediately following deglaciation, the Clearwater Spillway may have been below the southern outlet of Lake Agassiz, and acted as a temporary outlet for the lake.
It is concluded that the northern side of the Wapawekka Hills was the location of the Keewatin ice margin while the lake remained at the Upper Campbell level. Sometime after 9,350 BP, rapid ice retreat in the northwest exposed the northwestern outlet, allowing a rapid lowering of Lake Agassiz, and temporarily halting drainage through the Mississippi River system. Subsequent rapid isostatic rebound in the northwest would have raised the elevation of the northwestern outlet, causing its abandonment and reopening the southern outlet. (Abstract shortened by UMI.)