PORTLAND -- The Multnomah County River Patrol is beefing up its presence on area rivers in preparation for big crowds this week.

They're warning that while it may finally feel like summer, the rivers are running cold, high and fast - more like spring.

The Waterfront Blues Festival is one of the busiest times for the Willamette River with scores of boats making their way into downtown Portland. Authorities are strictly enforcing a no-wake zone.

Rescue crews on Wednesday were also practicing drills at the High Rocks swimming hole on the Clackamas River, one of Oregon s most dangerous swimming areas.

It takes your breath away for a few seconds, swimmer James Matthews commented after taking a dip.

Crews have already rescued four people from the fast-moving water at High Rocks.

The water temperature is so different than the outdoor air temperature that people could experience cold shock as soon as they get in the water and it can incapacitate the swimmer, said Anthony Angelo with AMRRiver Rescue.

It feels like it's pulling you, like pushing you downstream, Matthews added. Like when it jumped from up there and swam this way it pretty much carried me.

That's why rescuers said the best advice is to always wear a life-jacket if you swim in area rivers this time of year. And never go swimming by yourself.

Some popular outdoor swimming holes also have lifeguards, including High Rocks and Glenn Otto Park.

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