People and pets were urged to steer clear of water contact at Lost Creek Lake after tests showed a potentially toxic bloom of blue-green algae there.
Tuesday's issue of a volunteer public-health advisory is this year's first at Jackson County's largest reservoir, which has seen annual blooms and advisories against ingesting its water in recent years.
Water tests taken Thursday by the U.S. Army Corps of Engineers came back Tuesday showing concentrations of Anabaena flos-aquae at more than four times the threshold considered by Oregon Public Health officials to be potentially dangerous to people and pets.
The reservoir, along the upper Rogue River, had been flirting with a potential algae outbreak in recent weeks, but tests showed only low levels of blue-green algae, which technically is a cyanobacteria that can produce dangerous toxins if ingested.
"We've seen a few cells here and there the past couple of weeks, but it started to turn at the end of last week," said Justin Stegall, the Corps' acting natural resource manager in the Rogue Basin.
Samples showed Anabaena flos-aquae levels in the reservoir at 405,900 cells per milliliter of water, Stegall said. The threshold for advisories is 100,000 cells per milliliter.
Tests on water released from the reservoir into the Rogue came back at 247 cells per milliliter, he said.
Corps workers began installing signs Tuesday warning of the advisory.
Visitor use has been slight since the end of the unofficial boating season over Labor Day weekend, Stegall said.
"We were very fortunate the bloom held off for that," Stegall said.
Visitors should avoid skin contact with the water and swallowing or inhaling water droplets, according to the advisory. Boating is safe as long as speeds do not create excessive water spray, which could lead to inhalation risk, the advisory says.
The toxins associated with cyanobacteria cannot be boiled away or filtered, so drinking the water should be avoided.
People who want to eat Lost Creek Lake fish should remove all the fat, skin and organs before cooking.
The advisory will be in place until further notice, according to the Corps.
The advisory is the seventh in Oregon so far this year and only the second in southwestern Oregon. A bloom discovered in July in Jackson Creek in Central Point triggered an advisory that lasted five days.
There never has been a confirmed human illness in Oregon caused by the algae or its related toxins.
However, two dogs have died in past years from algae-related toxins along the Umpqua River near Elkton.
Reach reporter Mark Freeman at 541-776-4470 or email at firstname.lastname@example.org.