Updated November 1, 2026

Portland's water starts as 135 inches of rain on Mount Hood

Few big-city water supplies are as pure at the source as Portland's. Its main supply, the Bull Run Watershed, sits in the Mount Hood National Forest 26 miles east of the city and serves nearly one million people. That's about to change in one important way.

A forest managed for drinking water

The Portland Water Bureau and the US Forest Service manage Bull Run together for one purpose: drinking water. In a typical year the watershed gets about 135 inches of rain and snow. That water runs into the Bull Run River and two reservoirs behind dams, which together hold nearly 10 billion gallons. Because the watershed is so protected, the state's 2019 source assessment named only naturally occurring microorganisms as contaminants of concern: Giardia, Cryptosporidium and coliform bacteria.

The second source: Columbia South Shore wells

Between Portland International Airport and Blue Lake Park, 25 active wells draw from three aquifers. The bureau uses them in two situations:

• to supplement Bull Run in summer, when demand rises and the reservoirs drop
• to temporarily replace Bull Run during turbidity events after storms, during maintenance and in emergencies

You may notice when the wells are on. Bull Run is very soft, at 7–11 ppm hardness, while the groundwater is about 80 ppm. The bureau offers notifications when the source changes, which helps people who use water for brewing, aquariums or dialysis.

Filtration is coming

Portland currently treats Bull Run with chlorine, which is stabilized with ammonia into a longer-lasting disinfectant, but it doesn't treat for Cryptosporidium. Because of repeated low-level detections of that parasite, the city must filter by 2027 and is building a plant to meet a September 2027 date. For most households the change will be invisible. People with weakened immune systems who have been following Cryptosporidium guidance from their doctors should ask whether that advice changes once filtration starts.

Corrosion control in soft water

Very soft, low-mineral water can be corrosive to metal plumbing, so Portland adjusts its chemistry. Sodium carbonate and carbon dioxide go into Bull Run water, and sodium hydroxide into groundwater, keeping pH typically between 8.0 and 9.0. The bureau tests more than 50 homes known to have lead solder, which are the homes most at risk. In the 2024 round:

• Lead: 90th percentile 4.4 ppb, below the 15 ppb action level. 1 of 101 homes was above it, and the highest result was 145 ppb.
• Copper: 90th percentile 0.14 ppm, with 0 of 101 homes above the action level.

Portland has no known lead service lines, so the solder in homes plumbed in the late 20th century is the main risk. Running the cold tap until it's cold before drinking and using only cold water for cooking help. An NSF/ANSI 53 filter offers more protection. See our filter selector.

PFAS and disinfection byproducts

Portland has tested both of its sources for PFAS and hasn't detected any. Running annual averages for disinfection byproducts at individual sites were 20.2–28.0 ppb for haloacetic acids (limit 60) and 20.3–32.1 ppb for total trihalomethanes (limit 80). If chlorine taste bothers you, a basic carbon pitcher works well. Our filter vs bottled cost calculator shows how far ahead it comes against buying water.