SUMMER FLOODING

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Photo: NWS Grand Forks

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Photo: NWS Grand Forks

When we hear of river flooding in the Red River Basin, most people think of the spring floods which have a melting snowpack contributing significantly to the situation. However, there have been several major floods in the region which have occurred during the summer months. This discussion will briefly look at some of the factors which contribute to summer flooding and some historical summer floods in prior years.

  • Conditions during the late spring are important factors for summer flooding. These antecedent conditions include the amount of moisture in the soil column and amount of water already present in the river channels (baseflows) prior to the flood.

  • The heavy, organic soils across much of the Red River Basin have a relatively, high clay content. This clay fraction allows the soil to retain water more easily than soils which are predominantly made of silt or sand.

  • Weather conditions, such as a cool spring with above normal rainfall, prevent significant evaporation of moisture from the soil and thus maintain a wet soil column. The high moisture content in the soil column results in the underground water table being closer to the surface than usual. This, in turn, causes the amount of base flow water in the river channels to rise.

  • Summer flooding is also dependent on heavy rainfall and what are known as “training” echoes. The term “training” refers to a series of heavy rainfall thunderstorms passing over the same general area in a relatively short period of time.  Such a combination of factors can result in multiple flood crests for a location, as occurred for the Fargo/Moorhead area in May-June of 2007.

  • Training echoes can also result in flash flooding.  A flash flood is a rapid and extreme flow of high water into a normally dry area. 

Long-term residents of the Red River Valley may remember the significant summer flood years of 1975, 1993, 2000, 2002, 2005 and 2007. Each of these summer seasons produced widespread flooding across the region. A devastating flood for Roseau, MN occurred in June 2002 (see photos). Since flooding can occur throughout the year, residents of the Red River Basin should make flood preparedness a normal and routine exercise when developing personal disaster plans.

For those who are interested in a more technical and detailed analysis of the June 2002 flooding events, the U.S. Geological Survey and the National Weather Service worked together on a summary report which may be found here.

Source: Brad Bramer, Science Operations Officer, National Weather Service Grand Forks, ND