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Chapter 3: Geologic Structure and Landforms

3.85: Landforms of Southwestern Manitoba

This image was obtained by one of the early land sensing satellites—Landsat I—from an altitude of about 900 km (559 miles) above the earth’s surface. Consequently a large part of southwestern Manitoba is displayed, although at a small scale of 1:1,000,000. The image was obtained on April 23, 1974 when most of the snow that had fallen during the preceding winter had melted, although some still remains on the higher land of Riding Mountain in the north 1. Some lakes within Riding Mountain National Park are still frozen and have a thin snow cover, resulting in an off-white colour; e.g. Clear Lake 2, Bottle Lake 3, Lake Audy 4, and Whitewater Lake 5. This also applies to Lake Manitoba 6, Pelican Lake 7, Rock Lake 8, and Swan Lake 9. On some other lakes the ice has at least partially melted resulting in a bluish colour, for example, Proven Lake 10, Seagull Lake 11, Oak Lake 12, and Whitewater Lake 13. Meltwater from the winter snow has accumulated in low-lying areas and produces a dark blue colour; for example along the Assiniboine spillway 14, the Souris spillway 15, and the Souris Pembina spillway between the white lakes 16. Extensive flooding is also occurring in the Jackfish Lake, Seagull Lake, and Chandler Lake area 17 as well as along the Whitemud River downstream from Westhope 18.

Of the many deltas deposited into glacial Lake Agassiz, the Assiniboine is the biggest and best documented.[i] It has its apex at Brandon 19 and fans out to the east—its boundaries are indicated by the dashed lines. On this image it is represented in buff/orange tones that represent the combined effect of radiation from vegetation—mainly parkland—with grass and tree patches—and the surficial deposits—mainly sand. Patches of coniferous trees are orange 20, resulting from the fact that they are reflecting infrared radiation that is recorded by the sensing system used. A darker patch in the northeast 21 represents an area where a late ice advance deposited till on top of the deltaic sands. The leading edge of the delta is marked by a strandline that continues to the northwest 22. A higher strandline that represents an early period of stillstand of Lake Agassiz can be seen to the west 23. To the east are numerous lower level, newer strandlines 24 left as the lake retreated to the east.

The Assiniboine has incised itself into the delta below Brandon responding to a reduced base level as Lake Agassiz retreated. Immediately below Brandon the valley floor is narrow with no room for flooding, but further east the meandering river has created a flood plain which was flooded on April 23, 1974 25. The river’s course is now near the southern edge of the delta, possibly because of greater isostatic uplift to the north where ice was thicker. Two former distributary courses can be seen; Douglas Marsh 26, Sewell Lake 27, and Epinette Creek 28 now occupy one to the north. Glenboro Marsh and the headwaters of Cypress River 29 occupy the other, to the south—dark blue because of flooding. An area with a similar colour to the Assiniboine Delta can be seen in the southwest around Oak Lake 30. Here too the colour results from a combination of vegetation and surficial deposits, silts and sands deposited into glacial Lake Hind, but without the deltaic form. Flooding—dark blue—is extensive in the low land around Oak Lake 31.

Several prominent valleys, in addition to those already mentioned, can be identified: the valleys of Birdtail Creek 32 and the Little Saskatchewan 33 spillways emanating from the Riding Mountain upland; the Polonia trench 34; the Whitemud Valley, south 35 and east 36 of Neepawa; the valley of Squirrel Creek 37, flowing from the Assiniboine delta; and the Souris Valley west 38 and north 39 of the elbow of capture 40.

Riding Mountain in the north is the most obvious upstanding landmass, but also visible in light colour are the Brandon Hills 41, part of an end moraine; the Arrow Hills, an esker 42; and the Bald Head Hills 43—also light coloured—sand dunes on the Assiniboine delta. The hummocky lake-strewn area of the Tiger Hills 44, also part of an end moraine, can be seen south of the Assiniboine delta, and less obvious near the southern edge of the image is Turtle Mountain 45, another area of end moraine indicated by a slight difference in colour from the flat land to the north.

On this image physical features are more obvious than the results of human activity except that over the whole area wherever agriculture is practised the checkerboard pattern of the DLS system is obvious.

Figure 3.85: Landforms of Southwestern Manitoba

Figure 3.85: Landforms of Southwestern Manitoba

Figure 3.85

Landsat I Image April 23, 1974

This is a colour composite of Bands 4 wavelength 0.5-0.6 mm, (green) band 5; 0.6-0.7 mm (red); and band 8 0.8-1.1 mm (infrared)

Scale: 1:1,000,000 (approx.)


[i] For details see Teller, J. T. and Clayton, L. eds. Glacial Lake Agassiz, St. Johns: Geographical Association of Canada, Special Paper 26, 1983.