According to officials, five teenagers were seen at Mount Tabor Park in Portland early Wednesday morning. One of them urinated through the iron fence.
Portland Water Bureau spokeswoman Jaymee Cuti said Thursday that test samples of water taken from the reservoir came back clean.
Officials acknowledge the urine posed little risk, but say they're not taking any chances and will flush the water anyway.
They note that they have plenty of water to meet demand and didn't want to deliver deliberately tainted water to customers.
The 19-year-old was cited for trespassing and public urination. The Multnomah County District Attorney's Office will decide whether to pursue criminal charges.
(The Associated Press contributed to this report)