Updated at 6:43 p.m.

Gov. Kate Brown declared a state of emergency Thursday for Marion and Polk counties, opening the door for national guard troops to truck in fresh water for residents grappling with the discovery of low-level toxins in Salem's tap water.

Though officials say the water is safe to drink for most people, the governor's declaration allows the Oregon National Guard to open 10 water stations in the affected areas. The troops will deliver the clean drinking water in 2,000-gallon tankers called "hippos."

The water advisory, issued Tuesday, came after small amounts of cyanotoxins were found in the Detroit Reservoir. The advisory will stand through at least Saturday, because the city must receive two clean water samples before it can be lifted.

The water doesn't pose a risk for healthy adults, but officials recommend that it be kept from children younger than 6; the elderly; people with compromised immune systems; nursing and pregnant women; and pets.

Dr. Richard Leman, Oregon Health Authority's chief medical officer, said the water's fine to wash dishes, water plants and other activities.

The toxins are caused by blue-green algae blooms, which have been turning up in water sources throughout the Pacific Northwest with increasing regularity. Such blooms are more pronounced in the Midwest, which is why Salem's water quality officials have been rush-ordering their water samples to an Ohio laboratory for testing.

Lacey Goeres-Priest, a water quality supervisor, said officials first detected something amiss in the water supply on Saturday. The city stepped up monitoring of the reservoir and its source, North Santiam Water. It also began running tests during the treatment process.

The algae is usually just an inconvenience, Goeres-Priest said, because of its ability to clog pipes and create taste and odor issues. Treatment staff are trying ways to bring the cyanotoxin levels down. They have added water from groundwater reservoirs and are considering reverse osmosis to more aggressively treat the water system.

The city uses a rare filtration system for the U.S. because Detroit Lake has been a reliably clean water source. The city has a European-style slow sand filtration system, where water is passed slowly through a fine layer of sand, which relies on natural processes to remove bacteria.

"We take our water quality very seriously," City Manager Steve Powers said during a Thursday news conference. "We test daily. We test for this toxin when it's not required we test for it."

Public Works Director Peter Fernandez said that it is an excellent and inexpensive way to treat water. But, if problems with algae continue, he could consult city councilors and the city manager about backup options.

Fernandez said the city could expand its groundwater stores, build connections with agencies that provide reverse osmosis treatment and improve the city's ability to test for cyanotoxins on-site.

Salem officials have not been clear about why they waited four days to issue a public health notice about the water. However, the U.S. Environmental Protection Agency suggests a 10-day window between detection to public notice so that water authorities can hone their treatment system. That 10-day period is supposed to be the timeframe when symptoms might start showing up.

The two types of cyanotoxins found in Salem's water can cause kidney or liver injury if someone already has a condition. For other vulnerable people, it could cause vomiting, diarrhea or nausea.

County officials said they don't know if anyone has been made sick from the water.

The advisory applies only to Salem, Turner and customers of the Suburban East Salem Water District and Orchard Heights Water Association. Stayton is also following Salem's advisory. Other places that draw water from the North Santiam are not affected, including Gates and Jefferson.

Marion County has partnered with private home health care providers and residential care facilities to distribute water to elderly or sick people who can't travel to a water distribution site. Nonprofits Oregon Food Bank and Marion-Polk Food Share have helped supply 18 pallets of water.

The Salem-Keizer School District is providing bottled water for students from preschool to first grade, medically fragile students, teen parents and staff members who are pregnant or nursing.

The Oregon State Penitentiary is providing bottled water to inmates who have liver or kidney issues, while the rest are still supposed to drink tap water. If they complain of feeling sick, they will be evaluated by the prison's medical staff, said spokeswoman Tonya Gushard.

Meanwhile, Oregon Attorney General Ellen Rosenblum said her office has received several price-gouging complaints about bottled water sellers in the past two days. Under Oregon law, sellers are barred from raising prices more than 15 percent above what they would have charged before the emergency declaration.

Andrew Phelps, director of the Oregon Office of Emergency Management, warned retailers to sell their water for reasonable prices.

"If you're a retailer selling water, knock it off," Phelps said.

He also said the runs on bottled water illustrate how ill-prepared Oregonians are for any kind of emergency. Residents are advised to have a 14-day stock of potable water at all times.

"As best you can, prepare for whatever bad day lies ahead," Phelps said.

-- Molly Harbarger

mharbarger@oregonian.com

-- Corlyn Vorhees

cvorhees@oregonian.com