The Oregon state water quality standard provides that a single sample shall not exceed 406 E.coli colonies/100 mL of water and the geometric mean of 5 samples within 30 days shall not exceed 126 colonies/100 mL. The EPA recommended standard is more protective and provides that a single sample shall not exceed 235 colonies/100mL and the geometric mean of 5 samples within 30 days shall not exceed 126 colonies/100mL. The Swim Guide will utilize the EPA standard for Oregon beaches.

There is currently very limited water quality monitoring of Oregon's inland swim beaches. The federal Beach Act does not cover freshwater or riverine beaches, and the state of Oregon does not monitor swim beaches or recreation sites on the Willamette River .

Willamette Riverkeeper monitors many major swim beaches along the length of the Willamette River system through their Volunteer Water Quality Monitoring Program. Sites are sampled twice monthly June through September. Between October and May, Beaches are sampled once per month.
In Portland, The Portland Boathouse Dock (adjacent to the Eastside Swim Bowl), Riverplace Marina Dock (adjacent to Tom McCall Bowl) and the Sellwood Park Dock are monitored by the City of Portland Bureau of Environmental Services Water Quality Division.

The Swim Guide's safety ratings are based solely on E.coli levels. While E.coli concentration is a useful indicator of fecal contamination, there are many other potential sources of pollution on the Willamette that are not reflected in the Swim Guide. These include other bacteria, heavy metals, pharmaceuticals, high temperatures and pesticides.