Manitoba has an abundant supply of surface water with many large lakes and thousands of smaller ones (chapter 2). However, much of the water is in the north or drains to the north where there is a sparse population. In the south dams and reservoirs have been constructed to conserve water in some areas, whereas in other places projects were designed to drain water from the land to make it suitable for cultivation. The province’s rivers offered enormous potential for hydroelectric power, much of which has now been exploited, starting with a small-scale project on the Little Saskatchewan River and progressing to large projects on the Winnipeg, Saskatchewan and Nelson rivers. Two thermal power stations at Brandon and Selkirk on the Assiniboine and Red rivers illustrate the need for an assurred water supply. Flooding has been a problem since the earliest days of the province, especially along the Assiniboine and Red rivers culminating in the 1997 “Flood of the Century” in the Red RiverValley. Dams, diversions and ring dikes are used to divert the floodwaters to less critical areas.
16.42: Comparison of a Radar Image and a Vertical Air Photo of Flooding in the Assiniboine Valley Near Virden
A. Radar Image
Radar has the advantage over air photographs in that it is not light dependant; images can be obtained at night, and if the correct wave length is used it can penetrate clouds. Shown here is a large-scale radar image with a vertical photograph taken at the same time to the right of it. As in figure 16.41 the very dark tones delimit the flooded area 1, the river channel 2, and one oxbow 3. The very light-toned areas on the flood plain are the tree-covered areas 4; the rough surface of the treetops sends back a strong signal. The east bank, facing the direction from which waves were propagated, is light-toned 5 compared with the west bank 6. PR 259, which has a smooth surface and is therefore dark-toned 7, crosses the Assiniboine spillway. The very light-toned spots, north of the road, are pylons 8 carrying an electrical power line. Two other lines of pylons can be seen further south 9 , 10.
B. Air Photo
The air photo taken at this same time has a slightly larger scale. Although most of the river channel 1 and the flooded area 2, which are light-toned, are visible, some parts are obscured by cloud 3. In contrast to the radar image, treed areas are dark-toned 4, and PR 259 is light-toned 5 due to light reflection from its gravel surface. There is no evidence of the power lines.
Figure 16.42: Comparison of a Radar Image and a Vertical Air Photo of Flooding in the Assiniboine Valley Near Virden