At the end of the Wisconsinan glaciation, melting ice produced enormous quantities of water that accumulated in massive lakes in many parts of the prairie provinces. Once a lake reached an overflow level, it spilled into lakes at lower levels, possibly in as short a time as a few months or years. The erosive power of the overflow water was great, resulting in the creation of steep-sided, flat-floored valleys—spillways—usually cut into easily eroded glacial sediments but occasionally reaching down to the underlying bedrock. Spillways of various sizes are ubiquitous throughout Alberta, Saskatchewan, and Manitoba. This photograph shows a spillway, now occupied by the Assiniboine River, which carried water from glacial Lake Assiniboine in Saskatchewan to glacial Lake Agassiz.[i] At this location just south of Shellmouth, the flat spillway floor 1 is between 0.7 and 1.0 mile (1.1 to 1.6 km) across. The steep valley walls, over 200 feet (61 m) high, can be seen to the west 2 and east 3. The dark-toned west wall is more obvious because it is completely wooded, a result of the relatively wet microclimate on the east and northeast facing slopes. The eastern wall—west facing—is drier and therefore covered by scrub. Tributary valleys, short on the west side 4 but much longer, in the form of the valley of Thunder Creek 5, in the east, dissect both sides. Thunder Creek enters the Assiniboine River 6 near the southern edge of the photo. Its valley has the characteristic v-shape of recently eroded valleys, contrasting sharply with the flat-floored spillway. The freely meandering Assiniboine occupies the spillway floor. In three places the channel impinges on and runs along the valley sides 7. Recent cutoffs can be seen at several locations 8, and at one location a whole river segment has been abandoned 9.
West of the spillway is characteristic ground moraine topography with a definite north-northeast/south-southwest orientation with long narrow sloughs in depressions 10. In the east is a drier plateau with little surface drainage except two lakes 11 and with the characteristic ripple markings of outwash 12.
The imprint of the DLS system can be seen, but in this area with two major valleys, the topography influences road directions more than is usual in southern Manitoba 13. The same is true of the CN line 14, which crosses Thunder Creek at right angles 15, then swings south on the eastern plateau 16 before running obliquely down the spillway side 17. Two prominent cuttings can be seen along the railway’s route 18.
[i] For details see Klassen, R. W. “Wisconsin events and the Assiniboine and Q’Appelle valleys of Manitoba and Saskatchewan.” Canadian Journal of Earth Sciences, vol. 9, 1972, 544-560.