Dan Sligh and his wife were in their pickup truck on Interstate 5 heading to a camping trip when a bridge before them disappeared in a "big puff of dust."

"I hit the brakes and we went off," Sligh told reporters from a hospital, adding he "saw the water approaching ... you hold on as tight as you can."

Sligh said his wife was "doing OK" and that he had "lots of cuts."

"You're kind of pinching yourself and realize you're lucky to be alive."

Sligh, his wife and another man in a different vehicle were dumped into the chilly waters of the Skagit River when the span collapsed Thursday evening. They were injured, but miraculously, authorities said it appeared nobody was killed in the bridge failure that raised the question about the safety of aging spans and cut off the main route between Seattle and Vancouver.

"We don't think anyone else went into the water," said Marcus Deyerin, a spokesman for the Northwest Washington Incident Management Team. "At this point we're optimistic."

Sligh and his wife were taken to Skagit Valley Hospital with non-life-threatening injuries. The other man was reported in stable condition at United General Hospital in Sedro-Woolley, hospital CEO Greg Reed said.

People were perched on top of their vehicles waiting for rescue after the bridge over the Skagit River collapsed around 7 p.m. PT Thursday. (King 5 News)

Authorities are trying to determine what caused the bridge to collapse about 100 kilometres north of Seattle in Skagit County.

State Patrol detectives and the patrol's commercial vehicle enforcement bureau troopers spoke to a commercial truck driver whose rig struck the structure.

"We do have the truck driver who remained at the scene. We've had initial conversations with him to get an indication as to what occurred," said State Patrol Chief John Batiste.

Sligh said his shoulder was dislocated in the drop into the water, and he found himself "belly deep in water in the truck." He said he popped his shoulder back in and called out to his wife, who he described as being in shock initially as they waited for rescuers to arrive in boats.

Safety board sending investigative team

Traffic along the heavily travelled route could be affected for some time.

The bridge is about 130 kilometres south of Vancouver — about halfway between the Canadian border and Seattle — and the interstate is a major transportation route and the main link for Canadians driving between Vancouver and Seattle. The collapse came just ahead of abusy Memorial Day weekend in the U.S.

"The I-5 corridor is totally disrupted," said Gov. Jay Inslee, who went to the scene Thursday night.

He said work has already started to design detours, but state Transportation Secretary Lynn Peterson asked people to avoid I-5 in the area for the next several days.

The National Transportation Safety Board was sending an investigative team.

Police said a portion of the four-lane bridge over the Skagit River collapsed about 7 p.m.

Jeremiah Thomas, a volunteer firefighter, said he was driving nearby when he glimpsed something out of the corner of his eye and turned to look.

"The bridge just went down, it crashed through the water," he said. "It was really surreal."

The bridge was about 15 metres above the water. Deyerin said it appeared that two vehicles — a car and a pickup with the travel trailer attached — fell into the river. He said the water depth was about 4.5 metres, and the vehicles half-visible in the water likely were resting on portions of the collapsed bridge.

Crowds of people lined the river to watch the scene unfold.

"It's not something you see every day," said Jimmy O'Connor, the owner of two local pizza restaurants who was driving on another bridge parallel to the one that collapsed. "People were starting to crawl out of their cars."

He said he and his girlfriend were about 365 metres away on the Burlington Bridge when they heard "just a loud bang."

"Then we looked over and saw the bridge was down in the water," he said.

He pulled over and saw three vehicles in the water, including the camping trailer that landed upside-down, he said.

Bridge built in 1955

The bridge was not classified as structurally deficient, but a Federal Highway Administration database listed it as being "functionally obsolete" — a category meaning that the design is outdated, such as having narrow shoulders and low clearance underneath.

The bridge was built in 1955 and has a sufficiency rating of 57.4 out of 100, according to federal records. That is well below the statewide average rating of 80, according to an Associated Press analysis of federal data, but 759 bridges in the state have a lower sufficiency score.

According to a 2012 Skagit County Public Works Department report, 42 of the county's 108 bridges are 50 years or older. The document says eight of the bridges are more than 70 years old and two are over 80.

Washington state was given a C in the American Society of Civil Engineers' 2013 infrastructure report card and a C- when it came to the state's bridges. The group said more than a quarter of Washington's 7,840 bridges are considered structurally deficient or functionally obsolete.

Democratic Rep. Judy Clibborn, who leads the transportation committee in the state house of representatives, said the bridge wasn't one that has been a focus for lawmakers.

"It is shocking that I-5 would have something happen like this," she said.

Clibborn said the collapse will call attention to the issues facing bridges — especially the old bridge over the Columbia River that connects Vancouver and Portland, Ore.