A valve deep in the Third Powerhouse at Grand Coulee Dam was opened last week, partially flooding an elevator shaft and an elevator control room, authorities said.

Police Chief John Tufts, of the Grand Coulee Police Department, was called to the Third Powerhouse at about 4 p.m. March 19, his written report states.

Tufts was told that sometime between noon and 3 p.m. someone had opened a water...