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Ancient lake’s legacy
May 20, 2011
At the turn of the 20th century, the boast was that all railroads led to Winnipeg, giving rise to the belief that the city would eventually become the “Chicago of the North.” While this didn’t quite come true, a great city did arise on the open prairie. 
Another statement that equally applies to the city is that all rivers led to Manitoba and Winnipeg. In effect, Winnipeg is in every sense of the term a “river city.” As such, Winnipeg emerged in the 1860s and 1870s as a significant prairie inland port served by steamboats from south of the border. Steamboat traffic then spilled over from the city into Lake Winnipeg, Lake Manitoba and westward via the Assiniboine and Saskatchewan rivers.
The two great rivers of the eastern prairie are the Assiniboine and Red, while the 
major lakes are Winnipeg, Manitoba and 
Winnipegosis. The Red drains an area of 48,000 square miles, while the Assiniboine drains an area of 63,500 square miles. 
All this water is the legacy of glacial Lake Agassiz, a legacy that is both a blessing and a bane to the province. At its maximum, Lake Agassiz was larger than any other glacial lake in North America, covering most of what is today’s Manitoba, and parts of Saskatchewan, North Dakota, Minnesota and Northern Ontario. Fed by melt-water from the Laurentian Ice Sheet, Lake Agassiz once spread out for more than a half-million square kilometres and was over 200 metres deep. By comparison, such an expanse of water makes today’s so-called Great Lakes little more than puddles, according to Bob McDonald of CBC’s Quirks and Quarks.
Lake Agassiz catastrophically drained twice. The first massive drainage of the lake occurred 13,000 years ago when it discharged through the McKenzie Valley into the Arctic Ocean, causing a disruption of ocean currents and a subsequent drop of about 10°C in air temperature  that resulted in the Young Dryas cooling period. The cooling caused the ice sheet to again advance and when a warming period eventually arose melt water recreated Lake Agassiz.
The second sudden drainage of Lake Agassiz occurred 8,400 years ago. Researchers worked out that the freshwater from Lake Agassiz burst forth from the bottom of the ice sheet, spilling a massive volume of water into Hudson Bay and sending a surge outward that swept across the Atlantic. The surge separated the British Isles from Mainland Europe and then raged into the Mediterranean Sea. The sea was so inundated with water that it forced a breach through the Bosporus Strait, turning what had been dry land around a small inland lake into the Black Sea.
The drainage was catastrophic for other regions of the world, but once the water was depleted in Manitoba, rich deposits of sediment remained behind that made the province’s soil a boon to agricultural exploitation. Other remnants of the huge lake are the rivers that criss-cross the province and lakes Winnipeg, Manitoba and Winnipegosis that make it a recreational wonderland. 
But the ancient lake bed periodically comes back to haunt the province — Lake Agassiz’s former basin causes water to flow into the province from the east, west and south. Water originating in the Rocky Mountains foothills even finds its way into Manitoba, as does water from Northern Ontario and Minnesota and North Dakota.
Being flat means Manitoba is especially prone to flooding. Any rise in water levels from too saturated soil, too much winter snowfall and too much rainfall at inappropriate times — within or beyond the province’s borders — means flooding will occur regardless of the best intentions of attempting to control Mother Nature. The effects of floods may be mitigated by human intervention, but they cannot be prevented. Lake Agassiz’s legacy of a flat broad basin assured that this would be the case for millennia into the future.
The flooding is invariably not dramatically rapid. Instead, water levels slowly creep upward until rivers spill their banks, as is the case today with the Assiniboine River. But once a river spills its banks, the very flatness of the plain spreads the water outward to create a disproportionate amount of havoc than is normally associated with a river flood. During 1997’s “Flood of the Century,” it gave rise to the massive inland sea, and now another expanding body of water is confronting Assiniboine Valley residents. Even in the Red River Valley this year, communities and farms found themselves surrounded by water. In fact, Highway 75, Manitoba’s major artery to the U.S. border, only recently re-opened following a weeks-long closure due to flooding.
This year’s once-in-300-years Assiniboine flood has had the double effect of causing unusual flooding along the southern shores of Lake Manitoba, as officials attempt to mitigate the flood damage along the river by allowing water to flow into the Portage Diversion in a capacity the system was never designed to contain. The water level on Lake Manitoba has crept upward and shoreline is disappearing. Where once a cottage, farm or home owner saw stretches of beach and open space, there is now only water moving forward onto the land. Sending more water into the diversion is helping people living along the river — though far from everyone — but it’s bad news for those living along the lake.
The other manmade attempt at flood mitigation was the cutting of a breach in the dike near Hoop and Holler. Its a controlled breach, so the evidence of the legacy of Lake Agassiz is there for anyone to observe. It’ll take days for the water to spread out and reach the creeks and rivers that eventually reach the waterways that flow into the Red River. 
The only way to stop this excruciatingly slow movement is for the anticipated volume of water to diminish through natural intervention. If Mother Nature bestows her blessing of sunshine and warmth upon the land from Saskatchewan into Manitoba, it will do more to lessen the flood damage than any measures conceived by government officials.
What has to be remembered is that floods are a natural consequence of living near prairie rivers and taking advantage of the agricultural bounty that the surrounding land provides. But it should be remembered that this bounty is the result of flooding that recharges the soil with nutrients, adding to its fertility. 
The problem is that water is one of the  mightiest forces of nature, seeking ways to breach any barriers that attempt to confine it. The power of rivers to flood the Manitoba prairie is never tamed, but only delayed until the inevitable occurs, such as a “Flood of the Century” or this spring’s once-in-300-years Assiniboine Valley flood .
Exceptionally rich agricultural land and vast tracts of valuable recreational properties that are highly susceptible to flooding is the legacy of Lake Agassiz.