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.
Agriculture in the area is intensive, frequently aided by irrigation with water from either the Assiniboine or from buried palaeochannels. This is most noticeable north of the Assiniboine River 17 where many small fields can be seen, probably supporting vegetables and fruits. The Portage area is the main strawberry growing region of Manitoba,[ii] and it seems likely that strawberries are grown in some of the fields close to the river. Similar intensive growing areas are located in Hoop and Holler Bend 18 and along the Elm River 19.
The twinned Trans-Canada Highway 20 can be seen in the north; it veers off to the south to bypass Portage la Prairie which is just off the photo to the west. PTH 26 21 is north of the Trans-Canada Highway and the CP railway line 22. Between the highway and railway are faint indications of the High Bluff palaeochannel 23. The CN line 24 trends west-northwest/east-southeast crossing the Assiniboine River in the process 25. In the west a road out from Portage la Prairie 26 approaches the Assiniboine River. There is no bridge, but the trend continues south of the river 27. In the extreme southwest are the ends of runways 28 at the Southport airbase (CFB Portage la Prairie). Two Hutterite colonies can be seen, the Elm River Colony 29 and another in the extreme southeast 30.
Figure 3.92: The Assiniboine River, the La Salle River, and the Elm River Palaeochannels East of Portage la Prairie
Vertical air photograph: MB89021-6-176
Flight height: 30,800 feet a.s.l.; lens focal length: 152.26 mm
Date: August 7, 1989
Scale: 1:57,600 (approx.)
Location: Townships 10, 11, 12; Ranges 5 and 6 WI
Map sheets: 1:250,000 62G Brandon
1:50,000 62G/16 Portage la Prairie
[i] Rannie, op. cit., 1990, 183.
[ii] Portage la Prairie claims to be the strawberry capital of Canada and has a strawberry festival each year.