"The US primaries ... have quite clearly indicated there is a real possibility NAFTA will be reopened and renegotiated and if that's the case, we certainly need to be much better prepared," said Tony Clarke of Polaris.

"And one of the issues that needs to be on the table is taking water off the table," he told reporters.

Clarke said many US cities could face critical water shortages by 2015 and noted the Southwest was already clearly in trouble.

"If the drought conditions continue to accelerate with climate change over the next few years, you can expect that there is going to be a major set of demands somewhere for that (Canadian) West coast water to flow ... directly into Washington state and Oregon."

Trade Minister David Emerson said there was no truth to the suggestion that NAFTA could one day be used to force Canada to export water to the United States.

"Water under NAFTA is acknowledged not to be a traded good and indeed there is a clear prohibition in Canada on any removal of bulk water from trans-border water basins," he told reporters.

While figures vary, Canada, with less than 1 percent of the world's population, is estimated to have as much as 9 percent of the world's renewable fresh water supply.

However, environmentalists say there is no truth to the idea that the country has massive reserves of fresh water. The Sierra Club says Canada has only 6.5 percent of the world's renewable supply, much of which flows north into the Arctic or into Hudson Bay.