The US city of Portland in Oregon is ditching more than 140 million litres of drinking water after a teenager was caught on camera urinating into a reservoir.

The water - enough to fill nearly 60 Olympic-size swimming pools - will be discarded despite tests finding no evidence of contamination.

The 19-year-old man, seen with two friends carrying skateboards in the early hours Wednesday, was filmed urinating through an iron fence into the reservoir in south-east Portland.

Portland Water Bureau administrator David Shaff said the water will be discarded and replaced with fresh water from the Bull Run water supply.

"Our customers have an expectation that their water is not deliberately contaminated," he said.

"We have the ability to meet that expectation while minimising public health concerns."

Portland Water Bureau spokesman Tim Hall dismissed suggestions authorities had overreacted.

"Any time there is human contamination of our drinking water, as a precaution, we shut the reservoir down and begin to drain and clean it," he said.

"We're in an enviable position of having an abundant supply ... Therefore releasing this water did not create any financial impact in the city or for our customers."

But the decision raised eyebrows further south down the US west coast, where California is suffering from its worst drought in decades.

"Really, Portland Water Bureau officials, do you have to flush 38 million gallons of potable water for the sake of a cup or two of human urine?" one Los Angeles Times commentary asked.

"Have a little pity on your neighbours to the south. Remember California? That big, dry behemoth ... how do you think we feel, when we hear the whoosh of 38 million gallons going to waste?

"We're in a drought, remember? Not good. No, not good at all."

The three men involved have been excluded from the reservoir area, while police were reviewing the video surveillance footage to decide whether to press criminal charges, Mr Hall said.

AFP