Souris River flood of 2011
The flood of 2011 in Southwestern Manitoba was record breaking in many ways. The Assiniboine River, the Souris River and almost every creek and river south of Riding Mountain flooded at least three times, each peak bigger than the one before.
The stage for these floods was being set in the summer of 2010. Rainfall on the southern prairies was about 30% above normal in 2010 and just 1 mm short of the record. As the map shows soil moisture throughout the watershed at freeze up was 130 to 175% of normal. Also see the snow map showing the snow pack up to February 20. Chart 1 shows the monthly precipitation amounts and chart 2 shows percent of normal from August 2010 to August 2011. For the year, August 1, 2010 to August 1 2011, Weyburn received 167%, Minot 135% and Melita 112% of normal precipitation.
Chart 3 shows the daily precipitation for Melita, Minot and Weyburn averaged over the three sites to roughly show what fell on the watershed. The average of the three for the year adds up to just over 400 mm yet only 75 mm ran off indicating that more than 300 mm soaked into the ground, evaporated and went into reservoir storage. Also see the precipitation maps for May and June 2011.
As chart 4 shows there was first a peak in April primarily due to snowmelt. Then in June another higher peak due to rain. More rain in June caused Saskatchewan reservoirs to overflow and very high peaks all along the Souris flooding Minot and threatening many other communities. All three peaks were higher than every other flood since 1913 except 1976 see chart 5.
Until this year 1976 was considered the benchmark flood and nothing on record came even close. Although the peak flows in Manitoba weren't much different than in 1976 the volumes were spectacular see chart 5 and chart 6.
The flood peak at Wawanesa was 29,000 cubic feet per second on July 6, 2011. This was about a one in a 130 year event. The total volume of flow was about 4.59 million acre feet (that's equivalent to 4.59 million acres (7178 square miles) covered by a foot of water or the Souris River watershed covered with 3.6 inches of water and that is about a one in 500 year event. So the good news is that the odds of that happening again in your life time are pretty small.
The drainage area upstream of Wawanesa is 23590 square miles. The total runoff was about 3.7 million acre feet or about 3 inches over the entire watershed. The average annual runoff is equivalent to 0.1 inches off the entire watershed.
Here are some photos that show the progress of the flooding over Oak and Plum Lakes click here