The Assiniboine River is a typical meandering river with a single main channel embanked within a flat, shallow valley. The river flows south-east from its source in Saskatchewan towards Virden, Manitoba. At Portage la Prairie, some of the flow of the Assiniboine is diverted into Lake Manitoba. Since 1967, the flow of the Assiniboine River has been controlled by a dam located at Shellmouth. However, several important tributaries of the Assiniboine River are drained downstream of the dam thus increasing the stream flow during a runoff period.
The Flood of '95
This year during the spring, while the majority of southern Canada was snow free, the south-western part of Manitoba was still experiencing winter conditions. The Assiniboine River basin and its main tributary, the Qu'Appelle River basin received above normal snow fall during the winter which meant that the snow depth was also much greater than usual. The below normal temperatures recorded from March to the beginning of April increased gradually to above freezing point which resulted in a tremendous snow melt runoff and a flood that will be remembered for generations.
The Assiniboine flow rates have been recorded since the early 1900's at three stations along the river. Flow rates recorded at these stations during the peak of the flood were the highest on record since the construction of the dam. Since 1913, the mean flow rate recorded at the three stations for April are 32 m3/s, 81 m3/s, 115 m3/s respectively. The 1995 peak flows for the Assiniboine River were 360 m3/s, 566 m3/s, and 300 m3/s. Note that the peak flows of 1995 occured even with the flood control operations at the Shellmouth dam. (The actual flow rates recorded at the three stations are available as a table.)
CCRS C-Band SAR
On April 22 and May 04, 1995, the airborne radar operated by CCRS surveyed the river from its source to Portage la Prairie. The image presented here was acquired during the first flight with a narrow mode imaging configuration (C-band HH polarization). A larger version of the image, centred on the town of Virden and showing more of the river valley is also available (304Kb, jpg).
On the image, we can distinctly see the flood extent of the river which covers the entire Assiniboine River Valley (A). The standing water appears very dark on the image while the backscatter of the flooded vegetation is enhanced by the presence of water underneath the canopy (B). These combined factors clearly delineate the meandering river channel (C), and the oxbow lakes (D). The pseudo corner reflector effect between standing elements and flooding water increase the backscatter of power line and fence posts which allow field boundary delineation under flood conditions (E).
This mosaic of hand held aerial photographs was taken coincidentally with the SAR data from an altitude of 3,000 feet.
As with the radar image, we can see the road, the meandering channel (C), the tree cover, etc. However, on the photograph, it is difficult to delineate the flood extent under the cloud shadow (F) and moreover under the flooded vegetation. Another disadvantage is that we can not see the fence posts and/or power line which are clearly visible on the radar image (E).
This demonstration clearly illustrates how SAR imaging systems can be used for delineating flood boundaries, and determining flood damage under any atmospheric conditions. RADARSAT, with its high temporal frequency coverage will improve flood monitoring techniques and provide a powerful tool for flood control, forecasting and prevention.
|Location||Peak Flow, 1995
|Mean Flow, April '95
|Mean Flow, May '95
|Max Flow, Date
April 29, 1922
May 07, 1923
April 27, 1916