COULD THIS SPRING’S FLOODING BE ANYTHING LIKE 2011?

craven_floodSometimes people talk about the 2011 spring flood as if it were just part of an ongoing “natural” cycle. Some who have been around for past floods compare 2011 to the severe floods of 1974 or even 1955. As a teenager in 1955 I well remember canoes going down the streets near the overflowing Wascana Creek.

Some record high runoff levels still stand from these floods; the highest recorded levels for Buffalo Pound and Wascana Lake were in 1974. And, even after the 2011 flood, the 1955 record levels stand for Last Mountain, Round and Crooked lakes in the Qu’Appelle Valley. On the surface then it doesn’t seem that the 2011 flood was particularly special. Until we carefully look at the figures!

HISTORICAL EXTREMES

The 2013 Spring Runoff Outlook from the Water Security Agency (WSA) includes statistics on “Lake Levels at Selected Lakes and Reservoirs in Saskatchewan.” This includes information on “normal summer levels”, “recorded historical extremes”, 2011 and 2012 peak levels, as well as 2013 forecasts. These are all reported as meters above sea level.

The recorded historical extreme for Crooked Lake remains 454.38 meters in 1955. That same year Round Lake reached its highest level of 445.3 meters and Last Mountain Lake reached 492.09 meters. In 1974, Wascana Lake and Buffalo Pound reached their recorded historical extremes, 572.23 and 511.45 meters respectively.

There are other peak years for other Saskatchewan water bodies. In 1979 the Boundary and Cookson Reservoirs reached their historical highs of 561.15 and 753.35 meters. In 1985 Jackfish Lake had its historical peak of 530 meters. Anglin Lake had its historical extreme of 515.84 meters in 1994 and in 1997 Reindeer Lake reached its recorded historic high of 336.8 meters but this came after summer downpours.  In the spring runoff of 2007, Wakaw Lake reached its highest level of 509.9 meters and in 2010 Good Spirit Lake reached its historical extreme of 485.68 meters, again from summer downpours.

Eight of the 23 monitored Saskatchewan water-bodies had their historical extremes in the 30 years between 1955 and 1985, whereas in the 28 years since then, 15 reached their highest recorded level. But, most noteworthy, the following 11 of these 15 water-bodies set their historic peak highs just two years ago in 2011, most after spring flooding, a few after summer downpours: Lake Alameda – 568.58 meters (summer), Lake Diefenbaker – 556.90, Echo and Pasqua Lakes – 480.98, Fishing Lake – 530.92, Katepwa and Mission Lakes – 479.58, La Ronge Lake – 364.98 (summer), Lenore Lake – 527.79, Moose Mountain Lake – 621.71, and Rafferty Lake – 554.05 meters (also summer).

THE 2013 RUNOFF

The 2011 spring flood was clearly exceptional, but are there simply big weather cycles or is something else happening? Will the spring 2013 runoff extend a pattern of evermore extreme flooding, including from summer super-rains, as seen in other places on the planet?

Perhaps it’s too soon to know but in their March 5, 2013 forecast, the WSA says the “Areas around Moose Jaw, Regina, Melville, Saskatoon and North Battleford have potential for very high runoff.” They report that there’s “very high” snowpack, over 200% above the normal, in central southern Saskatchewan, a large area running from Moose Jaw to Regina to Indian Head. The same conditions are reported in a smaller area north of Saskatoon and southwest of Prince Albert. And all around these areas of “very high” snowpack, snow levels range between 150-200% above normal.

There’s going to be a lot of snow melting and running into the fairly few river basins in southern Saskatchewan. It might even set some new records, for example in 2013 some lakes are forecasted to be fairly close (within 0.2 meters) to their highest recorded levels. This includes Anglin Lake (515.7 meters), Boundary Reservoir (561), Lake Diefenbaker (556.8), Jackfish Lake (529.9) and Wakaw Lake (509.7).

The WSA qualifies its forecasts, saying it all depends on the melt and whether there is more snow or rainfall in the spring. But if we look at the Assiniboine and Qu’Appelle Rivers basins, the warning is fairly clear:  “…the existing snowpack in the Qu’Appelle River basin is well above normal. Typical spring precipitation and a typical rate of melt is (sic) expected to produce flows approaching those experienced in 2011.”

HISTORIC FLOWS

Can the historic and forecasted flows tell us anything about potential flooding this spring? The historic flows on the Assiniboine River were set in 1995 at Sturgis (111 cubic meters per second), Canora (247 m3/s) and Kamsack (488 m3/s).  The 2011 flows at these points were far above the average: 91 m3/s compared to an average of 28 at Sturgis, 234 compared to an average of 29 at Canora and 369 compared to an average of 59 at Kamsack. The 2013 flow forecasts (70 m3/s at Sturgis, 75 at Canora and 170 at Kamsack) are all well below both the historic high and the high 2001 levels.

The Qu’Appelle River basin tells a slightly different story. The historic high flows were in 1974 for various sites: Lumsden, Craven, Thunder Creek, Burdick, and Regina. Near Abernethy the historic flow was in 1976, for Spy Hill it was in 1995 and for Boulder Lake it was in 2006. However, near Loon Creek, Hyde and Welby, the historic flow was in 2011 (163, 254 and 345 m3/s respectively). The 2013 forecasts at all these locations are below their historic highs.

QU’APPELLE VALLEY

However there’s been a record snowpack in the Wascana Creek basin and the WSA forecasts a 1 in 50 year “snowmelt runoff event with a peak flow estimate of 90 cubic meters per second” for the City of Regina. This forecast is even higher than the peak flow of 76 m3/s in 2011, though still below the record high of 102 in 1974. And the 2013 forecast of 210 m3/s for the Moose Jaw River at Burdick is higher than the 2011 level of 197. Likewise the 2013 forecast of 38m3/s near Abernethy is slightly up from the 2011 level of 35. These forecasts however remain below their historic highs in the 1970s. The Craven forecast of 100 m3/s is just below the 2011 level of 107 and well below the 1974 record high of 141. The WSA notes that Craven’s control structure “will be fully opened prior to spring runoff”.

The forecasted peak levels and some peak flows nevertheless give reason to prepare, especially in the Qu’Appelle Valley. While forecasting is risky, it’s a better basis for planning than selective memory or conjecture. The WSA reminds us: “A slow melt, similar to what we experienced in 2011, could reduce the peak significantly. Conversely, a rapid melt similar to 1974 and/or significant precipitation between now and runoff could result in a higher peak flow than currently estimated”. At the March equinox we’ve already had this “significant precipitation” and more is on the way.

Lots of uncertainty remains, but the record snowpack and preliminary forecasts are a wakeup call. A common view is that because of our fairly dry fall, the ground has a great capacity to absorb moisture and this may be partially true, depending on the nature of the melt and any additional precipitation. Others speculate that because lake levels were low coming out a relatively dry fall, there’s little chance of lakeshore flooding. But according to the WSA, the size of the snowpack and the nature of the melt remain the major factors influencing the magnitude of runoff. We’ve ordered sandbags in our village.

Happy world water day!

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