The floods in Manitoba

The flooding of the lower areas of the Red River has always been associated to the snowmelt. Although only 17% of Manitoba's total annual precipitation comes from the snow, doubled with the other factors, it's been recognised as the main reason for the Red River's overflowing.

The following are the factors influencing spring floods in the Red River basin:

  • the extent and humidity level of the snow blanket;
  • the speed of the snow melt;
  • the coinciding of the moment of the snow melt with heavy rains;
  • soil humidity and surrounding temperature;
  • the low permeability of the Red river's clay soils;

Furthermore, the unusual orientation of the Red River's flow from the South to the North is another factor, which could contribute to the flooding. In effect, the upstream water being situated to the South thaws out before the downstream part of the river, situated in Manitoba, thus creating ice jams, blocking the passage of the water and increasing local flooding.

One of the factors contributing to the flooding of the Red River basin is the area's topography. The flatness of the region brings on a very slow drainage of the waters and also limits the formation of large natural water reservoirs. In this way, even if it is possible to build small dams, no large reservoirs can be built to retain the floodwaters. Moreover, nothing can hold back the Red River when the water level rises. The whole valley is a flooding plain. The waters can stay there for days and even weeks before receding.

Generally, the dry climate of winter and the low rate of precipitation don't create favourable conditions to flooding in the Red River basin. Normally, 75% of the annual 50 centimetres of precipitation in the basin fall between April and September. More specifically, the months of May, June and July are generally the rainiest. The period between November and February is the driest, the average monthly precipitation being of only 1,3 centimetres.

Because it is located in an area characterised by a subhumid to humid continental climate, the Red river basin has moderately hot summers, cold winters and, from one day to another, rapidly shifting meteorological conditions. Extreme weather variations are frequent. The average monthly temperatures in the Red River basin vary between -15°C and 20°C.

When strong precipitation mark the previous autumn, important and deep frost followed by abundant snowfall in the winter, all these factors put together cause a huge rise in water levels. The flood is even more of a problem when the thaw out is sudden and accompanied with strong rainfall or melting snow during the spring's ice breaks.