Manitoba Water Stewardship reports the 2009 spring flood potential is high on the Red, Souris and Pembina rivers and moderate in the southern Interlake. It is average on the Assiniboine River and average to below average in the Westman area from Riding Mountain to The Pas and in the north.
Flooding of a 2006 magnitude is likely on the Red River but even with unfavourable weather, peak water levels on the Red are predicted to be significantly lower than 1997. Under present conditions, there is less than a five per cent chance of a flood comparable to 1997.
The spring flood potential is very dependent on weather conditions through to the end of spring. The amount of additional precipitation and the timing and speed of the breakup will have a significant effect on the flood potential. Minor, localized flooding could occur during the early part of the run-off due to ice jams and, while the probability of major ice jams on larger rivers is only average, these larger ice jams cannot be ruled out.
Climatic Conditions:
Precipitation during late 2008 was unusually heavy in much of southern Manitoba. A major rainstorm over southeastern Manitoba and adjacent portions of the United States during the first week of November saturated soils in the Red River watershed. Well-below-average temperatures from mid‑November through January resulted in a deep frost. Heavy snow occurred from late December through early February, mainly in areas near the U.S. boundary. A significant storm in early February brought rain to southeastern Manitoba and heavy snow to western Manitoba from the Duck Mountain area to the U.S. boundary. A mild La Nina condition is expected to continue until early summer somewhat favouring above-average precipitation from now through the spring.
Soil Conditions:
An aerial soil moisture survey in December showed moisture in the top 7.9 inches (20 centimetres) of soil was well above average in southeastern Manitoba and close to average in south-central and southwestern Manitoba. Fall soil moisture in the root zone (up to 1.2 metre depth) appeared to be average to above average in agricultural areas of Manitoba and well above average over the Red River Valley based on a survey by Manitoba Agriculture, Food and Rural Initiatives.
Soil frost information suggests the soil is frozen to a depth of .6 to .9 metres (two to three feet) in many areas, which is deeper than average due to the very cold weather in December and January. Wet frozen soils do not allow meltwater to soak in and increase spring run-off.
Snow Cover and Winter Precipitation:
An airborne snow survey was conducted over the Red, Assiniboine and Souris river watersheds Feb. 1 to 6. The survey showed snow water content ranged from 25 to 50 millimetres (one to two inches) over most areas of southern Manitoba, below the average since 1995. It is also noted that additional precipitation has fallen since that survey. A Feb. 13 to 17 airborne snow survey over the U.S. portion of the Red and Souris river watersheds showed snow water content ranged from 50 to 75 mm (two to three in.) over most areas, with a few areas near 100 mm (four inches). This is generally above average for mid-February. A conventional snow survey in February showed snow cover was well above average in the Pembina River watershed and average to below average in most other areas.
Spring Run-off:
The spring run-off, based on average future precipitation, date of ice breakup and the melt rate, is expected to be somewhat above average for most of southeastern Manitoba and the Interlake, and average to below average for most other areas of southern Manitoba. It is expected to be well above average for the Pembina River watershed. A much-above-average run-off is predicted for the U.S. portions of the Red and Souris river watersheds.
Areas of western Manitoba from Swan River north as well as most areas of far northern Manitoba are expected to have average to below average run-off this spring based on normal weather conditions.
River Forecasts:
The spring flood outlook is based on future weather scenarios relating to additional snow, melt rates and spring rain. These scenarios are based on climate statistics over 40 years. The scenarios are referred to as favourable, average and unfavourable and relate to the statistical lower, median and upper decile conditions. There is a one-in-10 chance of weather being favourable or better and there is a one-in-10 chance of it being unfavourable or worse. In addition to precipitation differences, lower decile and upper decile conditions also assume the breakup will be slow and relatively fast, respectively, while average conditions are assumed for the median. Forecasts do not include effects of possible ice jams which are unpredictable. Brief flooding can occur where ice jams develop, even with below-average river flows.
Red River Main Stem:
The potential for spring flooding is high due to high soil moisture levels in most of the watershed and above-average snow cover in the U.S. portion.
• A flood similar in magnitude to April 2006 is expected with average weather from now through April. The 2006 flood was very similar to 1996. Homes in the Red River Valley would not be flooded due to flood protection works. Transportation would be disrupted with closure of PTH 75 near Morris and closure of smaller roads in that area. Operation of the Red River Floodway would keep levels in downtown Winnipeg below 5.8 metres (19 feet) as compared to 7.5 metres (24.5 feet) in 1997.
• With unfavourable weather (one-in-10 chance) peak stages would be about .6 metres (two feet) higher than 2006 levels at most locations. Peak levels would still be significantly lower than 1997. Homes would not be flooded but transportation and access to properties in the flood plain would be greatly restricted for two to three weeks. The crest in downtown Winnipeg would be about 6.1 metres (20 feet). The probability of a 1997 magnitude flood (or greater) occurring this spring is less than five per cent at this time.
• With favourable (lower decile) weather, such as little additional precipitation and a gradual snowmelt, a flood similar to that of 2001 is expected. This would cause only minor over-bank flows from St. Jean to Morris, with bank-full conditions elsewhere.
• Peak stages from Selkirk to Breezy Point will depend a lot on whether ice jams develop. Ice‑cutting and breaking operations by North Red Community Water Maintenance Incorporated and Manitoba Water Stewardship will reduce the chance of serious ice jams. With favourable weather conditions and ice breaking, serious ice jamming is unlikely. Flooding of low-lying cottages at Breezy Point is likely even without serious ice jams but, elsewhere, flooding is not expected without serious ice jams.
Red River Tributaries:
With favourable weather from now through spring, flooding would be minimal and localized. This is due to snow cover being somewhat below average at this time.
• With normal weather conditions, minor over-bank flows would occur 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, somewhat in excess of 2006 but still significantly less than in 1997.
Pembina River:
The flood potential is high on the Pembina River due to high soil moisture and above-average snow cover. The snowpack is heavier in the United States portion and this may result in unusually high flows and flooding on streams such as Badger and Cypress creeks.
• With average weather conditions, peak water levels along the Pembina River and tributaries will be less than 2006, but with the upper decile (1-in-10) weather, flooding could exceed that of 2006 and approach that of 1974.
• Flooding of the Pembina River Valley is expected unless weather conditions from now on are very favourable.
Assiniboine River Main Stem:
With median weather conditions, there should be no flooding along the Assiniboine River. The drawdown of the Shellmouth Reservoir will minimize the risk of flooding, while providing sufficient water supplies to meet downstream needs later. This will greatly reduce spring peak stages along the Assiniboine, especially the portion between the Shellmouth Reservoir and St. Lazare where flooding will be avoided for all but unfavourable weather conditions.
• With favourable weather from now on (1-in-10) there would be no flooding along the Assiniboine River this spring.
• Average weather conditions from now on would result in minor flooding from Millwood to Brandon for a short period during April.
• Unfavourable weather (upper decile condition) would result in flooding of valley lands from Shellmouth to Brandon similar to 2007 and bank-full flows from Portage la Prairie to Headingley. Flooding could occur in the Spruce Woods area if ice jams develop.
• The Portage Diversion will be operated as needed to prevent ice jams and flooding from Portage la Prairie to Winnipeg. Minor flooding could occur in this portion if heavy ice flows enter the Portage Reservoir, mainly under the unfavourable weather scenario.
Assiniboine River Tributaries:
Flooding is not expected with average weather conditions from now through the breakup. An adverse weather scenario would produce minor flooding on most tributaries.
Souris River:
The flood potential is unusually high for the Souris River due to high soil moisture and much‑above‑normal snow cover in the North Dakota portion of the watershed. Significant flooding of agricultural lands near the river is likely.
• A favourable weather scenario would produce a flood similar to 2001.
• With normal weather conditions, a flood similar to 1999 is likely. Low-lying portions on the south edge of Melita would require some diking.
• The unfavourable weather scenario would produce levels somewhat higher than 1999 but still significantly lower than peak stages of the record flood of 1976. Some low-lying homes near Souris might require diking in addition to the area next to PTH 3 at Melita.
Souris River Tributaries:
Flooding is unlikely on Manitoba tributaries of the Souris River under the normal weather scenario. Minor flooding is expected with the adverse weather scenario.
Interlake Region:
Soil moisture is high and most small depressions are relatively full as a result of last summer’s heavy rains. Snow cover is average to below average at this time.
• With favourable weather from now on, flooding would be limited to low agricultural lands and streams would remain within their banks.
• With average weather from now on, minor flooding similar to that of 2005 would occur.
• With unfavourable weather, serious flooding similar to 1986 could develop. There could be difficulties due to a high water table, resulting in seepage into basements in rural areas.
Eastern Region:
Soil moisture is above average but snow cover is below average at this time.
• Flooding should be minimal on the Brokenhead, Whitemouth, Whiteshell and Winnipeg rivers with favourable weather conditions from now on.
• With normal weather conditions, minor flooding is likely to occur along streams and lake shores.
• The unfavourable weather scenario could produce significant flooding of agricultural lands, but still much less than previous record floods. It is unlikely homes would be affected although there could be water on some yards and seepage into basements.
Westlake, Dauphin and The Pas Regions:
Soil moisture is near normal in most areas, except for a small area near Dauphin. Snow cover is generally below average.
• Flooding is unlikely with normal spring weather conditions.
• Localized flooding could develop if unfavourable weather develops. Brief, localized flooding is also possible due to blocked or ice-filled streams, drains or ditches.
Northern Manitoba:
Run-off in most of far northern Manitoba (north of latitude 54) is likely to range from average to below average. Run-off should be more substantial in northeastern Manitoba.
This forecast will be updated in mid-March or earlier if heavy precipitation develops.
