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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.
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
Existing conditions must be considered when determining the
magnitude of a spring flood. The main conditions include:
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
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.
Article Courtesy: Grand Forks National Weather Service
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