With a sweltering, sunny forecast set for Memorial Day weekend, authorities are warning people about the dangers of rivers and lakes that are still shockingly cold.

If you choose to swim, officials urge you to be aware of water temperatures and hazards, wear lifejackets and refrain from drinking alcohol. You should also keep an eye on children near the water, abide by posted rules, swim with others and be aware of your swimming skill, officials say.

Such warnings are commonplace as temperatures rise and lakes and rivers exert a seemingly gravitational pull on swimmers. Cold water — which can seem warm near the surface — can shock swimmers, cause a lack of muscle function in their limbs and make it exceptionally difficult for them to swim to safety.

The safety reminders come amid the season's first deadly stretch of water incidents.

Authorities suspended their search Tuesday for a missing swimmer who's presumed drowned at Three Pools, a popular swimming hole in Marion County. A man who jumped into the North Umpqua River in Douglas County is also presumed drowned. And a Portland man is presumed drowned after he jumped into the Clackamas River on Tuesday.

Twenty-one people drowned in public, natural waterbodies in Oregon and southwest Washington last summer — down from 36 the summer before. An average of 27 have died over the past five summers.

Clackamas Fire District No. 1 spokesman Steve Hoffeditz said the Clackamas River is cold and flowing fast — difficult conditions for even good swimmers.

Clark County officials reported more of the same, saying southwest Washington rivers are high and fast because of rain and snowmelt. Debris in the water can trap swimmers, the county said in a news release.

The U.S. Forest Service warned people of "extremely cold" water temperatures that can cause swimmers to become hypothermic.

And American Medical Response, which will post river rescue technicians at Troutdale's Glenn Otto Community Park and Gladstone's High Rocks Park beginning Saturday, said people shouldn't expect rivers to be exactly like they were last summer.

"The very wet winter means that we have a lot of snowpack and high water this year," the company said in a news release. "High water levels from snowpack mean fast currents, unexpected drop-offs and cold water temperatures."

— Jim Ryan

jryan@oregonian.com

503-221-8005; @Jimryan015

Tony Hernandez of The Oregonian/OregonLive staff contributed to this report