Condition Reports

Spring Flood Outlook for Southern Manitoba

March 20, 2008

Table of Contents:

• Overview
• Climate Conditions
• Soil Conditions
• Snowcover and Winter Precipitation
• Spring Runoff
• River forecasts
  • Red River Mainstem
  • Red River Tributaries
  • Assiniboine River Mainstem
  • Assiniboine River Tributaries
  • Souris River
  • Interlake Region
  • Eastern Region
  • Westlake - Dauphin Region
  • Duck Mountain to The Pas
  • Northern Manitoba
• Lake Forecast
• Forecast Updates
• Attachments

Overview

The potential for spring flooding in Manitoba has further decreased since February, 2008 due to continued below average precipitation in most areas. The flood potential is low in southern Manitoba due to a below average snowpack and average to below average soil moisture. Much of the meltwater is expected to soak into the ground. A near normal runoff is expected in a portion of western Manitoba from the Porcupine Mountain to the Carrot River and in portions of far northern Manitoba where snowcover is heavier but flooding is unlikely even in these areas.  

Minor localized flooding could occur during the early part of the runoff period due to ice jams or snow blockages in drains, ditches and small streams as usual. The probability of major ice jams on larger rivers cannot be ruled out but it is lower than usual due to low expected river flows or due to ice breaking activities.

The spring flood potential is still very dependent on spring weather conditions. The amount of additional precipitation and the timing and rapidity of the breakup will have a significant effect on the flood potential.

Unusually wet weather in May and June of 2007 gave way to generally dry weather during most of the summer and autumn. Winter precipitation has been well below average in the southern prairies. It was unusually mild until mid January, 2008 but unusually cold during the following six weeks. Mild weather returned during the second week of March. The causes of these major changes in the weather are not well understood but appear to be in large part due to a fairly strong La Nina condition in the tropics. The National Oceanic and Atmospheric Administration (NOAA) in the United States predicts that the La Nina will continue through the spring of 2008. An analysis of nine moderate to strong La Nina conditions since 1950 shows that April precipitation during such events is more often below average than above average.

An aerial soil moisture survey conducted on October 31, 2007 showed that moisture in the top 20 cm of soil was below average in all portions of southern Manitoba with the exception of the southern portions of the Riding Mountain. Topsoil moisture is believed to be high in southeastern Manitoba including the Whiteshell due to heavy October rains; this area is not covered by the airborne survey. A map showing the results of the airborne soil moisture survey is attached (NOHRSC logo).

Fall soil moisture in the deeper root zone (30 cm to 120 cm), was somewhat below average in virtually all agricultural areas of Manitoba based on a field survey conducted by Manitoba Agriculture, Food and Rural Initiatives in late October, 2007. The attached map shows that soil moisture in this zone was less than 50 % of capacity in most areas of southern Manitoba. The survey also showed below average soil moisture in the top zone.   

An index of soil moisture called the antecedent precipitation index (API) is commonly computed by Manitoba’s Hydrologic Forecast Centre based on weighted precipitation from May to October expressed as a percent of normal. The attached API map suggests that soil moisture on October 31, 2007 was generally below average in southern Manitoba. However it suggests above average soil moisture in areas close to the Ontario boundary, the Duck and Porcupine mountain area and in portions of far northern Manitoba.

Soil frost information is sparse but what data is available suggests that the soil is frozen to a greater than average depth due to the meager snowcover and the very cold February weather. The effect of frozen soil on runoff is not easy to predict as is can impede water infiltration but can also cause soil cracking which aids infiltration.

The March snow survey was cancelled in most areas due to a lack of significant snowfall after the February, 2008 survey. However a new survey was conducted March 10-12 in the area from the Red River east to the Ontario boundary. Snowcover in this region varied generally from 50 % to 95 % of normal as shown on the attached map. Snowcover is quite variable due to land use and wind effects so some of these values may not be representative.  

The results of the February 11-14 airborne survey over the Red, Assiniboine and Souris River watersheds is attached (map with NOHRSC logo). The survey showed that snow water content ranged from 1-2 inches (25-50 mm) over most areas of southern Manitoba, which is significantly below the average since 1995 when these surveys began.

Results of the February 11-14, 2008 surface based survey conducted by Manitoba Infrastructure and Transportation staff under the direction of Manitoba Water Stewardship are attached. A map showing the water content as a percent of the long term normal is attached. The water content of the snowpack was found to be below average in most areas of southern Manitoba with some areas having only half the normal snowpack. The snow water content was above average from the Porcupine Mountain to The Pas. Percent of normal snowcover as of mid March is less than that based on the mid February survey due to lack of significant additional snow during this time interval.

Snowcover is also available from a satellite survey provided by Environment Canada. A map for March 10, 2008 is attached (EC-Climate Research Division logo). This survey suggests that snowcover is below average in most of southern Manitoba but is average to above average in portions of western and northern Manitoba and Saskatchewan. Satellite estimates of snow water content may be inaccurate at times due to sudden changes in weather and crystal structure of the snowpack. Environment Canada has indicted that the heavy snowcover in western Manitoba and eastern Saskatchewan as shown on the map is over-estimated.

Cumulative winter precipitation from Environment Canada climate stations is also used as an estimate of snowcover. A map showing percent of normal precipitation from November 1, 2007 to March 19, 2008 is attached. The number of sites reporting winter precipitation has diminished in recent years and snowfall is difficult to measure accurately in gauges due to wind effects; therefore these estimates must also be treated with caution.
 

A map showing the expected percent of normal runoff potential for the spring of 2008 is attached. This prediction is based on watershed conditions as of March 20 and average future conditions for precipitation, date of breakup and melt rate.

The 2007 spring runoff is expected to be well below average in much of southern Manitoba. Dry soils resulting from below average precipitation from July to October, 2007 should absorb most of the meltwater from a below average snowpack. Runoff in the Whiteshell and Nopiming Park areas should be close to average, in large part due to high soil moisture remaining from heavy rain last October.

Average or slightly above average runoff is expected in an area from the Porcupine Hills to just south of The Pas where soil moisture and snowcover are somewhat above average.

Above average runoff is also expected in the portion of northern Manitoba from Thompson northeast to Hudson Bay where snowcover and soil moisture are above average.

Spring runoff could deviate significantly from that shown on the attached runoff potential map if future precipitation and breakup conditions differ greatly from the average. An early runoff with below average spring precipitation would result in little or no runoff in many areas. However a late breakup with above average spring precipitation, and a quick melt could still produce minor flooding in many areas of southern Manitoba this spring.

The spring flood outlook is based on three future weather scenarios relating to additional snow, melt rates and spring rain. These scenarios are based on climate statistics for the past 40 years. The three scenarios are superimposed on the present basin conditions for input to river forecasting procedures. The three future weather scenarios are sometimes referred to as ‘favourable’, ‘average’ and ‘unfavourable’ and relate to the statistical 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 for this outlook ranges from about 10-15 mm for the lower decile, 25-30 mm for the median and 50-60 mm for the upper decile condition with some variation due to location. 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. The forecast based on lower decile weather may be omitted in subsequent portions of this report if the flood potential is very low.

Forecasts do not include effects of possible ice jams which are generally unpredictable. Brief flooding can occur in locations where ice jams develop, even with below average river flows. However ice jams are likely to be of lesser significance when river flows are low as expected this year under the average or dry weather scenario.

Flood outlooks for international and inter-provincial streams include information obtained from the U.S. National Weather Service and the Saskatchewan Water Authority. Their contribution is acknowledged herewith. 

The following sub-sections describe the forecast and flood potential for specific watersheds. Numerical peaks stage forecasts for the Red River, Assiniboine River and Souris River are shown on Table 1. Predicted peak flows for smaller streams are shown on Figure 2.

The potential for spring flooding is quite low due to below average snowcover and below average soil moisture.

The potential for spring flooding is quite low due to below average snowcover and below average soil moisture.

With average weather from now through April the Red River is expected to remain within its banks and operation of the Red River Floodway would not be required.

With unfavourable weather (1:10 chance) minor overbank flows would occur in the portion from Emerson to just south of Morris. Operation of the Red River Floodway would result in a crest near 17.3 feet (744.9 feet asl) in downtown Winnipeg while the crest upstream of the Floodway Inlet would be near 756 feet. Construction on the floodway channel would not interfere with floodway operation should an operation become necessary.

Serious ice jamming from Selkirk to Breezy Point will depend a lot on weather conditions. Present ice cutting and breaking operations by North Red Community Water Maintenance Incorporated and Manitoba Water Stewardship will reduce the chance of serious ice jams. Ice is thicker than average this year but not as thick as in 2003 when no ice jams occurred due to low river flows. With favourable spring weather conditions, ice jamming is unlikely due to ice breaking activities and due to expected low river flows which would not have enough force to create serious ice jams. Flows for the median (or greater) weather condition could be sufficient to create jams but hopefully these will not be major jams because of the ice breaking activities.

Flooding is not expected on Red River tributaries with average weather from now on. This includes the Manitoba portion of the Pembina River watershed. The unfavourable weather scenario could produce minor localized flooding of low lying areas.

With median weather conditions, there should be no flooding along the Assiniboine River this spring. The drawdown of Shellmouth Reservoir is being conducted in a manner which will minimize the risk of flooding while providing sufficient water supplies to meet downstream needs in 2008 and beyond. Reservoir outflows were reduced on March 18 in order to balance the risks of these two objectives. There will be sufficient storage space to accommodate reservoir inflows for the upper decile condition without having spillway flows and downstream flooding. The Shellmouth Reservoir Regulation Liaison Committee is providing advice regarding operations and their potential impacts.

Operation of the Portage Diversion may not be required for the median weather condition, unless ice action causes a temporary surge in inflows.

An unfavourable weather scenario (1:10 chance) would result in bankfull flows along the Assiniboine River from Shellmouth to Brandon with crests lower than those of 2007. Operation of Shellmouth Reservoir would prevent spring flooding along the river or reduce its duration to a minimum in order to enable seeding of agricultural lands.

Flooding would be unlikely from Spruce Woods to Winnipeg due to low expected flows on the Souris River and due to operation of the Portage Diversion under this weather scenario. However ice jams could develop under this scenario and cause temporary flooding in this portion as occurred in 2007.

Flooding is not expected with average weather from now on.

An unfavourable weather scenario could produce minor overbank flows in low lying areas.

Flooding is not expected along the Souris River with average weather conditions from now on.

The unfavourable weather condition could produce minor flooding in the Coulter area where the bankfull capacity is very low.

Flooding on Souris River tributaries is very unlikely this spring.         

Neither the average nor the unfavourable weather scenarios are likely to produce flooding on the Fisher River (including Peguis Townsite), Icelandic River and other streams in the southern Interlake.  Flooding could occur in the event of a more serious spring rainstorm.

Flooding of low-lying agricultural lands near the Shoal lakes could develop under the unfavourable weather scenario, but would remain significantly less than in 2006.

Spring and early summer peak flows in the Fairford River and Dauphin River are expected to be lower than those of 2007 when flooding was avoided. Flows in the Waterhen River are expected to be lower than last year. Unless the unfavourable weather scenario unfolds. While the Waterhen River and the Dauphin River contain a lot of ice buildups which developed last autumn, ice jam related flooding is unlikely during the spring.

Under the dry weather scenario there would be some concern about low flows and lake levels in southeastern Manitoba and the Whiteshell, since snowcover is well below average at this time.

Flooding is not expected on the Brokenhead River, Whitemouth River or Whiteshell River with average future weather conditions and the Winnipeg River would show only a modest rise from present levels.

The unfavourable weather scenario could produce minor flooding of low lying areas, in large part due to the high soil moisture remaining from heavy rain last October.

The median weather scenario would result in minimal runoff and no flooding on the Whitemud River and streams flowing into Dauphin Lake.

The unfavorable weather scenario could produce minor localized flooding of low lying areas along streams especially if the melt is rapid.   

The favourable weather scenario would produce little or no flooding on streams flowing off the Duck Mountain, on the Swan River and on streams from the Bell River north to the Carrot River. Brief localized flooding could occur where channels have winter ice buildups.  

The average weather scenario could result in some localized flooding of the Red Deer River, Overflowing River, Pasquia River and smaller streams in the area but it would be significantly less than in 2007. Flooding would still be unlikely on the Swan River, Carrot River and the Saskatchewan River.

The unfavourable weather scenario could produce minor flooding of streams in the region. However peaks on the Red Deer River, Saskatchewan River, Carrot River, Saskeram area and Pasquia Polder area would still be lower than those of 2007. Streams flowing east and north off the Duck Mountain and smaller streams in the Porcupine Mountain area could experience some flooding, especially if a rapid melt were to develop.

Runoff in northern Manitoba (north of latitude 54) is likely to range from average to above average as shown on the runoff potential map.  Flooding is not expected on the Churchill River or the Nelson River. Manitoba Hydro may have more specific information on the flood potential for certain areas in northern Manitoba such as the Churchill River near Churchill.  

Predicted spring peak levels for 23 lakes in Manitoba, based on three future weather conditions, are shown on Figure 3.

Lake Winnipeg and Lake Manitoba are expected to be within their operation ranges at the end of spring runoff, although the upper decile weather condition would result in levels very near the top of the range. Lake Winnipegosis is expected to decline under the normal weather condition, but will still be three feet above its normal level this summer and would be little lower than last year under the upper decile weather condition. Flooding of low-lying haylands along Lake Winnipegosis could again occur in 2008 if strong winds develop. The level of Lake St. Martin will decline during April when ice moves out of the Dauphin River. The level this summer should be somewhat lower than that of last year based on normal weather conditions.

Lake levels in western Manitoba, such as Dauphin Lake, Oak Lake, Pelican Lake, and Whitewater Lake will likely be somewhat below their summer target levels after spring runoff unless the upper decile spring weather condition develops. Red Deer Lake should be several feet lower than last year and far below the record level of 2006. Lake of The Prairies is expected to be near its target summer level based on average spring precipitation. Lakes in the vicinity of The Pas may be somewhat above desirable levels following spring runoff. Reindeer Lake is expected to be near the top of its operating range.

Lake Of The Woods may be somewhat below its summer target due to dry conditions in the Rainy River watershed. Lakes in Manitoba’s Whiteshell and Nopiming Provincial Parks should be near desirable levels after spring runoff based on normal spring weather.  

Lakes in far northern Manitoba, such as Southern Indian Lake, are expected to be at average or somewhat above average levels after the 2008 spring runoff.

This will be the last spring flood outlook for 2008 unless heavy snow develops. Daily flood reports will be issued for streams with a significant flood risk when spring runoff gets underway.

• Topsoil Moisture Map (NOHRSC) - Oct. 1 - Nov. 1, 2007
• Sub-Zone Soil Moisture (MAFRI) - Fall, 2007 (late October)
• Antecedent Precipitation Index Map (Man. Water Stewardship) - Oct. 31, 2007
• Airborne Gamma Snow Water Equivalent Map  (NOHRSC logo) - Feb. 11-14, 2008)
• Snow Survey Water Content Map (Man. Water Stewardship) - March 10-12, 2008)
• Snow Survey % of Normal Map (Man. Water Stewardship) - Feb. 11-14, 2008)
• Satellite Snowcover Map. (Climate Research Branch, EC) - March 10, 2008)
• Precipitation Map:  Percent of Normal (Nov. 1, 2007-March 19, 2008)
• Spring Runoff Potential Map (Man. Water Stewardship) - March 20, 2008
• Forecast Spring Peak Stages for Red, Assiniboine, Souris - March 20, 2008
• Forecast Spring Peak flows for Smaller Watersheds - March 20, 2008
• Forecast Spring Peak Levels for Lakes - March 20, 2008