The Clearwater-lower Athabasca spillway was eroded by a catastrophic flood and extends 233 km from northwestern Saskatchewan into northeast Alberta, ending at the Late Pleistocene Athabasca braid delta, 85 km north of Fort McMurray. At the head of the spillway, an anastomosing complex of erosional channels are separated by streamlined-shaped erosional residual islands (hills). Smaller localized upper scoured zones consisting of channels and erosional residuals are regularly spaced along margins of the spillway.

Flood deposits within the spillway are sparse, but are present where the spillway widens or along its upper scoured zones. Representative flood facies consist of (1) boulder lags; (2) poorly sorted boulder gravel; (3) cross-stratified sands and gravels; and, (4) upper flow regime sandy plane beds. The principle depositional feature is the Late Pleistocene Athabasca braid delta which was built into glacial Lake McConnell.

Paleohydraulic calculations of the flood from the northwest outlet of glacial Lake Agassiz provide estimates of peak discharges, ranging from 1.2 to $9.1 \times 10\sp6$ m$\sp3$s$\sp{-1}$.

The combination of shorelines and glaciolacustrine sediments indicate a lake transgression in the upper Churchill watershed of west-central Saskatchewan. Geomorphic and sedimentological evidence for a transgression, and six radiocarbon ages (average 9869 B.P.) from associated flood deposits strongly supports glacial Lake Agassiz as the source in the upper Churchill river watershed during a high stand of the Emerson high-water phase.

Following closure of the eastern outlets at the onset of the Emerson Phase, glacial Lake Agassiz transgressed into the recently deglaciated and isostatically depressed upper Churchill watershed to a present elevation of 490 m at the head of the Clearwater-lower Athabasca spillway, an elevation coincident with the lowest point along the Churchill/Mackenzie drainage divide. The drainage divide consisting of till or the Beaver River moraine, was breached and incised resulting in a lowering of glacial Lake Agassiz by 52 m to a stable elevation of 438 m. In the process, 21,000 km$\sp3$ of water was released to the Arctic Ocean via the Mackenzie River, which raised global sea level by 5.8 cm.