A U.S. lawmaker is calling for an investigation into the City of Montreal's decision to dump eight billion litres of raw sewage into the St. Lawrence River.

New York state senator Patty Ritchie wants the International Joint Commission — the group tasked with protecting the St. Lawrence River and Great Lakes — to look into the plan.

"The City of Montreal plans to dump the equivalent of 2,600 Olympic-sized swimming pools filled with wastewater from homes and businesses into North America's most beautiful river," Ritchie wrote in the letter, sent to the commission Friday and published on her website.

"While I realize that the dumping will occur in Canadian waters, downstream from any U.S. communities, I am very concerned by the precedent Montreal is setting for other communities along the St. Lawrence and the lakes."

Pierre Desrochers says the city has studied all other alternatives and this is the only option. (Stephen Rukavina/CBC) Pierre Desrochers, chairman of Montreal's executive committee, said Friday that emptying a major sewer interceptor into the river is the only viable option that will allow necessary construction work to be completed.

The city estimates it would cost $1 billion to build a temporary diversion or storage area for wastewater.

The work is being done as part of work on the Bonaventure Expressway.

Montreal Mayor Denis Coderre had temporarily put the brakes on the plan after widespread public criticism, but ultimately decided to go ahead.

The sewage will be allowed to flow into the river beginning Oct. 18, and that will continue for about a week.

City spokesman Philippe Sabourin said last week that contamination is not a big concern because of the sheer size of the river and how quickly it flows.

Quebec's Environment Ministry has approved the decision but Environment Canada has said the plan is prohibited under the Fisheries Act.

Montreal not alone

Until the 1980s, it was common practice to clear Montreal's sewers this way, but it is no longer considered an acceptable practice.

Montreal isn't the first Canadian city to release untreated sewage into oceans, lakes and rivers.

For years, untreated sewage flowed freely into Halifax Harbour.

Victoria still discharges millions of litres of raw sewage into the Strait of Juan de Fuca every day, while Winnipeg's outdated sewer system has led to 185 million litres of raw sewage being dumped into the city's waterways.