Potentially deadly river flows expected as Portland hits the upper 80s this week

Portland will see a 29-degree swing in temperature this week, causing rapid snowmelt and potentially deadly river conditions.

PORTLAND, Ore. (KOIN) — Portland will see a 29-degree swing in temperature this week, causing rapid snowmelt and potentially deadly river conditions.

KOIN 6 Meteorologist Josh Cozart forecasts a high of 89 degrees on Saturday. While people may be eager to go for a swim during the hot, summer-like weekend, Oregon state climatologist Larry O'Neill told KOIN 6 News that a combination of rain runoff and snowmelt will cause swift, icy and unpredictable currents that can carry heavy debris and quickly put swimmers in danger.

“We expect many boaters, fishers, and swimmers this weekend given the forecast for extremely warm weather,” O’Neill said. “We want to emphasize that we rarely experience these high river levels during water-recreation season and people will likely not be as prepared to identify hazardous situations.”

Portland's National Weather Service office warns that water temperatures in rivers, streams and lakes are still too cold for swimming.

"As temperatures rise this week, outdoor water activities become more enticing," the NWS said. "Despite the warming trend, the water will stay dangerously cold, ranging from 40 to 50 degrees Fahrenheit. This poses a risk of cold-water immersion and potential drowning."

Many Oregon streams are already running at unseasonably strong, high levels. As a result, people are advised to use extra caution on the water during the coming days of sunny weather.

“Hazards can be difficult to identify and can become deadly with little warning,” O’Neill said.

Metro echoed O'Neill's warning, saying the water temperature of the Sandy River at Oxbow Regional Park - a popular location for water recreation in the area - was 44°F Wednesday afternoon.

Those temperatures are low enough to induce cold shock, in which sudden immersion in cold water can cause hyperventilation, loss of motor control and potential drowning. In some cases, it could even lead to heart failure and stroke, Metro said.

“People don’t always realize that even when the air has warmed up, the glacier-fed rivers around here can still be dangerously cold,” said Jon Blasher, Director of Metro Parks and Nature. “That’s why it’s so important to understand the risks of natural water and to take proper safety precautions.”

While water temperatures are low, and water levels are high for May, no local rivers are currently close to flood stage. However, some minor flooding remains possible later this week.

“The threat of flooding from rapidly melting mountain snow is always there with the first rapid warming trend of the season,” Cozart said. “It doesn't always happen, but recently saturated soil from rain in lower elevations can't hold as much water as it otherwise would. So, snowmelt water could give way to minor flooding as temperatures warm close to 90 around Portland by the end of the week.”