Portland is well-known as a tree-hugging, outdoorsy city, but the river that powers through its downtown has never been part of that green reputation.

For decades, residents have been repulsed by the idea of swimming in the Willamette River because of weekly sewage overflows that created a bacterial stew.

Now, the recent completion of a $1.4 billion sewage pipe has flushed those worries – and the river once shunned by swimmers is enjoying a rapid renaissance.

The city has partnered with a civic group called the Human Access Project to entice residents into the Willamette this summer with a roster of public swimming events and a flood of announcements that the river, finally, is safe for human use. The campaign is aimed at reversing the impact of decades of public health warnings in an eco-savvy city with a hard-earned green reputation.

The push mirrors efforts to revive ailing rivers in other US cities, from the Charles River in Boston – where occasional city-sanctioned swimming started in 2013 – to the concrete-lined Los Angeles River, where efforts have been underway in recent years to reverse decades of environmental damage along an 11-mile (18-kilometer) stretch.

In Portland, the movement has clearly found its moment.

The river is the city's largest public space, but less than 5 percent of the city's footprint has access to the waterfront, said Willie Levenson, who heads the Human Access Project and is working closely with Portland to expand swimming options.

Beaches in other communities along the river attract crowds, but swimmers in downtown Portland have nowhere to dive in despite increasing demand. Since the completion of the sewage control project in 2011, swimmers have been congregating on a floating esplanade for bikers and runners and sneaking onto city docks reserved for fire boats.

"We cannot pretend that swimming isn't happening in downtown Portland anymore. It's a livability issue, and Portland cares about livability," Mr. Levenson said. "It's time for our community to stop making jokes about our river and start digging in and looking to make a difference."

The Human Access Project has been working for several years to generate interest in the Willamette and has found a willing partner in new Mayor Ted Wheeler.

This week, a new beach with lifeguards and safety ropes opened on the city's south waterfront, within walking distance of hipster-friendly cafes and shops.

An inner tube river parade planned by the Human Access Project for this weekend is expected to attract several thousand participants, and members of a river swim group cross the Willamette several times a week in fluorescent green swim caps bearing the name River Huggers.

Mr. Wheeler, himself a swimmer, laid out a multipoint plan for increasing access to the river earlier this year and plans to swim the river later this month with 500 residents in the inaugural "mayoral swim." The city hopes to open two more beaches in coming years, install floating docks along the riverbank, and place public restrooms, picnic benches, umbrellas, and showers on site.

In a recent state-of-the-city address, Wheeler even spoke of one day eliminating Interstate 5 where it snakes along the Willamette's east bank to improve river access.

"We have a chance to reshape the face of our city," he said. "I also believe we have a chance to reshape our spirit."

Portland's relationship with the Willamette River hasn't always been easy to navigate.

For decades, the river was considered a watery highway, and industrial pollution severely contaminated its waters. This winter, after a 16-year wait, federal environmental officials released a plan to clean a 10-mile (16-kilometer) stretch near its confluence with the Columbia River in a project that will take decades of work and billions of dollars.

But in the heart of Portland, the primary problem has been human excrement. Residents grew accustomed to seeing near-weekly warnings about water quality during the winter rainy season, where even one-tenth of an inch (2.5 millimeters) of rain could trigger overflows.

Now, the city issues just a handful of warnings in winter and none during the peak swimming months of July and August, said Diane Dulken, spokeswoman for Portland's Bureau of Environmental Services. Testing at sites where people are already using the river show the water is safe, she added.

"We are really making a push to publicize our weekly testing because there is absolutely still a public perception out there, 'I will not go in the river.'"

On a recent blazing afternoon, Portland resident Alex Johnson was ready to take the city at its word.

The 24-year-old swim teacher and lifeguard began diving into the Willamette with the River Huggers swim group this month.

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On this day, he joined 30 others as they swam from the Hawthorne Bridge to the Morrison Bridge – through Portland's bustling business district – and back in the 70-degree (21 Celsius) water. Teenagers lounged like harbor seals on a nearby dock and jet skis zipped by as the swimmers completed the more than half-mile (0.8-kilometer) journey.

"I've heard stories that it's pretty polluted. It tastes a little funny, but it is river water," Mr. Johnson said. "It's a huge resource, and we don't take advantage of it – and it feels great."