Flood waters in Quebec and Manitoba may continue to cause environmental harm long after they recede, with experts warning of potential damage to water quality and wildlife.
Scientists say a churning mix of human waste, manure and pesticides poses ecological threats to downstream water bodies. The contents of overflowing urban sewage systems and big quantities of farming chemicals in rural areas are being flushed into connecting ecosystems.
Some of the contaminants flowing through Quebec's Richelieu River and Manitoba's Assiniboine River can also promote the spread of choking algal blooms and accumulate on fragile fish habitat.
A University of Manitoba ecologist said a bypass used to relieve the swollen Assiniboine has been redirecting flood waters into Lake Manitoba for several weeks. He said the flow could continue for another month.
Gordon Goldsborough said the pollutant cocktail is packed with nutrients such as phosphorus and nitrogen, primary culprits behind the proliferation of blue-green algae.
"You're going to get those green, scummy growths that we think of when we think of a nutrient-poor lake," Goldsborough said in an interview from Winnipeg.
"There's a variety of chemicals in the water that's flowing into the lake, so I think over the next few months you're going to see a dramatic change in the water quality of Lake Manitoba."
And as algal blooms rot over the winter, they suck up even more oxygen and can kill off fish, he added.
In Quebec, some 3,000 homes were flooded by the rising Richelieu, which has begun to slowly drain. The region just outside Montreal has seen flood waters reach highs unseen in over a century.
Human waste from the more-populated areas is believed to be blending under the waves of the Richelieu with chemicals and sediments from nearby agricultural operations.
A biologist with Quebec's Environment Department expects the swollen river to dilute any additional contaminants in the system, enough to keep water quality within the norms.
"For sure, when a river gets out of its bed and floods a territory, it creates problems and the overall water quality deteriorates," Marc Simoneau said.
"But … the substances in question will exist in much weaker concentrations."
He added that pesticides and fertilizers weren't applied this year before the floods, so fewer farming chemicals will reach the river.
Still, Simoneau said sediments that collect in rivers and lakes along the system could threaten fragile fish habitat.
When such sediments come from sources such as animal waste, pollutants are released over a long period of time, says an expert from the University of British Columbia.
"In my view, that's probably the biggest problem which you can think of," said Hans Schreier, a professor with the school's Institute for Resources, Environment and Sustainability.
While flooding is a natural event, Schreier had several suggestions on how humans can limit the amount of contaminants that find their way into a water ecosystem. He said a key to protecting water bodies is to make land around lakes and rivers less impervious to water.
Some of his ideas include encouraging the growth of more vegetation along shorelines, adding wetlands to soak up pollutants and leaving more space between human activities and the waterfront.
"Everybody in the city wants to have a house right on the riverside," Schreier said. "We need to have a fairly wide buffer zone, so that we can actually absorb all the sediment and all the pollution before it goes into the water."
He blamed generations of politicians who have permitted land usage within flood zones.
"We have been totally negligent in enforcing this," Schreier said. "People build in these flood plains and then they get flooded and we help them rebuild and then they get flooded again."