The following article was taken from the Winnipeg Free Press, and was a 2-part series printed in the July 16th and 17th editions. This article has been re-printed here with the generous permission of Box Cox, Publisher of the Free Press:
Pulling the plug on Lake Manitoba
By Scott Forbes, an Ecologist with the University of Winnipeg
Bill Rannie, my colleague at the University of Winnipeg — a hydrologist and quiet sage — noted that this year was the third highest flood recorded on the Red River but that you didn’t hear much about it. His point was that we have prepared so well for high water on the Red River that we now handle near record levels with relative ease.
Not so in the Assiniboine basin where we are still in the midst of disastrous flooding, with the worst yet to come along the Souris and around Lake Manitoba.
Forecasters have referred to this year’s flooding on the Assiniboine a one-in-350 year event. This is a mathematical construct based upon models of water level variability, not a chronicle of history. Indeed it is quite probable that the Assiniboine has reached comparable levels four times in the last 175 years — in 1826, 1852, 1882 and 2011. I say probable because we did not have modern flow gauges along our rivers in the 19th century and have to rely upon imperfect observations about the extent and timing of flooding.
The flood of 1882 contains lessons we might learn from today. It was similar to 2011, with high water on both the Souris and main stem of the Assiniboine. The Assiniboine overflowed its banks near Portage la Prairie and some of that water traveled overland to Lake Manitoba. The same thing would have happened this year without the Portage Diversion.
In 1882, Lake Manitoba water levels rose at least as high as the current level of 817 feet. It was then that something interesting happened. Water from the southeast corner of the lake started to flow back to the Assiniboine, reentering the river about 30 kilometres east of Portage la Prairie.
The Assiniboine falls in elevation as it moves from west to east. South of Portage la Prairie it is higher than Lake Manitoba (which is why the Portage Diversion works), but downstream near St. Eustache the Assiniboine is about 20 to 30 feet lower than Lake Manitoba depending on the time of year.
In 1882, at least some of the water from Lake Manitoba traveled back to the Assiniboine through the Long Lake Channel. It runs from west to east and empties in to the Assiniboine near St. Eustache.
What we now call the Long Lake Lateral Drain was somewhere between 4,000 and 7,000 years ago a channel of the Assiniboine.
Is the fact that the Assiniboine River has meandered through different channels over the last few thousand years merely a historical curiosity or might it be something more?
Today Lake Manitoba continues to expand and floodwaters creep toward Portage la Prairie from the south end of the lake. Lake Manitoba is now less than four kilometres from the upper reaches of the Long Lake Lateral Drain just south of Highway 227 and east of Oakland. It will get closer as the lake continues to rise over the next month.
There is no obvious reason why Lake Manitoba could not be reconnected to this ancient river bed — with a bit of help from heavy equipment to move some dirt — to provide a new and temporary outlet for the flood waters.
A similar possibility might also exist in the north. Lake Manitoba at a level of 817 feet extends beyond its normal banks northeast to Highway 6 at Steep Rock Junction. From there it is a downhill run into Birch Bay on Lake St. Martin, suggesting another possible path for water to exit Lake Manitoba.
Lake Manitoba is now almost certain to reach 818 feet in coming weeks. The flood waters that have devastated Minot, N.D., will reach us in early July and that water will be shunted north into Lake Manitoba via the Portage Diversion. The diversion will need to remain open through July and as long as it is, Lake Manitoba will continue to rise.
Given that Lake Manitoba this fall will be about 30 feet higher than the exit of the Long Lake Lateral Drain into the Assiniboine River, if we create the connection between the two, gravity should do the rest.
Right now the lower Assiniboine is full to the brim and can carry no extra water without overtopping its banks. But in two months time when our summer floods have mercifully abated, it will have ample surplus capacity to carry more water until next April when its levels begins to rise anew. If we could move 5,000 cubic feet per second from Lake Manitoba through the Long Lake Drain from September through March, that would lower the level of Lake Manitoba almost two feet.
I don’t know if we can get that much water through the Long Lake Drain. But we should find out and soon. And we should also find out if there are alternate pathways from Lake Manitoba to the south and north that would help pull the plug on the lake. The reason is simple. Without a new exit, the level of Lake Manitoba is not going to drop much any time soon.
High water levels on Lake Winnipegosis guarantee that Lake Manitoba will experience high inflows from its main tributary, the Waterhen River, for an extended period. A rough guess is that the Waterhen and other smaller inputs will account for 12,000 to 17,000 cfs of inflow until next year. The Fairford Water Control Structure has a maximum outflow capacity of just over 20,000 cfs, though that goes up and down with the lake level. A ballpark estimate of the surplus outflow capacity of the Fairford Water Control Structure might be 5,000 cfs between August and next April, perhaps a bit more depending on how quickly Waterhen flows subside.
Even if the Fairford Water Control Structure could remain wide open from August until next April (ice conditions might make this impossible) a surplus outflow of 5,000 cfs would only drop Lake Manitoba water levels about two feet. And only then if we get lucky.
The other main outflow from the lake is evaporation. It normally accounts for just under half the water loss from the lake over the year. But evaporation is counterbalanced by precipitation that accounts for 43 per cent of the normal inflow to the lake. To compare it to stream flow, evaporation is the rough equivalent of an outlet releasing 2750 cfs of water year round, though it would really only occur during the ice-free season. Precipitation represents the rough equivalent of about 2500 cfs of input year round.
If we got a wet, cool summer and fall, evaporation may not lower the lake level at all, as precipitation would exceed evaporative losses. If we catch a break (and we haven’t so far) with a hot, dry summer and fall, evaporation might drop the lake one to 1-1/2 feet. Between evaporation and a maximum outflow from a wide-open Fairford Water Control Structure over the fall and winter, even in the best case Lake Manitoba will still sit well above its flood level of 814 feet next April when high river flows arrive again.
This year’s flood damage around Lake Manitoba has easily surpassed a quarter billion dollars. If we are to avoid continued flooding into next year and beyond, we need to find a new path to dump water from Lake Manitoba and soon.
As a rising Lake Manitoba starts to bleed water through new exits at both the south and north ends of the lake, it might just be telling us where to look.
With rivers, lakes and reservoirs overflowing all across the Prairies this year, it seems almost certain that high water is in our future next year, as well.
It means we will likely need to use the Portage Diversion again next spring to protect people and property on the lower Assiniboine from Portage la Prairie to Headingley. This is very bad news for Lake Manitoba.
The lake will almost certainly reach a level of 818 feet later this summer, about four feet above flood level, and five feet above its highest desirable level.
In the best-case scenario, with a dry summer and fall over the rest of this year, water levels might drop three feet from the peak to perhaps 815 feet. In the worst case, the lake will remain above, perhaps well above, 816 feet next spring when high river levels arrive again.
With the lake already at flood level going into next spring, additional inflow from the Portage Diversion would be a death sentence for the farmers, ranchers and communities around Lake Manitoba.
A second full year of flooding becomes unavoidable and with it another year of damage from windstorms to whatever structures are still standing, another year of flooded roads, ranchlands, pastures and farms.
Why is this important? Seasonal residents provide the bulk of school and municipal taxes in lakeside communities. In St. Laurent, the figure is 70 per cent.
I am no expert, but my guess is that property sitting under two feet of water for more than two years is worth less than property normally high and dry. With the Lake Manitoba flood persisting for a second year, many — perhaps most — cottagers and permanent residents along the lake will choose to leave. The result will be collapse of the tax base of rural municipalities along the lake.
Support of the schools and infrastructure will necessarily fall to the province. Local economies will wither: Restaurants, grocery, convenience and liquor stores, golf courses, building centres, boat launches, campgrounds and gas stations are all supported primarily by seasonal residents and visitors.
The current dollar value of flood damage around Lake Manitoba will exceed $250 million when it is finally tallied. It is probably considerably more. But that already enormous cost will soar even higher if you add to that the collapse of communities, farms and ranches around Lake Manitoba. All necessary steps need to be taken to avert this unthinkable outcome.
The long-term solution is to reduce the lake level and to do that we need increase the outflow capacity of Lake Manitoba through the Fairford Water Control Structure. Such projects on the Fairford and Dauphin rivers are already at the planning stage but will take years to complete and provide no relief in the near term. This solution will come too late for most.
Our best strategy over the short term is to reduce our reliance on the Portage Diversion to keep a very bad problem from growing worse and just maybe give Lake Manitoba enough time to recover.
The diversion was opened on April 6 this year, and with Souris flood waters yet to arrive, will need to remain open through July. By the time it is closed, it will have more than doubled the normal annual water input to Lake Manitoba, causing a massive rise in lake levels. And its use is more or less unavoidable because of a bottleneck of river flow on the Assiniboine.
Upstream of Portage la Prairie, the channel capacity exceeds 50,000 cubic feet per second. Below Headingley, it is 30,000 cfs. The bottleneck lies between Portage and Headingley where the channel capacity is just 18,000 cfs.
This year, the flow upstream of the diversion peaked at 52,000 cfs. The surplus flow that cannot be handled within the lower Assiniboine channel is forced to detour through the Portage Diversion and park in Lake Manitoba. When flows on the Assiniboine at Portage reached 52,000 cfs this year, flow through the Portage Diversion reached 34,000 cfs to keep flows on the lower Assiniboine at 18,000 cfs.
In the 1970s, however, the capacity on the Assiniboine was considerably greater than it is now, roughly 24,000 cfs. An immediate priority must be to raise that channel capacity back to 24,000 cfs.
Had that extra 6,000 cfs capacity had been available this year, it would have reduced flows through the diversion by that same amount, lowering the rise on Lake Manitoba by about 1.25 feet.
We need also to explore — quickly — moving flood water from the Assiniboine into the La Salle drainage: creating a La Salle Diversion.
The La Salle is an old channel of the Assiniboine and joins the Red River downstream of the floodway. A portion of the Red River flow would need to be diverted through the floodway to accommodate additional flow from the La Salle. That means additional flow can be routed safely around Winnipeg when there is surplus capacity. And this year there was.
The 2011 flood on the Red peaked before the Assiniboine flood waters arrived from the west. Any water that could have been diverted south of Winnipeg through the La Salle River would have been carried safely around Winnipeg in the Red River and floodway.
Why is this important? It again reduces our reliance on the Portage Diversion.
We came very close to the tipping point on the lower Assiniboine this year. With flows on the Assiniboine upstream of Portage reaching 52,000 cfs and with the Portage Diversion straining to carry flows far in excess of its design capacity, we narrowly avoided extensive flooding on the lower Assiniboine between Portage and Winnipeg.
The Hoop and Holler cut was a last-minute improvisation designed to move water overland from the Assiniboine to the La Salle when flood managers had run out of other options.
Now imagine if the flow on the Assiniboine had been not 52,000 cfs but 58,000 cfs. Our flood defences would have been swamped and that trickle of water through the Hoop and Holler cut would have been a deluge.
Why is 58,000 cfs important? It is the level of flow that hydrologists estimate could occur on the Assiniboine if everything breaks wrong. This year has been, and remains, very, very bad. But it could have been worse.
Downstream of Portage la Prairie, the La Salle River sits next to the Assiniboine. There are several locations where a short channel connecting the two could be located. This would allow more water to travel through the combined channels of the Assiniboine and La Salle to the Red River. For such a strategy to work effectively, we need to shore up dikes and add pump capacity to handle local flooding where needed.
In the long run, a La Salle diversion would help protect us against a maximum Assiniboine flood. In the short run, it would help Lake Manitoba by reducing the need to use the Portage Diversion.
If the La Salle could carry somewhere between 5,000 to 10,000 cfs of diverted Assiniboine flow, that would have reduced the level on Lake Manitoba one to two feet this summer. Restore the Assiniboine channel to its 24,000 cfs capacity and Lake Manitoba is another 1.25 feet lower in 2011.
If we could avoid using the diversion next spring and summer that might just give Lake Manitoba the breathing space for its water level to drop back to something approaching normal later next year.
But we need to start now. Such a project poses a significant challenge. In 1997, in the face of advancing Red River flood waters, we built the 42-kilometre Z-dike in just three weeks. Manitobans can rise to a challenge.
This is the second of a two-part analysis of flooding on Lake Manitoba. The first part, Pulling the plug on Lake Manitoba, is available at winnipegfreepress.com. Scott Forbes is a behavioural ecologist at the University of Winnipeg. He has owned lakeside (now lake) property at Twin Beaches for the last dozen years.