Manitoba Water Stewardship has issued a flood watch for areas of western Manitoba hard hit by heavy rains over the past three weeks.
A slow-moving storm system developing over Montana could bring heavy rain from Wednesday night to Thursday with amounts of 50 to 75 millimetres (two to three inches) possible.
The flood watch includes the Assiniboine, Swan and Red Deer river watersheds. It also includes streams in the Riding, Duck and Porcupine mountain regions and the Interlake.
Run-off from the additional rain will be heavy due to saturated soil conditions. Over-bank flows and extensive overland flooding are likely to occur if the rain falls as predicted. Specific river forecasts will be issued once the rain amounts are more certain.
Significant rain is also expected over the Manitoba portion of the Red River watershed and could cause renewed overland flooding in areas that experienced heavy rain in recent weeks.
Red River Floodway Operation
The Red River Floodway continues to be operated to reduce levels at the floodway inlet by about 0.3 metres (one foot) per day. Flows into the floodway are expected to end tomorrow but operation of the control gates will continue for about another two weeks to gradually reduce levels upstream of the floodway inlet.
The river level in downtown Winnipeg rose to 2.8 m (9.1 ft.) this morning and will change little for the rest of this week. The natural level without operation of major flood control works would have been 3.3 m (10.7 ft.) this morning. Increased flows on the Assiniboine River are stabilizing levels in the city.
Red River
Levels of the Red River declined 0.18 m (0.6 ft.) at Emerson and 0.3 m (1 ft.) at St. Adolphe during the 24-hour period ending this morning. Levels are unlikely to rise significantly due to rainfall later this week.
The flow in the Red River Floodway this morning was 1,160 cubic feet per second and the total flow at the floodway inlet was 19,630 cfs.
Assiniboine River
The Shellmouth Reservoir water level rose 0.1 m (0.35 ft.) since yesterday morning and stood at 428.7 m (1,406.6 ft) this morning. The computed inflow was 4,500 cfs based on the change in the reservoir level. Flows at Kamsack could be as high as 5,900 cfs and, if correct, would suggest a significant further increase in reservoir inflows during the next few days. The Water Survey of Canada will measure the flows at Kamsack tomorrow. The outflow was increased to 1,800 cfs early this afternoon and may rise as high as 2,200 cfs next week even without additional rain. The reservoir level is expected to rise to about 429.8 m (1,410 ft) by late June without further precipitation, well above the spillway elevation of 429.3 m (1,408.5 ft.). The level could rise much higher if rain later this week falls as predicted.
Minor flooding of low-lying valley lands occurs in the Shellmouth to Millwood area with outflows of 1,800 cfs. An operation strategy to minimize flooding of valley lands from Shellmouth to Millwood is being developed. Manitoba Infrastructure and Transportation has reviewed the possibility of raising the spillway with sandbags but has deemed this to be an unsafe procedure. An analysis shows that surcharging of the reservoir will not prevent flooding downstream of the dam. The effect of expected heavy rain during the next few days is being reviewed.
Portions of the Assiniboine River from Russell to Headingley are unlikely to experience over-bank flows unless rainfall from the next storm is widespread and heavy over both Saskatchewan and Manitoba portions of the watershed. Rainfall forecasts for the next storm are not encouraging.
Flows in the Portage Diversion are being reduced now that the river levels in Winnipeg are below the range where sewer backup would be affected by the river level. Flows down the Assiniboine River at Southport have been increased daily by 500 cubic feet per second since Saturday morning. River levels from Southport to west Winnipeg will gradually increase by an additional two feet or so during the rest of this week.
