The province released its first spring flood update yesterday (February 27). Little direct information was provided about Lake Manitoba other than noting that the current level of Lake Manitoba is above its operating range sitting at 813.1 feet.
But useful information was provided for lake residents. For Lake Manitobans and everyone downstream, the key information is in the forecast for the Assiniboine River. Even with unfavourable weather, the forecast looks reasonably good.
MIT anticipates that under unfavourable weather, we will experience Assiniboine river flows comparable to 2005. In that year, flows exceeded the bank capacity on the lower Assiniboine of roughly 17,000 cfs for only a few days on two separate occasions.
Yesterday, the province did not provide a forecast for Lake Manitoba levels in 2015. However, it is not too difficult to use the information they provided to look at what the spring and summer holds for Lake Manitobans. What happens on Lake Manitoba will be driven by whether the Portage Diversion opens for an extended period or not. If we do experience Assiniboine flows similar to 2005, we can use that as a guide to what to expect this summer.
In 2005, the Portage Diversion opened twice. It first opened in April for 19 days with a peak flow of 17,600 cfs (most days were well below this); and it was opened for a second time in June and July for 46 days, with lower flows peaking at about 10,600 cfs. Most days were well below this.
Most importantly, in 2005 the total flow through the Portage Diversion into Lake Manitoba was roughly 780,000 acre feet. That is enough water to raise the level of Lake Manitoba about 8 inches. Thus, even if we get the unfavourable 2005 conditions, Lake Manitobans are still in relatively good shape: the lake will be unlikely to reach flood level this summer, although it will remain high and above its operating range for an extended period.
The province forecasts that under normal weather conditions, Assiniboine flows will be comparable to 2002. The Portage Diversion did not open that year. And with average or favorable weather conditions, the level of the lake should fall steadily over the summer of 2015. It is too early to make firm projections about lake levels but if the diversion is not opened, or opened only briefly to clear ice on the lower Assiniboine, the level of the lake will be headed down, not up, which is better news than we have had in some time.
For the moment, Lake Manitobans should be cautiously optimistic about 2015, with the proviso always that Mother Nature is capricious and does not always play fair.