American lawmakers are increasingly concerned about pollution from British Columbia mines contaminating waters in the United States.
Eight U.S. senators have written a letter to B.C. Premier John Horgan over the threat mines in his province pose to rivers flowing into American states.
They say Canadian mines are badly assessed for their environmental impact and badly monitored once they are built.
Americans have long been upset over pollution from coal mines in southern B.C., which leaves rivers flowing into the U.S. with contaminant levels up to 50 times higher than recommended.
The letter, dated June 13, says four U.S. states receive water downstream from mines with poor oversight.
Last year, Americans on the panel that regulates cross-border waters said Canadian members blocked the release of information on contaminants that are many times above guideline levels.
The letter is signed by both Republicans and Democrats.
It is copied to Prime Minister Justin Trudeau and four federal cabinet ministers.