The gradual but steady pollution of the popular St. Croix River means it's no longer the sparkling algae-free gem it was four decades ago.

Already classified as impaired by the Minnesota Pollution Control Agency, the St. Croix is expected to land on a national top 10 list of endangered rivers that will be announced Tuesday by American Rivers, an environmental advocacy group.

"We're not happy with the news but getting some national attention in Minnesota and Wisconsin is good," said Dan McGuiness, interim executive director of the St. Croix River Association. "We're hoping that this information will help us bring more attention to the river."

From northern Washington County south to where the St. Croix flows into the Mississippi River, the St. Croix through the spring and summer becomes a mecca for thousands of speed boaters, canoeists, anglers, swimmers, cyclists, hikers and others drawn to the river's beauty. It's a major recreational destination in the metro area, probably second only to Lake Minnetonka in sheer numbers of outdoor enthusiasts.

Yet even as boats emerge after a long winter's nap, the river's troubles are mounting. The St. Croix is a federally protected wild and scenic river, the only one in Minnesota and one of only 165 such-designated rivers nationwide. Although a web of administrative rules and local ordinances protect it, land-use practices and zoning difficulties along the sprawling watershed continue to threaten the river, said Molly Shodeen, an area hydrologist for the Minnesota Department of Natural Resources. She said she has watched the St. Croix's steady decline in the 27 years she has worked for the agency.

"Water quality has definitely deteriorated," she said. "You can physically see it now where you can see algae blooms. It's always been renowned for its water quality, and that's what got people to use it -- enthusiastically. I think it's somewhat shocking that it's declined to the degree that it has."

The river landed on Minnesota's impaired waters list for the first time in 2007 because of high levels of phosphorous in Lake St. Croix, the deepest part of the river from Stillwater south to Prescott, Wis.

Stillwater's mayor, Ken Harycki, said he's not worried by the prospect of an "endangered" river flowing past one of Minnesota's premier tourist destinations.

"The reality is it's still a very pleasant river," he said. "Efforts are being made to clean it up. It's still a tremendous asset and attraction for Stillwater."

Shodeen and McGuiness say the St. Croix's decline -- mostly from Stillwater south -- has been long in coming and will worsen without a bigger effort to reverse the flow of sediments, fertilizers and other contaminants into the river. Because the National Park Service oversees the river north of Stillwater, much of the land along the water there is empty and the river is in better shape.

"This is a situation where it's not a big oil spill but it's a lot of little changes," said McGuiness, who thinks the river's condition would be even worse without the 40-year-old wild and scenic designation. He wants more commitment from the DNR and city and township governments to enforce zoning ordinances already in place.

Christopher Stein, superintendent of the Saint Croix National Scenic Riverway, said he's quite sure Congress didn't intend the St. Croix to become impaired and endangered.

"If this designation helps people take notice that we have a precious gem here and we need to take care of it then I'm glad," Stein said. "We have to be constantly vigilant about protecting these resources."

Environmental improvements to save the river are coming from several directions.

Stillwater, Harycki said, has built sophisticated settling ponds to control storm water runoff and has taken river-friendly measures such as installing "permeable" paver bricks at the remodeled city library. "We're spending a lot of time and money trying to clean up the lake water and river water that's dumping into it," he said.

Watershed groups have rallied, too. For example, last year the Valley Creek Watershed District, near Afton, repaired the creek's banks to stem the flow of tons of contaminants and sediments into the St. Croix.

Shodeen said the St. Croix is falling to the same trend that badgers waterways throughout the country -- the human impulse to replace smaller, decades-old dwellings with bigger structures close to the water.

More hard surfaces channel more dirty water into the river, as do municipal septic systems that leak in sandy soil, she said. Pollution, she said, moves faster than do solutions to stop it. At least 20 percent of the current pollution of the St. Croix would have to be reversed, she said, just to keep the river at its current degraded state.

"Without a large effort, the water quality will continue to degrade," she said.

Kevin Giles • 612-673-4432