Eight million gallons of water drained from reservoir after man urinates in itThe operation is costing the state's taxpayers $36,000 (£22,000) and was ordered after Joshua Seater, 21, was caught on a security camera relieving himself in the pristine lake.
Health experts said the incident would not have caused any harm to people in the city of Portland, who are supplied with drinking water from the reservoir.
They said the average human bladder holds only six to eight ounces, and the urine would have been vastly diluted.
But David Shaff, an administrator at the Portland Water Bureau, defended the decision to empty the lake.
"There are people who will say it's an over reaction. I don't think so. I think what you have to deal with here is the 'yuck' factor," he said.
"I can imagine how many people would be saying 'I made orange juice with that water this morning.' "Do you want to drink pee? Most people are going to be pretty damn squeamish about that."
Mr Seater had been out drinking with friends when he decided to relieve himself in the open air reservoir at 1.30am.
He has not been arrested or charged with a crime, but may ultimately face a fine.
He apologised publicly for his behaviour, adding: "It was a stupid thing to do. I didn't know it was a water supply, I thought it was a sewage plant.
"I wouldn't mind paying for it but I don't have a job right now. I'm willing to do community service to clean up the place because I feel bad and feel pretty stupid." Sergeant Pete Simpson, of Portland Police, said: "It's really an unfortunate incident that probably could have been avoided if he had just chosen a bush."