Northwest outlet channels of Lake Agassiz, isostatic tilting and a migrating continental drainage divide, Saskatchewan, Canada

Lake cores obtained from the northwest outlet of glacial Lake Agassiz in northwest Saskatchewan, Canada, provide a minimum date for the cessation of the flood from the northwest outlet, and a chronology for abandonment of mid-Holocene channels that presently straddle the Mackenzie and Churchill drainage divide. The stratigraphy of a vibracore taken from Long Lake consists of a lower pebble gravel fining to massive sand, silty-clay and then fibrous peat. Wood fragments from the base of the clay yielded an accelerator mass spectrometry (AMS) date of 9120 BP. Because the lake is scour in origin and is in the head of the spillway, the date is considered to be a minimum estimate for cessation of the flood from the northwest outlet at the beginning of the Emerson Phase. A vibracore taken at Haas Lake in an abandoned channel surrounded by muskeg with no influent streams, consists of 0.8 m of stratified, pebble gravel containing abundant shell and wood fragments, overlain by 1.62 m of gyttja with a sharp, conformable lower contact. AMS dates range from 5590 BP from the topmost gravel to 3080 BP within the gyttja. The gravel is interpreted as fluvial, recording a river draining Wasekamio Lake north into the Clearwater River across the present-day drainage divide. Today, a drop of 2 m occurs from Wasekamio Lake southeast to Lac Ile-a-la-Crosse, along 150 km of lake basins parallel to the Cree Lake Moraine. The dates from Haas Lake suggest that before 5200 BP, the drainage divide was about 100 km further southeast, implying that during the Emerson Phase, lake level was controlled by a sill near Flatstone Lake at about 430 m instead of between Wasekamio Lake and the Clearwater River, as was previously proposed. Holocene differential isostatic uplift caused the flow reversal in the upper Churchill basin. Anastomosed channels at the mouth of rivers flowing north into lakes indicate that uplift is still active in the area today.