Reuters cited the Japanese consulate in Portland as confirming Wednesday that the concrete and metal dock — which is reportedly 66 feet long, 19 feet wide and 7 feet tall — was from the tsunami.
Meanwhile, Japan has estimated that as up to 1.5 million tons of debris could still be afloat after the March 11 earthquake and tsunami, Reuters reported, much of it drifting toward North America.
In April, the BBC wrote, US Coast Guard were forced to sink a crewless Japanese ship that drifted to Alaska.
And a Harley-Davidson motorbike washed up inside a container on a beach in Canada, having drifted about 4,000 miles.
Hirofumi Murabayashi, deputy consul general, said the dock — which washed up on Agate Beach, just north of Newport — was one of four washed away in the tsunami, Australia's ABC reported.
"The other three we don't know where they are, if they're floating somewhere or they sank in the ocean or not," he said.
US west coast states were helping coordinated efforts to clean up the debris, according to .
According to the BBC, the dock featured a commemorative plaque indicating that it came from the fishing port of Misawa.
Nearly 16,000 were killed by the quake and tsunami.
Agence France-Presse cited Kirk Tite, visiting Agate Beach with his young son Trevor, as saying: "It's kind of scary seeing this wash up here, because we all surf.
"If this crossed the Pacific Ocean and it's this big, that means that just about anything of our worst nightmares could cross the Pacific Ocean.
"So we're kind of frightened of what's to come."
A spokesman for U.S. Sen. Ron Wyden's office, Tom Towslee, as criticizing the lack of early warning about floating debris.
"I find it extremely odd that something this large could cross the ocean and not be spotted by anybody given the sophisticated equipment and all the attention that is supposed to be focused on this debris," OregonLive.com quoted Towslee as saying.
"This should be a warning to everybody that this stuff is coming a lot faster than we thought it was. There are obviously things in the ocean that are a danger to shipping. This is a vanguard.
"It's starting and at least nine months ahead of schedule as far as I can tell."