The accident happened between Gold Bar and Index around 1 p.m., said officials with the Snohomish County Sheriff's Office.
Sean Horst, who was in one of the rafts, said the water was "like buttercream" at the start.
"It was nice and easy, smooth. Then we got down," Horst said. "And then we got to the bottom wave, and both boats just got pummeled by a ledge wave."
A homeowner along the river saw the rafts overturn, thought the 10 on board were in distress and called 911. Deputies and dive team members raced to the scene after the initial reports indicated eight of the 10 rafters were unaccounted for.
But all 10 had managed to get themselves to safety along the river banks.
"There's a highway that runs along the river so it doesn't take much for somebody to be,' oooh!' " said Tyler Clabaugh, who was on board and has been guiding on the river for over 15 years.
The 10 eventually got back in their rafts and resumed their trip down river to the Big Eddy River Access Park.
"Couldn't have more of a story to take home than this one," Horst said, "because here I am staring into all these cameras and microphones."
Police said all 10 rafters were properly equipped with a life jackets, helmets and wet suits.
The area is popular for whitewater rafting and kayaking. Winter rains and snowmelt have swollen the river, which includes class 4 and 5 whitewater rapids.