PORTLAND — 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 U.S. 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," 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."