Recent issues have had articles concerning the Shellmouth Dam and its effect on flooding of farmland in the Assiniboine valley downstream from the dam. A basic fact concerning dams is that any day in which the reservoir level behind the dam rises, the outflow from the dam must be less than the natural inflow into the reservoir on that day and the outflow can be said to have been artificially lowered on that day.
Conversely, any day on which the reservoir level falls is a day on which the outflow can be said to have been artificially increased above the natural inflow into the reservoir on that particular day.
If one knows where to look online, the federal government has a site where one can view the elevation readings of Lake of the Prairies taken every five minutes by automatic equipment. These readings show that from mid-March until the end of June of this year, the reservoir rose on a daily basis. This means that during that period, the outflow from the dam was less than the natural inflow each day. It could therefore be argued that any downstream landowner who was flooded before July 1 has only Mother Nature to blame.
The problem with the foregoing argument is that since the end of June, the reservoir has been dropping on a daily basis, which means that the outflow from the dam has been greater than the natural inflow each day during that period, thereby artificially prolonging the flooding.
The principal purpose for the construction of the Shellmouth Dam was flood protection. Unfortunately it has proven to be fairly useless at that task because the reservoir does not have enough storage capacity and because the dam itself has totally inadequate low-level outlet capacity. To reduce the peak flow during a flood event, most of the pre-peak flow has to be passed downstream through the low-level outlet so that the there is capacity in the reservoir to hold back a good part of that peak.
Republished from the Brandon Sun print edition August 30, 2012