Debris from Japan's 2011 tsunami will continue to litter the North American coastline over the next three years, with everything from refrigerators to lumber and sports balls still floating offshore in the Pacific, an expert said on Tuesday.

About 1 million tons of debris was still lingering in the Pacific Ocean four years after a magnitude-9.0 earthquake, the most powerful ever recorded in Japan, set off a series of massive tsunami that devastated a wide swath of Honshu's Pacific coastline and killed nearly 20,000 people.

It also damaged the Fukushima No. 1 nuclear plant, leading to a series of explosions and meltdowns in the world's worst nuclear disaster since Chernobyl 25 years earlier.