Police said the man was caught on film early Wednesday morning at Mt. Tabor Reservoir Number 5 in Southeast Portland.
About 38 million gallons of drinking water are being discarded because of the incident.
The entire 50-million gallon reservoir is being tested for possible contamination.
Though there's not much risk to the public, the water bureau said it won't serve tainted water to customers. For the time being, water will come from the Bull Run watershed.
"Our customers have an expectation that their water is not deliberately contaminated. We have the ability to meet that expectation while minimizing public health concerns," said Portland Water Bureau administrator David Shaff. "We will continue to provide our customers with safe, clean and cold Bull Run water."
The man was filmed urinating through an iron fence into the reservoir. Two other men with him, who are 18 and 19, tried to scale the fence, with one making it in.
Police showed up soon after. All three were cited for trespassing, and the 19-year-old was charged with public urination.
Police haven't decided whether any of the three will be charged with a crime.