July 2020 Mini-Flood

With the recent rain there was some excitement on the North Saskatchewan this weekend:

On July 3rd the river peaked at a depth of 7.95m and a streamflow of 1,899m³/s. Just a few days earlier on June 30th it had been in a more normal range for this time of year at 5.1m and 635m³/s. So in 3 days the depth jumped by nearly 3m, and the streamflow tripled.

1,890m³/s+

With a peak of 1,899m³/s we didn't hit the 1,900m³/s mark. But since records began in 1911 on 16 years broke 1,890m³/s. Recently that included:

• June 24, 2013 was way up at 2,847m³/s and 9.25m.
• June 20, 2011 hit 1,897m³/s and 7.88m
• June 21, 2005 hit 2,612m³/s and 8.85m

The Biggest Floods 

This chart shows the largest recorded floods. Detailed depth measurement only started in 1999, which is why there is less history for the blue lines compared to the green:

• 1915 at 4,460m³/s and 13.73m
• 1986 at 3,990m³/s and 12m
• 1952 at 3,540m³/s and 10.59m

The 3-Day Rule?

The upper part of this chart compares the peaks of Edmonton's various big floods, and it shows what the streamflow was in the days just before and just after. (the lower part shows the rate of change from day to day).

For some reason, all of Edmonton's floods took 3 days or less to peak. 3 days before the levels were all in the normal~ish range of 800m³/s or less, then the big increases happened over 3 days, and on day 4 the numbers had started to drop. After a flood it usually takes a week or more for the levels to return to the normal range again.

None of these floods saw the levels jump up, then take a break or drop a bit, and then start climbing again to a new peak. It has always been a sudden ramp-up over 3 days, followed by a slower drop.

This "rule" isn't written in stone, because theoretically if a series of storms hit the province that could just keep adding more and more water into the system. But so far all of the major floods since 1911 have followed the 3-day rule.