Geologic interpretation of images begins with the identification of lineaments, “regional linear features caused by linear arrangement of regional morphological features such as streams, escarpments, and mountain ranges and tonal features that in many areas are the surface expressions of fractures or fault zones.”[i] The emphasis in this section is on geologic structures and landforms, both of which can be easily identified on air photographs and other images.
The sequence followed here is that often found in geomorphology books. Geologic structures and structurally controlled landforms are illustrated first, followed by images of mass wasting and of the results of the agents of erosion—running water, ground water, ice, wind and the sea. The meandering rivers of southern Manitoba are excellent examples of that river form. Several of them have deposited deltas into lakes. The effects of ground water are less easily illustrated, but examples of spring sapping and artesian erosion exist in the south. As all of Manitoba was covered by ice during the latest glaciation, examples of glacial erosion, and especially of glacial deposition, are widespread. Equally impressive are the suite of landforms created by the large glacial lakes that appeared as the ice melted. Glacial spillways, glacial lake deltas, strandlines and flat lake floors are found in many places. Wind action has created dunes on some of the deltas and on glacial outwash deposits. The Hudson Bay coast is rising as a result of isostatic rebound, recovery from the weight of the ice. One result is the existence of strandlines many metres above the level at which they were created. Finally Manitoba’s great lakes are large enough to illustrate many of the landforms normally associated with sea coasts.
[i] Lillesand, T. M. and Kiefer, R. W. Remote Sensing and Image Interpretation (Third Edition). New York: Wiley, 1994, 179.
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
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
1:50,000 62G/16 Portage la Prairie
[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.