Josh Seater could have done some serious harm when he stepped up to the wrought-iron fence around a Portland reservoir last month if he were holding something more ominous than a full bladder.

The open-air reservoir contains treated water that goes directly to people's spigots, and Seater's decision to urinate there after a night of drinking led Portland officials to drain the entire basin to keep from rattling the public's nerves about the purity of the drinking supply.

The saga delighted headline and joke writers, but it reveals a threat to urban water supplies in about a dozen cities.

Portland has five of up to 30 uncovered reservoirs around the country that contain treated water, some accessible to the public. The fear is that a terrorist could drop or somehow get a toxic chemical agent into a reservoir and sicken people.

"You can use your imagination. If somebody wanted to do something malicious, they could," said Richard Luthy, a Stanford University professor of civil and environmental engineering on a trip to a California reservoir.

Luthy and others told congressional panels after Sept. 11 about the vulnerability of infrastructure including water systems. Federal authorities ordered security evaluations, and water systems around the country have added fences, surveillance cameras, officer patrols or other measures.

Opinions about the extent of the risk that remains are divided.

In a 2004 paper for a NATO-Russia workshop on protecting urban infrastructure, University of Maryland Professor Gregory Baecher cited "a catalog of several dozen potential toxins, bacteria, viruses, protozoa and toxic industrial chemicals that have been identified as possible water contaminants that could be used by terrorists."

But Baecher said in a recent interview that dilution is one protection against harm from that sort of attack, and the nation's many open buildings are softer targets than water supply reservoirs. "If I were a terrorist, this is just not one of the easiest things to do," he said.

The dilution factor is what prompted some people to say that Portland overreacted in draining the reservoir. A pint of urine is a tiny drop in the bucket in a reservoir of 7.5 million gallons where ducks defecate as well.

It turns out that the federal government has been cracking down on reservoirs such as Portland's for reasons that have less to do with speculative threats from al-Qaida than with the known risk of serious health threats _ the biggest one being cryptosporidium, a parasite from the feces of infected animals or humans. In 1993 it got into Milwaukee's water, led to the deaths of as many as 100 people and sickened hundreds of thousands more.

Rules the U.S. Environmental Protection Agency rolled out in 2006 are putting an end to the sort of reservoir Seater used _ an open-air basin that holds treated or "finished" water to be distributed directly to consumers. The cryptosporidium parasite was a big motivating factor for the changes.

Many of these reservoirs date to horse-and-buggy days and were once celebrated in American cities.

They often exploited gravity to get water cheaply to growing populations. They provided a ready supply for firefighting. They were installed with architectural flourishes and lights and given central places in parks with surrounding pathways. That sentiment is strong in Portland, where neighbors who enjoy the scenery of the reservoirs call them a gem of the city and have been fighting for years to keep them open.

An estimate cited in a paper for the American Water Works Association says there were about 750 open, treated reservoirs in the 1970s. Recently, the Portland water bureau compiled a list of about 30, including some in such cities as New York and Los Angeles, Baltimore, Seattle and Tacoma.

Plans are well along in most cities to comply with the EPA's rules, although it will take years to finish in some. In Los Angeles, for example, the estimate is 2022. In New York, city officials have asked the Obama administration for a waiver to allow the billion-gallon Hillview Reservoir to remain uncovered _ or at least to delay the compliance deadline for the $1.6 billion project.

In 2007, New York City joined Portland in taking the EPA rules over cryptosporidium to federal court, but a federal appeals court slapped down their arguments as "either meritless, irrelevant, or both." As recently as June, Oregon state authorities told the city of Portland there's no such thing as a waiver to the rules.

The city is building two underground reservoirs expected in a few years to replace the Mount Tabor reservoirs.

As for Seater, prosecutors say they have not made a decision about charging him. And the city has finished draining the reservoir and scrubbing the walls.

Flushing the urine was a smart decision in the view of Stanford professor Luthy, who regards confidence in public water supply as an important social good. Mistrust could lead to social divisions along lines of those who can afford bottled water and those who can't, he said.

"We should expect that the water supplied to us is safe and wholesome and reliable."