Flood warning for Assiniboine River valley in Manitoba

This article was last updated on April 16, 2022

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Manitoba Water Stewardship advises it has raised forecasts for the Assiniboine River valley, from Shellmouth to Brandon, due to recent heavy rains in eastern Saskatchewan that produced 50 to 75 millimetres (two to three inches) of rain in the Yorkton area yesterday. The department also advises that a flood warning continues for the Assiniboine River.  The storms moved northeast and produced about 35 mm (1.4 in.) of rain in the Russell and Duck Mountain areas. 
The weather forecast calls for a high probability of additional thunderstorms in eastern Saskatchewan tonight, mainly in areas from Yorkton north to the area west of The Pas and Flin Flon. Southeast Manitoba is likely to experience thunderstorms late tonight or tomorrow, mainly in areas east of the Red River.
Heavy rain over the Shellmouth Reservoir area yesterday was largely responsible for a 5.2-centimetre (two in.) rise since Wednesday.  The reservoir was just above 430.4 metres (1,412 feet) this morning.  Additional heavy rain will increase inflows to the reservoir somewhat, but the effect may not be large since streams were receding prior to this latest storm.  The additional reservoir rise may by only three cm (1.2 in.), unless significant additional rain develops tonight or tomorrow. 
There is uncertainty in the Assiniboine River forecasts which will be refined in the next few days, however, it appears flooding of the Assiniboine valley will be more extensive than was expected prior to yesterday’s downpours.  Crest forecasts have been raised about 0.2 m (0.5 ft.) from Shellmouth to Millwood and about 0.5 m (1.5 ft.) from St. Lazare to Brandon. Crests will be about a week later due to the additional rain, with the crest expected at Shellmouth July 4 to 5, St.Lazare July 8 to 10, Virden July 12 to 14, Brandon July 14 to 17 and Portage la Prairie July 17 to 20.   Levels from Spruce Woods to Long Plains will be about 0.23 m (0.75 ft.) higher by mid-July due to yesterday’s rainstorm.
The Portage Diversion continues to be operated to reduce the risk of sewer backup in Winnipeg should heavy downpours develop over the city.  The diversion flow this morning was 5,870 cubic feet per second while the flow at Southport (downstream of the control
structure) was 3,070 cfs. Assiniboine flows toward Winnipeg will be increased next week at a rate which will keep river levels in downtown Winnipeg will be increased next weekbetween 2.9 to three m (9.5 to 10 ft.).  Total rises of 1.4 to 1.5 m (4.5 to five ft.) are expected from Portage la Prairie to Lido Plage next week with lesser rises of 0.9 to 1.2 m (three to four ft.) from Headingley to Winnipeg. 
Areas near Gypsumville experienced heavy rain yesterday, resulting in some overland flooding.  Small streams and drains in the Interlake continue to decline and larger streams such as the Fisher River have crested. Low areas on fields will continue to have standing water for a week or two even with favourable weather, and a high water table may cause seepage into some basements. The Shoal Lakes are at record high levels and will remain so for most of the summer.
Lowering of the gates of the Red River Floodway control structure continues.  The level of the Red River at the floodway inlet is expected to decline to the natural level by tomorrow. The flow at the floodway inlet this morning was 23,600 cfs. The river level in downtown Winnipeg stood at 3.4 m (11.1 ft.) at 9 a.m., a decline of 13.4 cm (5.28 in.) from yesterday. The level in Winnipeg is expected to decline very slowly this weekend, based on little rainfall. 
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