Manitoba Water Stewardship’s second spring flood outlook shows the 2011 spring flood potential remains high for much of Manitoba. This outlook includes the Red, Souris, Pembina, Assiniboine, Winnipeg, Saskatchewan and Fisher rivers as well as the Interlake region. Flood potential is high in these areas due to high river flows, very high soil-moisture conditions at freeze-up, above normal snow-water content in the snowpack, and an expected cooler and wetter spring.
Additional snowpack in southern areas of the Red River basin indicates that river levels may be slightly higher than what was predicted in the January flood outlook.
Average weather conditions along the Red River could result in a flood higher than 2009 while unfavourable weather conditions could result in Red River water levels reaching those experienced in 1997.
With average weather conditions along other rivers and streams in Manitoba, flooding is still anticipated and, with unfavourable weather, significant flooding could occur.
The spring flood potential is still very dependent on weather conditions from now until the spring melt. The amount of additional snow and rain, the timing and rate of the spring thaw and the timing of peak flows in Manitoba, the U.S. and other provinces will have a significant effect on flood potential.
Localized overland flooding is expected in most of central and southern Manitoba and could occur during the early part of the run-off period due to ice jams, snow blockages or frozen culverts in river channels, drains and ditches.
Climatic Conditions
• Precipitation during the autumn of 2010 was well-above normal in most of Manitoba, central and southern Saskatchewan, North Dakota and Minnesota.
• Cold temperatures from mid-November 2010 up to now have resulted in deeply frozen soil conditions in most areas.
• Widespread, heavy snow occurred from late November through early February in Manitoba, central and southern Saskatchewan, North Dakota and Minnesota.
• Recent warmer weather has slightly decreased the depth of the snowpack but the water content in the snow has remained relatively unchanged. Most of the snow water is contained in the snowpack or near the surface layer and is expected to remain in place until the start of the spring thaw.
Both the Assiniboine and Red rivers main stem channels rose less than three centimetres (1.2 inches), indicating a minor flow from snowmelt run-off.
• Significant meltwater is still trapped within the snowpack on fields, and in depressions and ditches.
• The onset of colder weather has slowed down or completely stopped any snowmelt.
Currently, a moderate La Niña in the equatorial Pacific is expected to gradually weaken through the spring months of 2011 and may bring below-normal temperatures and significant snowfall. El Niño conditions may also occur, bringing significant spring rain. Alternatively, a cooler-than-average spring might result in a staggered slow melt.
Soil Conditions
• As indicated in the January outlook, the soil moisture at freeze-up was well-above normal across most of central and southern Manitoba, central and southern Saskatchewan, North Dakota and Minnesota.
• Soil-frost information is sparse, but the data available up to mid-February for southern Manitoba and North Dakota suggests the soil is frozen to a depth of 20 to 100 cm (0.6 to three feet). Wet, frozen soils do not allow meltwater to soak in and, as a result, increase spring run-off potential.
Snow Cover and Winter Precipitation
• An airborne snow survey was conducted over the Red, Assiniboine and Souris rivers watersheds Feb. 18 to 20. The survey indicated that snow-water content ranged from six to 12.5 cm (2.4 to five in.), or an average of 9.6 cm (3.8 in.) over most of southern Manitoba. This is approximately 15 per cent above normal snow-water content for the region.
Spring Run-off
• Based on soil moisture and snowpack conditions as of Feb. 20, the 2011 spring run-off is expected to be above average over most of central and southern Manitoba.
• Well-above-average run-off potential is expected in the following locations:
1) along the Saskatchewan/Manitoba border south of Westray area to Virden,
2) the Killarney and Cypress River area,
3) the Winnipeg area and regions southwest of Lake Winnipeg,
4) the eastern portion of the Red River basin, and
5) the Winnipeg River watershed.
• The expected run-off is based on 2010 summer and fall soil moisture, snowpack conditions as of mid-February, and average future weather conditions for precipitation, date of ice breakup and melt rate.
Spring run-off could change significantly if future precipitation and breakup conditions differ significantly from the average.
River Forecasts
• The spring flood outlook is classified into three future weather scenarios related to additional snow, melt rates and spring rain. These scenarios are based on available climate statistics for the past 30 to 40 years.
• The three future weather scenarios are referred to as favourable, average and unfavourable.
• There is a one-in-10 chance of the weather being favourable and there is a one-in-10 chance of it being unfavourable. The favourable and unfavourable scenarios assume that breakup will be slow and relatively fast, respectively. Average weather and breakup conditions are assumed for the average scenario.
• Forecasts do not include effects of possible ice jams which are generally unpredictable. Localized and brief flooding can occur in locations where ice jams develop, even with below-average river flows.
Red River Main Stem
• The soil moisture index at 2010 freeze-up was the second highest recorded since 1948 and well above the level before the 1997 flood. However, it was slightly lower than the record high measured prior to the 2009 flood.
• Snow cover is well above average, especially south of Grand Forks, but still less than in mid-February 1997.
• Favourable Weather
1) With little additional precipitation and a gradual snowmelt, a flood close to that of 2006 is expected. This would cause only minor overbank flows from St. Jean Baptiste to Morris with bank-full conditions elsewhere.
• Average Weather
1) With average weather from now until April, flood levels are expected to exceed those of 2009 by between 0.3 to 0.76 metres (one to 2.5 ft.).
2) Flood protection works will prevent flooding of communities and most homes in the Red River Valley. However, transportation may be significantly disrupted with the closure of PTH 75 near Morris and the closure of many smaller roads in flooded areas.
3) Under this scenario, operation of the Red River Floodway would keep levels in downtown Winnipeg in a range of 6.4 to 7.01 m (20 to 23 ft.), depending on the flows from the Assiniboine River. In 1997, water levels in the Red River Floodway reached 7.47 m (24.5 ft.).
• Unfavourable Weather
1) There is a one-in-10 chance of unfavourable weather. With unfavourable conditions, peak stages would be comparable to or slightly higher than 1997 levels at most locations.
2) At most locations south of Winnipeg, Red River levels would not exceed those of 1997 by more than 30 cm (one foot). Due to protective measures in place, it is unlikely that homes would be flooded even under unfavourable weather conditions. However, transportation and access to properties in the flood plain would be reduced for up to three weeks. The crest in downtown Winnipeg would be similar to 1997, with a maximum of about 7.47 m (24.5 ft.).
3) Peak stages from Selkirk to Breezy Point would depend on whether ice jams develop. Ice-cutting and breaking operations will reduce the chance of serious ice jams. Currently, ice buildup is slightly less than the winter average due to high river levels and substantial groundwater flows which typically contribute warmer water to the Red River. With unfavourable weather, serious ice jamming will be reduced by ice cutting and icebreaking.
4) Even without serious ice jams, flooding of low-lying properties in the Breezy Point area is likely. Elsewhere, flooding is not expected unless serious ice jams develop.
Red River Tributaries
• With favourable weather from now through to spring, flooding would be fairly localized.
• If average weather conditions occur, significant over-bank flows would develop on portions of most streams and there would be overland flooding in low-lying areas.
• With unfavourable weather, extensive over-bank flows would occur on all streams, bringing them close to 1997 levels.
Pembina River
• With favourable weather conditions, there is the potential for minor flooding.
• With average weather conditions, peak water levels along the Pembina River and tributaries would still be close to those of 2006.
• With unfavourable weather (one-in-10-chance), flooding could exceed that of 2009.
Roseau River
• With favourable weather conditions, minor flooding is expected.
• With average weather conditions, flooding would occur at close to 1974 levels.
• With unfavourable weather conditions, flooding would occur at close to 2002 levels.
Assiniboine River Main Stem
• Favourable Weather
1) With favourable weather conditions, there would be minor flooding along the Assiniboine River.
• Average Weather
1) With average weather conditions, there would be flooding along the Assiniboine River. The drawdown of Shellmouth Reservoir now underway will minimize the risk of flooding while providing sufficient water supplies to meet downstream needs in 2011 and beyond.
2) Average weather conditions may also result in flooding from Millwood to Brandon with levels generally below those of 1995, but higher than 1979.
• Unfavourable Weather
1) Unfavourable weather would result in flooding of valley lands from Shellmouth to Brandon with levels likely higher than those of 1976 by about 0.05 to 0.6 m (0.2 to two ft.).
2) Shellmouth Reservoir is being drawn down this winter to provide maximum storage space for spring flood control along the Assiniboine River. This will greatly reduce spring peak stages along the river, especially for the portion between the dam and St. Lazare, where flooding will be avoided only if favourable weather conditions occur.
3) The Portage Diversion will be operated to prevent ice jams and flooding from Portage la Prairie to Winnipeg. Flooding may occur in this region with unfavourable weather conditions.
Assiniboine River Tributaries
• With favourable weather, localized flooding is expected.
• With average weather conditions, flooding is likely.
• Unfavourable weather conditions would result in significant flooding on most tributaries leading to over-bank flows.
Souris River
• The flood potential is high for the Souris River due to high soil moisture and higher-than-normal snow cover in the North Dakota portion of the watershed. Significant flooding of agricultural lands adjacent to the river is likely.
• A favourable weather scenario would produce a flood similar to that of 2001.
• With average weather conditions, a flood slightly less than that of 1974 is likely. Low-lying portions on the south edge of Melita would require diking.
• Unfavourable weather conditions would result in water levels higher than those of 1974 but lower than the peak stages of the 1976 record flood. Low-lying homes near Souris would require diking in addition to the area next to PTH 3 at Melita.
Souris River Tributaries
• With average weather conditions, flooding is likely on Souris River tributaries.
• Under unfavourable weather conditions, Souris River tributaries would experience significant flooding.
Interlake Region
• Soil moisture is well-above normal and most small depressions are relatively full from heavy rains last summer and fall. Snow cover is currently average to above average.
• With favourable weather, flooding would be limited to low agricultural lands and streams would remain within their banks.
• With average weather conditions, flooding would occur and the Fisher River would see flood levels close to those of 2006. Overland flooding would also occur.
• With unfavourable weather, flooding similar to 1986 (but lower than 2009) could occur. There could also be difficulties due to a high water table, resulting in seepage into basements in rural areas.
• Record high levels are likely on the Shoal Lakes, even with average weather conditions.
Eastern Region
• Soil moisture is currently above average and snow cover overall is above average in the region.
• With favourable weather conditions, localized flooding could occur on the Brokenhead, Whitemouth, Whiteshell and Winnipeg rivers.
• If average weather conditions develop, flooding is likely to occur along streams and lake shores. The Winnipeg River could experience levels similar to those of 1994.
• Unfavourable weather conditions could result in flooding of agricultural lands that would be close to previous record floods. In unfavourable weather conditions, water levels in the Winnipeg River would be similar to 2009 flood levels.
Westlake, Dauphin and The Pas Regions
• Soil moisture is above normal to well-above normal in most areas. Snow cover remains generally near normal to above normal.
• With average weather conditions, flooding is possible on the Whitemud, Turtle, Swan and Carrot rivers. With average conditions, the Saskatchewan River would be expected to see flood levels slightly higher than the 2005 flood at The Pas.
• With unfavourable weather conditions, general flooding could occur. Brief localized flooding is also possible due to snow-blocked or ice-filled streams, drains or ditches. If unfavourable weather conditions develop, flood levels higher than 1974 are expected on the Saskatchewan River at The Pas.
Northern Manitoba
• Run-off in most of Manitoba’s far north (54 degrees north) is likely to be average. Run-off is expected to be more substantial in northeast Manitoba.
Forecast Updates
• This forecast will be updated in late March when further precipitation and weather details are available.
Flood Preparations
• The Manitoba government and municipalities are continuing to prepare for spring flooding. This includes work with municipal emergency-management teams to review existing plans, gather information through conference calls, flood-preparedness meetings, Disaster Financial Assistance sessions and other related activities.
• The ice-mitigation program north of Winnipeg is well underway with six ice cutters and three Amphibex machines working along the Red River. Approximately 12 km of ice-cutting is complete.
• Vegetation has been removed from the floodway channel to improve flow in the floodway.
• Community ring dikes in the Red River valley are ready.
• The Shellmouth reservoir has been lowered by 5.9 m (19 ft.) to 424 m (1,391.4 ft) and is expected to be reduced by another 2.1 m (seven ft.) to 421.8 m (1,384 ft.) before the spring melt begins.
• Maintenance work at the Portage Diversion is underway including dredging the reservoir, erosion protection and removing excess vegetation.
• The Assiniboine dikes east of Portage la Prairie are being assessed and will be strengthened as needed.
• High levels of the Shoal Lakes north of Winnipeg are being assessed and improvements are being considered to the Hatchery Drain system, which is the only current outlet for the lakes. Vegetation removal on PR 419 to improve drainage capacity is being considered to divert water away from the lakes.
• New flood-fighting equipment is arriving, and equipment and material is being positioned across the province in preparation for any flood responses. Some of the equipment being deployed includes:
1) two million sandbags; – two new sandbagging units, bringing the number to five available sandbagging machines; – 20,000 additional super sandbags on order, bringing the total to 30,000; – 43 km of rapid-deployment cage barriers on order, with 14 km currently in inventory; – 20 additional heavy-duty steamers on order, which will bring the overall provincial complement to 57; – 21 new mobile pumps and two new large diesel pumps to be delivered in a few weeks; – 18 additional kilometres of water-filled barriers on order, bringing the overall provincial complement to 72 km, 30 km of which will be in rapid-response trailers.
Dredging of the Red River will be studied and assessed to look at its potential impact on ice jams, environmental factors and the capacity of the Amphibex machines to conduct a viable dredging program.
Information is also available www.gov.mb.ca/waterstewardship/floodinfo and: www.manitoba.ca.
