DES MOINES, Iowa — As someone who has spent countless hours rowing along the Des Moines River, Tonya Logan appreciates the city's vision to create a whitewater course that would draw kayakers to the Iowa capital.

But there's a dirty secret for Des Moines and many other U.S. cities that want to upgrade their urban waterways into scenic destinations: Much of the water is so polluted with manure that people fear it's not safe to dip their hands in the current, let alone to swim in it.

"I won't," said Logan, who doesn't touch the water that passes just inches from her as she sits in her slender rowing shell, even on blistering hot days. "The last time I went into the water, I took a long shower and then scrubbed myself with peroxide." Others have complained of intestinal problems, skin rashes and infections.

The unseen but potentially dangerous pollutants threaten to undermine the efforts of dozens of communities seeking to turn rivers into urban amenities that will attract tourists and businesses and become centerpieces of downtown life.

"It's an issue in any city trying to do this," said Rick Tollakson, a Des Moines developer who is leading the push to remove small dams along the Des Moines and Raccoon rivers to create whitewater courses as part of a larger regional "water trail" plan. "The rivers are not as clean as people would like them to be."

In fact, most U.S. rivers are far cleaner than in decades past, largely because of the federal Clean Water Act, which was approved in 1972. But many waterways still carry farm runoff and city sewage that contain nitrates, ammonia and E.coli bacteria, according to the Environmental Protection Agency.

In Des Moines, the problem comes mainly from animal waste and chemical fertilizers that drain into the rivers from farmland. The city treats the water to make it drinkable, but that doesn't help paddlers and swimmers who could be exposed to high bacteria levels, especially after heavy rains.

"There are times you're out there and it's so beautiful, and then you smell the pig manure from upstream and it's just disgusting," said longtime kayaker Scott Bandstra, who uses antiseptic wipes liberally and has experienced only the occasional mild rash.

His wife will kayak with him when they travel to Colorado's Rocky Mountains, but she won't get in the water in Iowa.

Promoters of the Des Moines plan say such fears are overblown, but they acknowledge that a century of pollution has created skepticism about their $117 million proposal. The plan is intended to draw new residents to Des Moines, a city with plenty of jobs but not a lot of glamor. It's being led by business executives, who promise to provide about a third of the funding, with the remaining coming from federal, state and local governments.

City leaders have embraced the general idea but have not committed money to the project, which is at least several years away from the start of construction.

The effort to improve waterways seeks to take advantage of surging interest in paddling sports such as kayaking, canoeing and rafting. An annual survey by the private Physical Activity Council found that 21.7 million Americans reported paddling in 2014, up more than 3 million from 2010. Kayaking was the most popular, especially among young adults, age 18 to 24.

In Columbus, Georgia, thousands of people flock to a 2½-mile whitewater course built in 2013 even though raw sewage still occasionally flows into the Chattahoochee River during big rainstorms.

Juliet Cohen, executive director of the environmental group Chattahoochee Riverkeeper, said the river's popularity makes it all the more important to ensure that it is free of health risks. The conservation group is pushing officials to improve the community's sewage system.

In Los Angeles, officials are open about bacteria in the LA River, even as they encourage residents to canoe or kayak through stretches of a 51-mile waterway better known as a movie backdrop.

The city samples river water twice a week and posts the results on a website with color-coded warnings of "open," ''caution" and "closed."

Department of Public Works Commissioner Heather Repenning said she hopes the growing number of kayakers will make officials care more about stopping the street runoff and removing its concrete lining to make the river more attractive.

"You have to get people to have some stake in it," Repenning said.

Possibly the waterway most stigmatized by pollution is Ohio's Cuyahoga River, which famously caught fire in the 1960s because of oily industrial runoff in Cleveland. Even there officials are encouraging boating. The river is no longer flammable and even swimmable upstream, but it's still not pristine in Cleveland.

In Denver, where the South Platte River flows out of the Rocky Mountains and through a city-built whitewater course, officials acknowledge the water at times exceeds standards for E. coli.

Jon Novick, a city environmental administer, said officials have a special responsibility to warn people to wash up thoroughly after a river outing because so many people are drawn to it.

"It's a huge challenge for us, especially because the city is actively developing infrastructure to use our waterways," he said.

People like Dave Hillman are excited about Des Moines' whitewater plans.

Hillman, who has kayaked for decades along most of Iowa's rivers, said he's had ear and sinus infections he blames on the state's "hot chocolate rivers." But he still gets out on rivers nearly every weekend.

He thinks kayakers exposing themselves to disease could help bring an answer.

"This is a big issue for paddlers, and it's a topic where there's not an easy answer," Hillman said. "But the water quality is not going to improve until we take ownership of it."