The creek forms a pool at a concrete bridge, and on most days, the water is so clear, you can see the rocky bottom 4 feet below the surface.

It's an inviting respite from the heat after a hike through Castlewood State Park, in St. Louis County just south of Ballwin. Thirsty dogs lap up the waters of Kiefer Creek. When the temperature rises, mothers wade in with their children. Teenagers drift lazily in the gentle current.

But while the popular swimming hole is no more than 20 yards in diameter, the question lately is just how safe it is to wade into the water.

A monitoring station about a half-mile upstream has for years has registered high levels of E. coli, bacteria that can cause symptoms similar to food poisoning.

"There are times when it (the creek) is OK. There are times when it's not," said Robert Criss, a professor of earth and planetary sciences at Washington University who has studied urban streams in St. Louis County. "It's not easy to tell those apart."

Just a few months ago, the Missouri Department of Natural Resources put up a sign warning "Caution — Swim at Your Own Risk."

This week, the nonprofit Missouri Coalition for the Environment will submit a report to state regulators, urging them to put the creek on a list of Missouri's most polluted waters and make it eligible for federal cleanup money.

"This is in a park with half a million visitors a year," said Lorin Crandall, clean water coordinator for the coalition.

The issue of E. coli in state waters has received its share of attention recently. Last year, Gov. Jay Nixon was dogged by criticism that members of his administration covered up tests showing elevated levels of E. coli at the Lake of the Ozarks.

But Kiefer Creek is a different situation. State officials note that because the creek is not a designated swimming area, the state isn't required to monitor the water quality. The Department of Natural Resources only knows about the E. coli in Kiefer Creek because the Metropolitan St. Louis Sewer District monitors it.

Judd Slivka, a DNR spokesman, said it would be nearly impossible to keep people out of the water. The sign the department has posted warns that the creek may contain "unacceptable bacteria," among other hazardous conditions.

"That's not out of the norm for what many states do," Slivka said.

But whether the state classifies it as such or not, the creek is indeed a 'swimming area," said Criss of Washington University.

"It's a state park with people in the water," Criss said. "Only a dang attorney would say, 'We don't care because it's not on our list.'"

MSD takes a few water samples a year, and the results have varied over the years. Last year — the most recent for which data were available — the monitoring station on two occasions found levels of E. coli higher than what the U.S. Environmental Protection Agency considers safe for full body contact.

E. coli bacteria can often be harmless, but some strains cause cramps, diarrhea or intestinal illnesses. E. coli is spread through contact with water or food contaminated with animal feces.

No one is quite sure what's causing the contamination in Kiefer Creek. Some people suspect that the septic tanks of older homes in the area have failed and are leaking into the water. Another cause could be wildlife in the park, or even horses and pets.

MSD spokesman Lance LeComb said the agency monitors the creek so it can have a better understanding of how development is affecting water quality. MSD makes the data available to anyone who asks.

Crandall, of the Missouri Coalition for the Environment, said it's unlikely the MSD data would have come to light if not for Steve Seyer, a Manchester resident.

Seyer used to run through the park with his giant schnauzer, Dolphus, two or three times week. After the jogs, Dolphus would cool off in the creek. Seyer says that about three years ago, his dog developed polyps on his back and had a black discharge from his eyes. When Seyer stopped letting his dog go in the water, Dolphus recovered.

About a year later, Seyer ran across the MSD water monitoring station about a half-mile from where Dolphus used to swim and soon tracked down the data.

Some samples over the years revealed levels of E. coli more than 10 times higher than those found in the Lake of the Ozarks last year. When he saw the data, Seyer said, he thought, "Gosh, that's what's killing my dog."

Seyer lobbied state and local officials to clean up the creek and warn people about the water.

"Everybody was shrugging their shoulders, saying, 'It's not my responsibility,'" Seyer said.

Eventually, the Department of Natural Resources agreed to put up the sign. But Seyer says it isn't specific enough.

"We need a sign that says frequent high levels of contamination," he said.

Nearly every time he jogs through the park now, Seyer sees people swimming in the creek and tries to warn them.

"You look at that stream, and it looks like something you would want to jump into," he said. "It's not."

People who swam in the water in recent days said they were not too concerned.

Mary Lou Auton of Ballwin brought her 12-year-old daughter and an 11-year-old friend to enjoy the creek on Monday. Her husband, Jeff, her son and neighbors went hiking in the park.

"My oldest son is 20, and we have been doing this for at least 15 years, and we've never gotten sick," Auton said. "When you come down here on the weekend, this place is packed."

Shannon Buehrle, 31, of Ballwin, brought her two kids and a niece to splash around. She was aware of the creek's hazard but was not worried.

"We go to the Lake of the Ozarks all of the time, and that water doesn't bother us," Buehrle said. "As long as my kids don't drink the water, it's OK."

Denise Hollinshed of the Post-Dispatch contributed to this report.