Water quality experts agree that the Ottawa River is a lot cleaner now than it has been in the past, but they are concerned that the tests are limited, and that little is known about the effects of some chemicals on wildlife.

Once treated, the Ottawa River provides drinking water for one million residents in cities and towns along its shore. That water is regularly tested, and exceeds all provincial and federal standards.

But municipalities only test for a limited number of pollutants. Scientists are just now starting to study fish in the river, looking for trace amounts of chemicals that people may be flushing into the system, such as caffeine, and also estrogen from birth control pills.

"We're seeing male fish produce egg yoke, which is a phenomenon only found in females," said Daniel Cyr, of the Armand Frappier Institute in Montreal.

"Certainly, there's a lot more questions than answers. Certainly, we don't know where this problem is occurring, what's causing these problems, but we do know that we are seeing effects," Cyr said.

Like Ottawa, many cities along the Ottawa occasionally release raw sewage into the river.

For instance, Arnprior, 50 kilometres upstream from Ottawa, had to release its sewage into the Madawaska River nine times this summer because its treatment plant couldn't handle the heavy runoff during a storm. From the Madawaska, the sewage flowed straight into the Ottawa.

Arnprior, like Ottawa, has an aging infrastructure, built at a time when waste- and storm-sewer lines were often combined.

"We all need to be concerned about what we put in the river, period. That's why we've made a commitment to upgrading our infrastructure, and it looks like Ottawa is making that commitment now too, which is important because they have people downstream from them," said Arnprior Mayor Terry Gibeau.

Water quality experts say recent sewage spills have been getting lots of attention, but the Ottawa River is much cleaner now than it was in past decades, when pulp mills and other heavy industries lined the shore.

"We know that at the turn of the 19th century and beginning of 20th century, people died of cholera in Ottawa, and the water was pretty bad," said Frances Pick, a professor of aquatic microbial ecology at the University of Ottawa.

Scientists agree that, with an average flow of almost 2,000 cubic metres per second, most of the river's pollution is simply flushed away downstream.

"There is that expression, 'the solution to pollution is dilution.' Does that really apply to the Ottawa?" Cyr said.

"Well, it has applied historically, and we've gotten away with it to a certain extent. But, as the city grows, it's not a solution.

"The physics of the water flow are such that you can keep some of that polluted water for a while, and it may impact a beach downstream," Cyr said.