UPDATED 2006 SPRING FLOODING OUTLOOK FOR MANITOBA

The potential for flooding is relatively low in most portions of southern Manitoba this spring due to an average to below average snowpack. The potential is moderate on the Red River mainstem, in portions of southeastern Manitoba, the Interlake region and in some areas of western Manitoba from Riding Mountain to The Pas due to high soil moisture and substantial snowcover. Spring runoff in northern Manitoba is expected to be generally above average, largely due to lingering effects of the very wet conditions in 2005.

The spring flood potential is still very dependent on weather conditions from now through May. Below average precipitation and an early breakup would result in little or no flooding whereas well above average precipitation could still result in serious flooding in many areas of Manitoba.

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An aerial soil moisture survey conducted in early November, 2005 showed that moisture in the top 20 cm of soil was average to somewhat above average in most portions of southern Manitoba. A map showing the results of this survey is attached (NOHRSC logo).

Soil moisture in the deeper root zone (top 120 cm of soil), is generally above average based on a field survey conducted by Manitoba Agriculture in late October, 2005. This is supported by an antecedent precipitation index (API) based on weighted precipitation from May to October, 2005. Soil moisture was found to be somewhat below average in the Dauphin area and in portions of the western Interlake.

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Manitoba Water Stewardship conducted an aerial snow survey and a conventional surface based survey in mid February, 2006. These surveys showed that the water content of the snowpack in southeastern Manitoba ranged from 75 to 100 mm (125-150 % of normal). Snow water content decreased to less than 25 mm (70 % of normal) in southwestern Manitoba.  The snow water content was high in the Riding, Duck and Porcupine Mountain regions where it ranged from 125-150 mm (150-200 % of normal). Maps showing the water content for the aerial survey and the percent of normal water content for the surface based survey are attached.

Snowcover is also available from a satellite survey prepared by Environment Canada on February 23, 2006. The snow water content may not be reliable in many areas of Manitoba this year due to the many episodes of thawing and freezing this winter. The satellite estimates are of particular interest in northern Manitoba where other surveys are not available. The estimated snow water content in the north ranges from 75 to 125 mm, which appears to be close to average.

Cumulative winter precipitation can be used as an estimate of snowcover. It may over-estimate the snowcover in southern regions this year due to periodic melting and some sublimation losses. A map showing percent of normal precipitation from November 1, 2005 to February 20, 2006 is attached. Snowfall is difficult to measure accurately in gauges due to wind effects.

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A map showing the spring runoff potential, as a percent of normal, based on present basin conditions and average future weather is attached.

Spring runoff is expected to be above average in portions of southeastern Manitoba, the eastern Interlake and in western Manitoba from the Riding Mountain to The Pas. Most other areas of southern Manitoba should experience average to below average spring runoff based on normal weather. Runoff in northern Manitoba is expected to be somewhat above average in most areas due to high soil moisture and high pothole storage remaining from the extremely wet late-summer and autumn conditions in 2005. While many areas in southern Manitoba were very wet in June and July of 2005, much of this moisture has since dissipated so the effect on runoff this spring will not be as great as in the north.

Spring runoff could deviate significantly from that shown on the attached map if future precipitation and breakup conditions deviate greatly from the normal.

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The spring flood outlook is based on three future weather scenarios that take into account probabilities of additional snow, melt rates and spring rain based on statistics for the past 40 years. These have been superimposed on the present basin conditions for input to river forecasting procedures. In terms of future weather, the terms ‘favourable’, ‘average’ and ‘unfavourable’ hereafter refer to the lower decile, median and upper decile conditions respectively. There is a one-in-ten chance of the weather  being ‘favourable’ or better and there is a one-in-ten chance of it being ‘unfavourable’ or worse. Additional precipitation ranges from 10-15 mm for the lower decile, 35-45 mm for the median and 70-80 mm for the upper decile condition. The lower decile and upper decile conditions also assume that breakup will be slow and relatively fast respectively while average conditions are assumed for the median. 

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 runoff in eastern Saskatchewan was obtained from the Saskatchewan Water Authority.

Predicted peak stages for the Red River, Assiniboine River and Souris River are shown on the attached Table 1, along with bankfull capacities, dike elevations and recorded peaks for past years.

Predicted peak discharges for smaller watersheds are shown in Table 2 together with bankfull capacities and peak discharges for past years. Numerical peak stage forecasts for smaller watersheds are not usually prepared until March.

The following sub-sections describe the forecast and flood potential for specific watersheds.

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There is a potential for flooding due to high soil moisture and moderate snowcover in most of the watershed including the U.S. portion.

With favourable weather (1:10 chance), crests from Emerson to the Floodway Inlet would be lower than during the spring of 2005 with no flooding.

With average weather, peak levels would be somewhat higher than those of last spring but somewhat lower than those of last summer and 2001. Minor flooding of low lying areas near the river would occur from Emerson to the Floodway Inlet. Floodway operation would keep the crest in downtown Winnipeg from exceeding 18 feet. Floodway construction activities will be able to accomodate this spring’s floodway operation needs.

The unfavourable weather condition could lead to a flood about two feet higher than that of 2001 and greater than that of last summer in areas from Emerson to St. Agathe. This would produce significant agricultural flooding along the river.

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Little or no flooding is expected with favourable weather from now on. With normal weather some flooding could occur in the headwaters of the Little Saskatchewan River and on the Shell River. With unfavourable weather (1:10 chance) flooding could develop on most tributaries from the Saskatchewan boundary to Brandon and on Sturgeon Creek.

Flooding is not expected along the Souris River with favourable weather.        

Median weather could produce minor flooding of agricultural lands from Coulter to Napinka for a short time.

The unfavourable weather condition (1:10 chance) could produce moderate flooding somewhat greater than that of last spring but a little less than that of 2001.

Tributaries of the Souris River could reach bankfull levels under the unfavourable weather condition.

With favourable weather there would be little or no flooding except near the Shoal lakes where flooding of farmland would continue until at least the summer.

With average weather, minor flooding could occur on the Fisher River at Peguis Reserve. Some flooding could also occur around marshes and small lakes even with normal weather conditions, due to the high water levels remaining from 2005. Flooding near the Shoal lakes would increase somewhat.

The unfavourable weather scenario (1:10 chance) could produce serious flooding on the Fisher River and other streams in the eastern Interlake approaching 1979 magnitudes. There would also be extensive overland flooding. Agricultural flooding near the Shoal lakes would increase in extent and could threaten some homes and farm buildings.

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Flooding is not expected with average future weather conditions.

The unfavourable weather scenario could produce flooding on the Brokenhead River, Whitemouth River, Birch River and streams in Whiteshell Provincial Park including the Winnipeg River.

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Snowcover is up to 200 percent of normal on the Riding, Duck and Porcupine mountains. Snow on the high ground melts relatively slowly over a period of several weeks to a month most years. Occasionally it melts very quickly in a few days. The flood threat for streams flowing off the high terrain is highly related to the melt rate as well as to additional precipitation.

Soil moisture is particularly high in the Carrot and Red Deer River Watersheds, but relatively low in the Dauphin Lake watershed.

Flooding is not expected in the region with favourable weather from now on.

Average weather conditions could produce minor flooding on streams flowing off the high ground and on the Carrot River, Red Deer River and Pasquia River.

Adverse weather scenario including a fairly rapid melt (1:10 chance) could produce serious flooding on most streams in the region. The Whitemud River and its tributaries could also sustain flooding under this scenario.

Flooding at Ralls Island in The Pas is unlikely although a fairly high first peak may occur from runoff downstream Nipawin, Saskatchewan together with possible ice jamming at the Pas. Peak water levels at Ralls Island and in the Bracken area should be several feet lower than the peak last summer. Forecasts from Saskatchewan and Alberta indicate that the crest from the headwater area will likely be below average, so the second peak from upstream should not present a problem. The flood potential is relatively high on the Carrot River and could pose some problems west of mile 18 on the Carrot River. Some agricultural flooding could develop in the Pasquia polders, even with average weather conditions. However flooding in the Pasquia area, even with the unfavourable weather scenario, should be much less than the record flooding resulting from torrential downpours last summer.

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Runoff in northern Manitoba (north of latitude 54) is likely to be somewhat above average based on normal weather conditions from now on. While snowcover appears to be close to average, soil moisture and depression storage is well above average due to heavy rain last summer and autumn. Localized flooding of low-lying areas is likely to occur.

Flooding is not expected along the Churchill River and the Nelson River this spring and early summer under the average weather scenario as both rivers would experience relatively minor rises. The unfavourable weather scenario (1:10 chance) could produce significant flooding of low lying areas and lake shores in northern Manitoba, although not likely as great as that of the late summer and autumn of 2005. Levels of the Churchill River and the Nelson River would still be lower than those of 2005. Manitoba Hydro may have more specific information for certain areas in northern Manitoba.

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Recent levels for 22 Manitoba lakes are shown in Table 3 along with anticipated peak levels this spring for three possible weather scenarios as previously defined. Many lakes are still at above average levels as a result of the unusually heavy precipitation in 2005. Lakes which are at above average levels could be susceptible to shoreline flooding if above average spring runoff develops in their watersheds.

Smaller lakes in southern Manitoba including the Whiteshell are unlikely to be seriously high in 2006 unless the unfavourable weather scenario unfolds. However lakes in the vicinity of The Pas and in northern Manitoba are likely to be higher than desirable due to the presently high levels and the significant runoff expected this spring. Lakes along the Nelson River will most likely be above the desirable range although likely lower than last summer. Lakes along the Churchill River should be above average but much lower than the record levels of last autumn.

Strong wind storms, which are more probable in the fall, could again pose a threat of shoreline flooding on large lakes in 2006 especially if the unfavourable spring weather scenario unfolds and is followed by adverse summer weather. However this scenario is unlikely to be repeated in 2006. The present Lake Winnipeg level of 714.3 feet is still 1.4 feet above average. With average weather conditions, the lake is expected to crest at about 714.8 feet in June. This level would be similar to the wind eliminated levels during the windstorms in the autumn of 2005. There is a one in ten chance that the Lake may rise to 715.5 feet in case the unfavourable weather scenario unfolds. The highest level without wind in 2005 was 716.6 feet in August. Manitoba Hydro has maintained the maximum possible outflow from the Lake since January, 2005 within constraints required to prevent ice related difficulties. The wind eliminated forecasts given above are preliminary estimates made in consultation with Manitoba Hydro.

The chance of flooding along the Lake Winnipeg shoreline this spring and summer is relatively low since strong winds seldom occur during this time when the level without wind will likely be at its highest. The one-in-ten year increase in levels due to wind set up and wave uprush at Gimli is only 3 feet during the spring and summer as compared to 6 feet in the autumn.

Flooding along the Lake Manitoba shoreline is unlikely with average weather this spring, as the wind eliminated level will be lower than it was last autumn and winds are likely to be lesser than in the autumn.

Future Information:

This spring flood outlook will be updated in late March, 2006, or sooner if a major storm occurs before that time. An update may not be issued if there is little additional snow or if runoff begins early. Daily reports will be issued when spring runoff is well underway. The Hydrologic Forecast Centre, Water Science and Management Branch, Manitoba Water Stewardship will keep a close watch on watershed conditions.