North Dakota says it faces severe drought within 50 years and needs to tap water from the Missouri, which normally flows into the Mississippi River and then on to the Gulf of Mexico.

When drought conditions hit, the proposed diversion would take an estimated 120 cubic feet of water per second out of the river, which flows at an average of 20,000 cubic feet per second.

The province of Manitoba, still smarting from losing a bruising row with North Dakota over draining low-lying Devil's Lake into a river that feeds into Lake Winnipeg, says the risks of the plan are just too big.

"Our concern would be that brings a risk of harm to Manitoba with the potential movement of harmful, invasive species," Dwight Williamson of Manitoba Water Stewardship told Reuters in a recent interview.

That's one of the arguments the Manitoba government put forward last year when it and the Canadian government resisted the Devil's Lake plan. North Dakota finally got its way after agreeing to add more rocks and gravel to its drain as filters to try to prevent the introduction of foreign species.

Gaile Whelan-Enns, spokeswoman for Sierra Club Canada, said the latest plan is an even greater concern as it would join two water basins that have been separated for 10,000 years and could bring foreign species from the Missouri River to Lake Winnipeg and then to Hudson Bay.

"The Boundary Waters Treaty between Canada and the US must be upheld," she said.

The treaty, which dates from 1909, says an independent agency known as the International Joint Commission should resolve cross-border water disputes.

In 1977, the commission recommended against diverting water from the Missouri unless the two countries could agree that risks can be eliminated.

Merri Mooridian, of the Garrison Diversion Conservancy District in North Dakota, said a treatment plant for the proposed water supply project would prevent invasive species from getting into Canadian waters.

"We feel this alternative, with the proper treatment, will not harm the water," she said.

Canada wants North Dakota to use water sources within the Red River Basin in Minnesota and North Dakota, but Mooridian said there will not be enough water available during the expected drought.

The Garrison Diversion is due to issue its final environmental impact statement by December, after which the US interior secretary will make a final ruling. Congressional approval will be required before construction can begin.

Construction, estimated to cost $500 million to $660 million, would probably not start before 2009 and the system would be operational by 2012 at the earliest.