The Flood Action Committee received the following answers from Manitoba Water Stewardship in response to questions asked about Lake Manitoba levels.
1) Concerning inflows into Lake Manitoba this year, I saw that there was a recent increase in the provisional flow data from Environment Canada for the Waterhen River, increasing the flow by at least 100 cubic metres per second. This was a shock to me, as this would mean that the inflow from the Waterhen and the outflow at Fairford would approximately negate each other. Does your department have any additional information in being able to confirm the current flows from the Waterhen, or is this the best data available?
The increase in levels seen in Waterhen River gauge is due to ice jams downstream. The automated conversion to flows through rating curves never reflect the reality of flows during ice jams. As the jams clear and/or settle, the water levels fluctuate within a short time and then generally are reduced as the ice cover builds up over the river.
2a) Could you please confirm the total volume of water that was channelled in through the Portage Diversion from April 6 until early August this year, and what this is equivalent to in terms of feet of water added to Lake MB?
In 2011 flow was conveyed from the Assiniboine River to Lake Manitoba via the Portage Diversion from April 7 to August 5; a period of 121 days. The total volume carried by the Portage Diversion in 2011 was 4.7 million acre-feet, or 5,800 million cubic metres. This is equivalent to 4.13 feet of storage on Lake Manitoba.
2b) I’m also aware that the actual artificial volume contributed was slightly lower (one non-government article I read said 20%) due to the fact that some of the water would have naturally flowed into lake. I imagine that it can be a difficult estimate, but is this 20% figure a good estimate?
This natural overflow volume is very difficult to estimate, but detailed modelling studies are currently under way to determine what that volume might be. Preliminary estimates are that the natural overflow from the Assiniboine to Lake Manitoba with 2011 Assiniboine River flows (adjusted to remove the effect of Shellmouth Reservoir operation) would be in the order of 1 to 1.5 million acre-feet (1200 to 1800 million cubic metres). That would be 20 to 30% of the total volume diverted in 2011.
