The artificial channel that protects Winnipeg from the Assiniboine River appears to be making Lake Manitoba sick.
The Portage Diversion, which sends most of the Assiniboine's flow toward Lake Manitoba during severe floods instead of allowing the full force of the deluge to pour into the Red River, is starting to degrade the water quality of the province's third-largest lake.
Unlike Lake Winnipeg, which is flirting with ecological disaster due to dangerously high levels of nutrients from natural and human sources, Lake Manitoba's waters are relatively pristine.
During an ordinary year, this lake receives water from two main sources -- the Waterhen River in the northwest and the Whitemud River in the southeast. Neither river is a massive source of phosphorus, the nutrient freshwater scientists consider most responsible for promoting the growth of harmful blue-green algae.
But the Portage Diversion brings water from all but the eastern reaches of the 182,000-square-kilometre Assiniboine River basin, which covers vast areas of western Manitoba, southeastern Saskatchewan and a corner of North Dakota.
During severe floods, the Assiniboine and its tributaries flow over nutrient-rich agricultural land, allowing floodwaters to absorb large quantities of phosphorus and other algae-promoting nutrients.
The Portage Diversion sweeps those nutrients into Lake Manitoba, said Gordon Goldsborough, an aquatic ecologist at the University of Manitoba and the former director of the university's Delta Marsh field station.
This is not a theoretical statement. In 2009, a flood year that saw the Portage Diversion operate for 27 days, U of M geography student Reanne Pernerowski analyzed the effects of the diversion on Lake Manitoba's water quality.
According to her observations, the Portage Diversion was responsible for 93 per cent of the phosphorus that flowed into Lake Manitoba in 2009. The two natural sources -- the Waterhen and Whitemud rivers -- accounted for the remaining seven per cent of the phosphorus Lake Manitoba received that year.
Pernerowski also found the diversion in 2009 brought Lake Manitoba 60 per cent of its nitrogen, another algae-promoting chemical, and 99 per cent of the suspended sediments -- dirt from flooded areas -- that make lake water less translucent.
"That really demonstrated the capacity of the diversion to affect water quality," said Goldsborough, adding the artificial channel's polluting effect should be even greater this year, as more water is being forced through it.
The Portage Diversion was built in 1970 to carry a maximum of 25,000 cubic feet per second of water to Lake Manitoba. This year, the province is forcing as much as 33,000 cfs through the diversion to prevent the Assiniboine River from overwhelming dikes between Portage la Prairie and Winnipeg.
Although Lake Manitoba has appeared healthy compared to Lake Winnipeg, where blue-green algae blooms are common, it won't take as long for the smaller lake to follow suit, Goldsborough predicted.
Nutrient-loading effects this year may not show up until 2012 or later, said Greg McCullough, a geographer with the U of M's Centre for Earth Observation Science.
McCullough's own research along the La Salle River has corroborated the relationship between flooding and the over-fertilization of lakes, a condition scientists call eutrophication. Flood waters don't just bring nutrients, but higher concentrations of the chemicals that promote the growth of algae, which in turn deprive lakes of oxygen when algae die and decompose.
The province is aware of the Portage Diversion's effects on water quality, said Water Stewardship Minister Christine Melnick.
"You are going to see nutrient-loading, and that's something we're concerned about," she said. "We are dealing with a real-time event."
Phosphorus loading into Lake Manitoba in 2009:
From the Portage Diversion: 16,543 tons (93 per cent)
From the Waterhen River: 779 tons (four per cent)
From the Whitemud River: 561 tons (three per cent
-- Source: The Portage Diversion and its Impact on Lake Manitoba's Water Quality, by Reanne Pernerowski