A section of the White Bluffs across the river from Hanford's former F Reactor, slumped off the hillside about 10:30 a.m. Wednesday, a Hanford worker reported.
He described the size of the landslide as about equal to the 2008 landslide of the clay bluff south of Ringold, if not larger, said Washington Closure Hanford spokesman Todd Nelson.
Hours later, Hanford workers could still see dust rising from the slide area on the east side of the Columbia River on the Hanford Reach National Monument.
Some of the slides that have collapsed sections of cliffs along the Columbia River have been blamed on seepage from irrigation water.
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This slide was on the national monument where there is no irrigation. However, there is a water wasteway that flows into the Columbia River farther north.