PORTLAND, Ore. (KOIN) — The late-spring storm that brought heavy rains to Portland Monday caused the combined sewage to overflow into the Willamette River, the Portland Bureau of Environmental Services announced.

In a release on Monday evening, officials said the Big Pipe system was only 12% full when a single outfall spilled over near Willamette Park shortly before 4:30 p.m. Environmental Services said occurred within a 30-minute period.

Big Pipe tracker from Tuesday, May 9, 2023 (Screenshot/City of Portland)

Although the overflow was concentrated at a single outfall, officials are urging Oregonians to avoid partaking in river activities, including fishing, downstream of the park for at least 48 hours.

A combined sewer overflow is about 80% stormwater and 20% sewage, according to Environmental Services, adding that these overflows are rare but tend to happen when there is heavy rain or snowfall.

Since the “big pipe” system was completed in 2011, the City of Portland says it has reduced sewage overflows in the Willamette by 94%. Despite this, spills can still occur when there are periods of excessive rain and snow.

Monday’s storm also wreaked havoc on a Southwest Portland neighborhood where Portland Fire & Rescue said a culvert, designed to channel water away from homes, ruptured and flooded not only the streets but also filled crawl spaces and basements of several homes.

In the coming days, the Willamette Valley is expected to heat up, with temperatures forecasted to reach the 90s on Mother’s Day.

These record-breaking numbers will pose the biggest threats to those living outside or without air conditioning units.