When the tsunami hit the northern coast of Japan last year, the waves ripped four dock floats the size of freight train boxcars from their pilings in the fishing port of Misawa and turned them over to the whims of wind and currents.

One floated up on a nearby island. Two have never been seen again. And one made an incredible journey across 5,000 miles of ocean that ended this week on a popular Oregon beach.

Along for the ride were hundreds of millions of individual organisms, including a tiny species of crab, a species of algae, and a little starfish all native to Japan that have scientists worried if they get a chance to spread out on the West Coast.

“This is a very clear threat,” said John Chapman, a research scientist at Oregon State University’s Hatfield Marine Science Center in Newport, Ore., where the dock float washed up early Tuesday morning. “It’s exactly like saying you threw a bowling ball into a china shop. It’s going to break something. But will it be valuable or cheap glass? It’s incredibly difficult to predict what will happen next.”

Plans were being considered by state authorities to scrape all the living things off the dock and bury them in the sand, so they would not spread, Chapman said.

While scientists expect much of the floating debris to follow the currents to the Great Pacific Garbage Patch, an accumulation of millions of tons of small bits of plastic floating in the northern Pacific, tsunami debris that can catch the wind is making its way to North America. In recent weeks a soccer ball washed up in Alaska, and a Harley Davidson motorcycle in a shipping container in British Columbia.

Just how the dock float — 165 tons of concrete and steel measuring 66 feet long, 19 feet wide and 7 feet high — happened to turn up on Agate Beach a mile north of Newport, Ore., was probably determined within sight of land in Japan, said Jan Hafner, a computer programmer in the University of Hawaii’s International Pacific Research Center, which is tracking the 1.5 million tons of tsunami debris estimated to still be floating across the Pacific.

That’s where the winds, currents and tides are most variable, due to changes in the coastline and the features of the land, even for two objects a few yards apart, he said. Once the dock float got into the ocean, it was pushed steadily by the prevailing westerly winds, and the North Pacific Current.

“If you have leaves falling from a tree … one leaf will be moving in a slightly different direction from another one,” Hafner said. “Over time, the differences get bigger and bigger and bigger.

“Something similar is happening on the ocean.”

After it ran ashore on Tuesday, the Japanese Consulate was able to track down the origin of the dock float from a plaque bolted to it commemorating its installation in June 2008. Deputy Consul Hirofumi Murabayashi said from Portland, Ore., on Wednesday that it was one of four owned by Aomori Prefecture that broke loose from the port of Misawa on the northern tip of the main island during the tsunami.