Yes, it’s safe to swim and play in the Willamette River most times of the year, according to the state Department of Environmental Quality. Exposure to bacteria is the greatest health concern for people swimming or enjoying other sports on the river, according to DEQ.
The City of Portland’s Bureau of Environmental Services tests for E. coli bacteria weekly at five popular locations all summer long. Check the Rec: the Willamette River Recreation Index to find out latest bacteria counts.
Even though Willamette River bacteria levels are low, it's never safe to swallow water from urban rivers and streams.
Oregon Department of Environmental Quality: Is it Safe to Swim on the Willamette River in Portland?
Oregon Health Authority recreational and fish advisories: This page also has advisories for harmful algal blooms if they occur. Occasionally, harmful algae blooms may form in the river or other water bodies. OHA advises that not all blooms are harmful but regardless of whether an advisory has been issued, if you see water that looks green or has a coating, the advice is “if in doubt, stay out.”