ANATOMY OF A RED RIVER SPRING FLOOD

By Allen Voelker National Weather Service, Grand Forks

The following article explains why Red River Valley floods are not caused solely by deep snow cover. Our point will be illustrated by looking at two spring floods which occurred in Fargo during the last 6 years. Although Fargo is used in this example, the following conditions can be applied to anywhere within the Red River Valley.

Geography

The Red River Basin is unique because the mainstem of the Red River flows north into Canada and eventually into Hudson Bay. Consequently, the spring melt, and the eventual runoff, differs from many other areas of our county. (as a side note, the Red River is not the only river in the United States which flows north, there are others such as the Genesee River in Upstate New York). In a typical spring thaw, warmest temperatures and initial runoff begin in the source region of the Red River, over Northeast South Dakota and West Central Minnesota. After this runoff enters Lake Traverse it begins to travel north. Weather conditions farther north or downstream, however, are usually quite different from those of the headwater region, with melting and runoff yet to begin.

Because of the relatively flat slope of the valley, the flow of the Red is slow, allowing runoff to backfill into tributaries, particularly when the downstream river channel remains frozen. In addition, localized ice jams, may impede the water flow, resulting in higher river levels.

Weather Factors

Existing conditions must be considered when determining the magnitude of a spring flood. The main conditions include:

    1. The freeze/melt cycle
    2. Early spring rains which increase melting of the snow pack or late spring snow storms adding to the existing snow pack
    3. The actual snow pack depth and water equivalency
    4. Frost depth
    5. Soil moisture content
    6. River ice conditions
Some of these conditions are known in advance such as frost depth, soil moisture content, and river ice conditions. However, the most critical conditions, the first three mentioned on the list, are not fully known until just before or during the spring thaw. A typical spring thaw occurs from the middle of March across southern portions of the basin and mid or late April across the north.

Naturally, great variability in conditions can exists prior to the spring melt and runoff period. For example, temperature, and precipitation amounts can vary greatly. By considering all the factors, there are numerous scenarios which may or may not cause flooding in the Red River Basin. Snow depth is just one of those factors. To illustrate let's look at two different years in Fargo. One event caused significant flooding and the other did not..

The Spring Thaw of 1994 - The Major Flood That Wasn't

The winter of 1993-1994 established a previous record snowfall at Fargo with 89.1 inches. With a record snowfall, one could certainly understand concerns about a possible significant flood that spring. Snow depth during the month of February ranged from 12 to 24 inches with a 15 inch snow depth heading into March. Water equivalency from the snowpack at peak depth ranged from 3 to 4 inches of water. Precipitation in February and March was normal with nearly an inch of precipitation in March.

The key to the spring flood that year was an ideal snowmelt scenario. Daytime high temperatures during the Month of March and into April were greater than 32 degrees on all but 5 days which allowed a gradual snow melt. During this period, daytime highs generally ranged from 35 to 45 degrees with five days at or greater than 50 degrees. Also important was that nighttime lows fell below 32 degrees on all but five days during March. This cooling allowed snow to freeze at night slowing down melting and runoff which occurred during the day. As a result of the ideal melt, the Red River crested in Fargo on April 3rd at a stage of 26.70 ft. Although flood stage in Fargo is 17.0 feet this was considered a minor to slightly moderate flood. Far less than what one would expect with a record snowfall. Farther downstream on the Red, Grand Forks crested at 33.09 feet again, a relatively minor flood.

The Thaw of 1996 - A Different Outcome

Different results occurred following the winter of 1995-1996. Snowfall was again heavy, 74.6 inches, which was the fourth snowiest on record. A mild February dropped snow depths from nearly two feet at the beginning of the month to less than a foot going into March. Mild conditions again occurred the second week in March dropping the snow depth to a trace! This series of weather events seemed to temporarily eased concerns for any significant spring flooding. However, a cool down occurred the second half of March into the first week in April which stalled melting of the remaining snow cover and ensuing runoff. From the 16th of March to the eighth of April temperatures failed to reached 40 degrees and climbed above 32 degrees less than 40% of the period. A series of precipitation events raised snow depths up to eight inches going into April.

Starting on April 8th temperatures soared, ranging from the middle 40s to middle 60s across the entire basin. These warm temperatures melted the remaining snow pack in a matter of a few days, which caused a rapid runoff. Since this melting occurred relatively late in the spring melt period, rapid runoff occurred across the whole basin. As a result of the rapid warming late in the spring melt season, moderate to major flooding occurred over a large portion of the basin. Fargo crested on the 16th of April at 28.73 ft. which caused moderate flooding. Grand Forks crested at a stage of 45.82 ft. which caused significant flooding. Significant flooding was observed at Drayton, Pembina and Winnipeg as well.

What Will Happen This Year?

Hopefully this discussion has shed some light on the difficulty of making pre-snowmelt flood forecasts. The many factors that are involved in the spring snowmelt make flood forecasting difficult and these factors make it almost impossible to make river crest forecasts two to three months in advance. We invite you to check out our home page for the results. And during the spring runoff season keep tuned to Weather Radio, or our Home Page for the latest river forecasts.


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