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Chapter 3: Geologic Structure and Landforms

3.91: Palaeochannels of the Assiniboine River East of Portage la Prairie

At Portage la Prairie the Assiniboine River emerges from a confined valley onto the flat floor of glacial Lake Agassiz. As a result of the lack of confinement and the decrease in gradient, an alluvial fan with a radius of 30-45 kilometres has been deposited. It was constructed by numerous palaeochannels of the Assiniboine River as the lake level declined beginning about 9500 years ago. The palaeochannels “are similar in size and geometry to the modern Assiniboine, a perennial, meandering suspended load river.”[i]

This photograph shows the Assiniboine River 1 and five of the palaeochannels, from north to south: High Bluff 2, Mill Creek 3, La Salle 4, West Branch La Salle 5, and Elm River 6. The meandering Assiniboine has left behind two cutoffs 7 that appear natural and two others 8 that appear artificial; the Norquay Provincial Recreational Area is located within the southernmost of the two 9. Meander scrolls left by the meandering palaeochannels can be seen in several locations 10. The evolution of the fan involved the Assiniboine repeatedly changing its position as a result of breaking levees during floods, possibly as a result of ice jams.[ii] The markings 11 were probably created by water rushing through a gap in a levee.

Most of the land in this area is intensively farmed and irrigation is practised using water extracted from buried palaeochannels.[iii] Land division varies from that seen on previous photos in that near the Assiniboine, division is according to the long lot system with narrow fields running back from the river 12. Further south the DLS system is used with sections and quarter sections being divided into small fields in this rich agricultural region 13. Common crops in this area are potatoes and vegetables grown under irrigation.

The most prominent routeway is the twinned Trans-Canada Highway 14. South of it PR 331 15 runs through Oakville 16 that also has the CN line near its southern edge 17. North of the Assiniboine, PTH 26 18 runs through Poplar Point 19, and the CP line 20 runs just north of that. A large Hutterite colony is visible in the west 21.

Figure 3.91: Palaeochannels of the Assiniboine River East of Portage la Prairie

Figure 3.91: Palaeochannels of the Assiniboine River East of Portage la Prairie

Figure 3.91

Vertical air photograph: A21852-19

Flight height: 24,100 feet a.s.l.; lens focal length: 85.55 mm

Date: August 12, 1970

Scale: 1:82,300 (approx.)

Location: Townships 11 and 12; Ranges 2, 3, 4, and 5 W1

Map sheets: 1:250,000 62G Brandon

62H Winnipeg

62I Selkirk

62J Neepawa

1:50,000 62G/16 Portage la Prairie

62H/13 Elie

62I/4 Warren

62J/1 MacDonald


[i] Rannie, W. F. “The Portage la Prairie ‘Floodplain Fan’.” in Alluvial Fans: A Field Approach, eds. A.H. Rachocki and M. Church, Chichester, England: John Wiley and Sons, 1990, 179.

[ii] For a detailed description of the origin see Rannie, W. F., Thorleifson, L. H. and Teller, J. T. “Holocene evolution of the Assiniboine River palaeochannels and Portage la Prairie alluvial fan.” Canadian Journal of Earth Sciences, vol. 26, 1989, 1834-1841.

[iii] Sawatsky, T. “Irrigation in the Central Manitoba Irrigation Association Region” in Irrigation in Manitoba: Past, Present and Future ed. J. Welsted. Brandon: Canadian Water Resources Association, 1998, 14-19.