Note: Updated to reflect fire burning inside the watershed, risks large fires pose to drinking water.

City of Portland officials say wildfires in the Columbia Gorge have encroached the city's Bull Run Watershed but so far have not reached or harmed the water. Nor, they say, has the ash falling throughout the area impacted water quality.

For now, they say, nothing is changing about the way water is delivered to customers of the Portland Water Bureau and water remains safe to drink.

Despite having entered the watershed, fire is not burning near the reservoir or the water supply infrastructure, officials reported about 4 p.m. Tuesday. Edward Campbell, the Portland Water Bureau's resource protection director, said the city's headworks facility, where drinking water first enters pipes, is about 18 or 19 miles from the watershed's northern boundary.

The city doesn't know exactly how far the fire has advanced or how much of the watershed has burned.

Campbell said that although he is hopeful winds in the gorge will ease, "it's concerning how far it can possibly move."

The bureau would switch to its back-up water supply at the Columbia South Shore well field if the headworks had to be evacuated, he said. That supply can meet the city's base demand, he said.

Water bureau leaders have authorized officials with the joint fire command to dip into Blue Lake and other water sources inside the watershed -- except the drinking water reservoirs -- to slow the spread of the fire.

It's too soon to say what, if any, effect the fire will have on a drinking watershed described as being one of the nation's most pristine.

Erosion increases after fires, and can lead to more sediment getting into the water, bringing higher treatment costs. Nutrients that run off after fires can cause algal blooms in reservoirs that make the water taste and smell bad, said Jeff Writer, an environmental engineering instructor at the University of Colorado.

"We've seen impacts persist for years," Writer said. "The utilities have to be ready and able to deal with this for many years."

But Portland is lucky. Even after hot fires that scorch the soil, rain tends to still infiltrate the ground on the western slopes of the Cascades, said Gordon Grant, a U.S. Forest Service research hydrologist. "It's good that it drizzles here," he said.

"As long as the fire's not in the vicinity of the infrastructure itself, I'd expect the impacts to be relatively minor," Grant said. "Call me back if the thing gets in there and burns really hot."

Water Bureau director Michael Stuhr, in a statement, expressed gratitude for state, federal and local agencies that are helping track and fight fires and providing city water officials with information.

As of Tuesday morning, the Eagle Creek fire in the Columbia Gorge had grown to 10,000 acres.

Winds push the fire westward, forcing evacuations of numerous communities and filling the skies with smoke and ash.

Wildfires are burning near the Columbia River in wide stretches of the Gorge. The city's Bull Run reservoirs are located about 15 miles south of the river.

-- Betsy Hammond and Rob Davis

Correction: This post was updated to clarify that the city does not know how close fire has gotten to the headworks facility.