ARLINGTON — They’re hard to see and potentially deadly.

Two treacherous logjams in the South Fork Stillaguamish River between Arlington and Granite Falls have officials worried.

Snohomish County Search and Rescue volunteers have moved a hovercraft near the river to be prepared in case rescues are necessary this weekend, Snohomish County sheriff’s spokeswoman Rebecca Hover said.

Warm summer weather and popular outdoor festivals and other activities may mean people will be tempted to cool off by floating in the river, she said. With the jams spanning the current, people need to be extremely cautious.

“People on any river should be very careful,” Hover said.

Logjams, called strainers by river experts, are created when downed trees, stumps and logs get stuck in the river. Strong currents rush under the trees and people easily can become stuck.

“The danger is that you could get trapped under water,” said Jerry Michalec, owner of North Cascades River Expeditions in Arlington.

Even highly trained river guides have been known to perish in logjams, he said.

“If you’re underwater but you’re stopped, and there’s good water flowing past you, it just fills up your lungs,” Michalec said.

People have been known to drown in less than half a minute, he said.

The two logjams are near a popular spot where many people put in to float the river and the county’s River Meadows Park, Hover said.

“These are the strainers we know about,” she said. “There are probably many more we don’t know about.”

The stretch of the South Fork Stilly along Jordan Road isn’t known for whitewater expeditions but is popular with people floating on inner tubes and rubber rafts, Michalec said.

Too often, many people float the river without proper safety precautions including helmets and life vests, he said.

“They’re the people who are most likely to get in trouble,” Michalec said.

Logjams, like other natural obstacles, should be scouted before the river is run.

“You can’t just go flying into it,” he said.

If it appears that the current will push a raft into the jam, the safest approach is to get out of the raft and walk around.

Even people familiar with the river should be careful, Hover said. The rivers change often overnight as debris is carried with the currents.

“We live in a beautiful part of the world and it’s only natural for people to want to be outdoors,” Hover said. “We’re just asking them to take safety precautions.”