Portland returned to the Bull Run Reservoir as its primary water source on Wednesday.
That was only one week after the potentially deadly parasite cryptosporidium was last found in a sample of water drawn from it.
Water and public health officials insist the risk of anyone getting sick from drinking Bull Run water is extremely unlikely, although they advise people with compromised immune systems to consult with their doctors.
"Our top priority is to protect public health," Water Bureau Administrator Mike Stuhr said when announcing the decision. "The evidence and data collected, along with input from our partners with the Multnomah County Health Department and regulators at the Oregon Health Authority, indicates the risk remains low."
Crypto, as the parasite is commonly called, has been found 13 times in Bull Run water since the beginning of the year. The most recent finding was in a sample drawn on March 8.
Additional positive findings are likely. But state and county health officials insist the general public is not at risk because the amount found has been so low.
Nevertheless, the Portland Water Bureau took the reservoir offline and switched over to the groundwater wells along the Columbia River owned by the city on Feb. 13.
They provided all of the water to the city's customers until Wednesday.
Customers with questions are encouraged to call the Water Line at 503-823-7525.