2007 SPRING OUTLOOKS
FOR RIVERS AND LAKES
The spring flood outlook is derived from three weather scenarios based on additional snow, melt rates and spring rain found in climate statistics for the past 40 years. The scenarios are based on existing conditions. The terms favourable, average and unfavourable refer to the lower decile, median and upper decile conditions.
Peak discharges and water levels have been computed for the Red, Assiniboine and Souris rivers based on tributary forecasts and flood routing. Information on expected peak flows for the Red River at Pembina, N.D., and for the Souris River at Westhope, N.D., was obtained from the U.S. National Weather Service. Information on probable run-off and flows in eastern Saskatchewan was obtained from the Saskatchewan Water Authority. Climatological and hydrometric data provided by Environment Canada and used in this forecast is also acknowledged.
Red River Main Stem
The potential for spring flooding is quite low, mainly due to below-average snowcover in the U.S. portion.
With average weather, the Red River is expected to remain within its banks and only very minor operation of the Red River Floodway would be required.
With unfavourable weather (one in 10 chance), minor over-bank flows could occur in the portion from Emerson to just south of Morris. Operation of the Red River Floodway would keep the crest in downtown Winnipeg from exceeding 18 feet. Construction on the floodway channel would not interfere with floodway operation.
Serious ice jamming from Selkirk to Breezy Point appears unlikely this spring. Ice thickness appears to be near average but the relatively low river flows will reduce the chance of serious ice jamming. The threat of ice jams will be further reduced by icebreaking with the Amphibex.
Red River Tributaries
Flooding is not expected on Red River tributaries with average weather from now on. The unfavourable weather scenario could produce minor flooding.
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Assiniboine River Main Stem
The unfavourable weather scenario (one in 10 chance) would produce significant flooding along the Assiniboine River from Shellmouth to Brandon with crests similar to those of 2006. The flooding would end before mid-May at all locations which is sooner than in 2006. Late flooding would be eliminated by releasing larger amounts of water from Shellmouth Reservoir during April and early May. There would be no flooding downstream of Brandon due to low flows in the Souris River. Operation of the Portage Diversion would prevent flooding from Portage la Prairie to Winnipeg.
With average weather, minor over-bank flows would occur from Shellmouth to near Virden during April. Operation of the Shellmouth Reservoir would prevent downstream flooding after April. A minor operation of the Portage Diversion would prevent flooding from Portage la Prairie to Winnipeg.
Flooding is not expected along the Assiniboine River with favourable weather from now on. Shellmouth Reservoir would successfully hold spring inflows. Operation of the Portage Diversion would likely be unnecessary.
Assiniboine River Tributaries
Little or no flooding is expected with average weather. Unfavourable weather (one in 10 chance) could result in flooding along the Qu’Appelle and Shell rivers and flooding of low-lying areas along many other tributaries.
Flooding is not expected along the Souris River with average weather conditions from now on. Unfavourable weather conditions could produce minor flooding similar to that of 2006 along the mainstem. Flooding along tributaries is unlikely.
With average weather, minor localized flooding of low-lying areas, somewhat less than that of last spring, would occur. Increased flooding of agricultural lands adjacent to the Shoal Lakes, many of which are still flooded at this time, would still occur but peak levels would be slightly lower than in 2006.
The unfavourable weather scenario (one in 10 chance) could produce minor flooding on the Fisher River (including the Peguis townsite) and other streams in the eastern Interlake, similar to that of 2006. There would be extensive flooding of agricultural lands near the Shoal lakes but only minor flooding of low-lying areas elsewhere.
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Under the dry weather scenario, there would be a concern about low lake levels in southeastern Manitoba including the Whiteshell. Flooding is not expected with average future weather conditions.
The unfavourable weather scenario could produce minor localized flooding, especially on Cooks Creek and the Brokenhead River. Flows on the Winnipeg River would be below average even for this weather condition.
Flooding is not expected on the Whitemud River and its tributaries or on streams flowing into Dauphin Lake with normal weather.
The unfavourable weather scenario could produce minor flooding on streams in the region but only if a rapid melt develops in the high ground.
Flooding on the Whitemud River would be less than last year. Manitoba Water Stewardship will arrange for icebreaking in the lower Whitemud River to reduce the chance of ice-jam flooding at Westbourne.
Levels of Dauphin Lake are unlikely to be as high as last spring unless the unfavourable weather scenario develops and, therefore, flooding is quite unlikely.
Duck Mountain to The Pas
The average weather scenario would result in moderate flooding. The Swan and Red Deer rivers would experience flooding but of somewhat lesser magnitude than that of 2006. The Carrot and Saskatchewan rivers would have peaks similar to those of last spring with some flooding. The Amphibex icebreaking machine would be deployed to the Ralls Island area where ice jams tend to develop.
An unfavourable weather scenario would likely produce a repeat of the record flood on Red Deer Lake and the near record level at Swan River experienced in 2006. The peak water levels at The Pas could be slightly higher than in 2006 and very close to overtopping the river bank at Ralls Island unless the Amphibex is successful in preventing ice jams. Significant diking would be required at the Opaskwayak Cree Nation. Levels of the Carrot River west of The Pas could be somewhat higher than in 2006 with flooding of agricultural lands and potential flooding of a few homes in the area.
The favourable weather scenario would result in minor flooding on most streams from the Fork River north to the Saskatchewan River. The flooding would be much less than during the spring of 2006.
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Run-off in northern Manitoba (north of latitude 54) is likely to range from average to above average.
Flooding is not expected on the Churchill River upstream of Southern Indian Lake or on the Nelson River. However, some difficulties may occur near the town of Churchill where ice jamming is not uncommon.
Lake Winnipeg is relatively low this year due to dry conditions last summer. A smaller-than-usual rise is likely this spring due to below-average inflows from the Winnipeg and Red rivers. With average weather conditions, the lake may rise to little more than 713 feet this year which is about one foot lower than average. The level was lower during the last drought in 2003 when the July level was 712.2 feet.
Lake Winnipegosis is likely to rise to last year’s levels even with average weather conditions. Flooding of low-lying agricultural lands adjacent to the lake is expected again this year.
Lake Manitoba should be somewhat lower than in 2006 unless the unfavourable weather conditions develop. It is expected to remain within the operating range of 710.5 and 712.5 feet. Lake St. Martin is expected to be at well-above-average levels but somewhat lower than last year.
Lake of the Woods and many other lakes in the Whiteshell area had below-average levels last summer.
Many lakes in this region are likely to be low again in 2007 unless precipitation is above average from now through May, 2007. This includes lakes on the Winnipeg River system.
Most lakes in southwestern and western Manitoba are at somewhat below-average levels due to dry weather last summer.
Lakes in the vicinity of The Pas are at above-average levels and are likely to rise higher than desirable this spring. Last spring’s flooding of low-lying lands near these lakes is likely to be repeated this spring.
The level of Reindeer Lake is expected to be well above average after spring run-off while most other lakes in far northern Manitoba are expected to be at average or above-average levels.
The full report is available at http://www.gov.mb.ca/waterstewardship.