The patrol says William Scott, a 41-year-old resident of Spruce Grove, was hauling a large load Thursday night and hit a girder of the bridge over the Skagit River on Interstate 5, about 100 kilometres north of Seattle.
Sgt. Kirk Rudeen says Scott was heading south with a large steel casing for a drilling rig when the right front corner of the load struck a support girder.
Rudeen says the collision appears to have led to the collapse of the span just seconds later and two vehicles plunged into the cold swift-flowing river.
Three people were rescued and were treated and released from hospital.
Rudeen says Scott made it off the bridge and remained at the scene to speak to investigators.