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CORVALLIS - The next time you slip down to your favorite swimming hole on a hot summer day, keep your eyes open and your mouth closed. That cool, soothing water may not be as safe as you believe.

A study of water quality at five natural swimming holes in the mid-Willamette Valley found moderate to high levels of bacteria that indicate possible disease-causing organisms. Ingestion of water laden with E. coli and fecal coliform bacteria may cause nausea, diarrhea and fever, researchers say.

Water quality is not necessarily apparent to the naked eye, they added, but swimmers watch the sanitation habits of other swimmers, especially those with small children.

"Swimmers tend to look at how clear the water appears and see a direct correlation to water quality," said Anna Harding, an associate professor of public health at Oregon State University. "Clear water isn't always an indication of clean water."

The study was conducted by Erica Van Ess, who completed a master's degree in environmental health management at OSU, under Harding's supervision. She collected samples of water at five sites throughout the summer of 1996.

Those sites were: the Mary's River at Avery Park in Corvallis, the Calapooia River at Monteith Park in Albany, a pond just east of Corvallis on Highway 34, the Luckiamute River on private property in King's Valley used frequently by 4-H and youth groups, and the Mary's River near 53rd Street southwest of Philomath.

At some locations, the level of bacteria was significantly higher than standards established by the Environmental Protection Agency - 126 "colonies" of E. coli and 200 colonies of fecal coliform per 100 milliliters of water.

At Monteith Park, for example, the 30-day log mean for fecal coliform was 293 colonies and 108 for E. coli in July. In August, the fecal coliform level rose to 314; the E. coli level dropped to 45. A broken sewer main - discovered after the study was completed and since fixed - may have contributed to the high readings.

The only other well-used site which exceeded the mean standards established by the EPA was the private swimming hole on the Luckiamute, which had fecal coliform levels of 260 in July and 176 in August.

Although 30-day log means were below EPA standards at the other sites, Van Ess said, daily readings ranged from very low to very high. The Mary's River at Avery Park, for example, had a 30-day mean of 92 colonies of fecal coliform last July, and 161 in August. Yet daily readings exceeded EPA standards 21 percent of the time.

"The readings are highly variable on a day-to-day basis," Van Ess said. "That could be due to the number of swimmers, stream flow, precipitation, or any number of factors."

The OSU researchers said the bacterial levels found in the study were comparable to those logged at Blue Lake near Troutdale, which has been the subject of intense study. The levels at which Blue Lake has been closed were exceeded at Monteith Park, they pointed out.

The cleanest site in terms of bacterial levels was the pond off Highway 34 east of Corvallis. Known informally as the Morse Bros. pond, it had low levels of both fecal coliform and E. coli, the researchers said. One possible reason: a very high pH level. "It's very alkaline," Van Ess pointed out.

In addition to causing general intestinal problems including nausea and diarrhea, the most likely effect of high bacteria levels are eye and skin infections, according to Harding.

"But these problems don't usually occur until a day or two later and most people don't think to associate them with swimming," she said.

The OSU researchers recommend more frequent testing of popular natural swimming areas, posted warnings when bacterial levels reach a certain point, and better public education.

"We would suggest monitoring of rivers upstream from heavily used swimming areas to determine if there are obvious contributors to bacterial contamination," Harding said.

Families can help, Van Ess added.

"Parents need to keep diapered children clean, keep their diapers off the beach, and encourage their children to use restroom facilities when they are available."