City of Lake Oswego 2026 Water Quality Report The City of Lake Oswego’s 2026 Water Quality Report is based on data collected during the 2025 calendar year. The City prepares this report in accordance with Federal and State regulations to bring our customers the best available information about the water they drink. Lake Oswego’s drinking water is tested every day and for the 2026 report, we can proudly announce there were no violations. More than 90 contaminants are regularly sampled for, both before and after the water is treated, to ensure it meets the more than 120 water quality standards for drinking water set by the Environmental Protection Agency (EPA) and the State of Oregon. All of the substances were either not detected or were detected at levels well below limits set by the EPA and State of Oregon for safe drinking water. If you have any questions about this report or water quality, please contact 503-635-0280.
Where does Lake Oswego’s water come from?
Lake Oswego’s drinking water originates in the Clackamas River watershed, which is one of the highest quality water sources in the state. This watershed encompasses 940 square miles and begins in the Mount Hood National Forest. Water is withdrawn from the Clackamas River in Gladstone, then pumped through a pipeline buried beneath the Willamette River to the Lake Oswego-Tigard Water Treatment Plant located in West Linn. A Clackamas River Watershed Source Water Assessment was completed in 2019. The report is available at www.deq.state.or.us/wq/dwp/docs/ uswareports/USWA_00457LakeOswego.pdf. An additional Clackamas River Water Providers report can be found at https://storymaps.arcgis.com/ collections/d107e2dad24e4a5d9c28dad37a835c16.
Sources of Contaminants
The Safe Drinking Water Act defines the term “contaminant” as any physical, chemical, biological, or radiological substance or matter in water. Therefore, the law defines “contaminant” very broadly as being anything other than water molecules. Drinking water may reasonably be expected to contain at least small amounts of some contaminants. Some drinking water contaminants may be harmful if consumed at certain levels in drinking water while others may be harmless. The presence of contaminants does not necessarily indicate that the water poses a health risk. To ensure that tap water is safe to drink, the EPA sets regulatory limits on the amount of certain contaminants in water provided by public water systems. The U.S. Food and Drug Administration sets limits for contaminants in bottled water which must provide the same protection for public health. Sources of drinking water (both tap water and bottled water) include rivers, lakes, streams, ponds, reservoirs, springs, and wells. As water travels over the earth’s surface or through the ground it dissolves naturally occurring minerals and, in some cases, radioactive material. Drinking water can pick up substances resulting from the presence of animals or from human activity. Contaminants that may be present in source water throughout the United States include: o Microbial contaminants, such as viruses and bacteria, which may come from wildlife or septic systems e.g. coliform and giardia. o Inorganic contaminants, such as salts and metals, which can occur naturally or result from urban stormwater runoff, industrial or domestic wastewater discharges, or farming. o Pesticides and herbicides, which may come from a variety of sources such as farming, urban stormwater runoff, and home or business use. o Organic chemical contaminants, including synthetic and volatile organic chemicals, which are byproducts of industrial processes and may come from gas stations, urban stormwater runoff, and septic systems. o Radioactive contaminants, which can occur naturally, e.g. radon.
Cyanotoxin Monitoring Rule
In 2018, the Oregon Health Authority (OHA) developed a drinking water rule that requires many drinking water systems in the state to test for cyanotoxins, and notify the public about the test results. Cyanobacteria naturally occur in lakes and streams. Under certain conditions, such as in warm water containing an abundance of nutrients, they can rapidly form harmful algal blooms (HABs). These blooms are capable of producing toxins known as cyanotoxins. Cyanotoxins are compounds that are capable of harming humans. The OHA has issued safe drinking water limits for the cyanotoxins microcystins and cylindrospermopsin. Lake Oswego’s results are listed below. During the 2025 testing, no cyanotoxins were detected in Lake Oswego’s water supply.
Algal Toxin
Health Advisory (Vulnerable Population)
Health Advisory (All Population)
Health Advisory (Recreational Advisory)
Sample Date
Raw Water
Total Microcystins
0.3 µg/L
1.6 µg/L
4 µg/L
2025
Not Detected
Cylindrospermopsin
0.7 µg/L
3 µg/L
8 µg/L
2025
Not Detected