OROVILLE, Calif. (AP) — An Associated Press examination has revealed federal and state managers made a series of questionable decisions before and during a recent crisis at America's tallest dam.
For nearly a week in February, Oroville Dam managers assured the public there was no immediate danger after a massive crack opened in the main concrete chute that carries water from the dam.
Then, a backup spillway began breaking apart, and nearly 200,000 people were suddenly ordered to evacuate. A timeline of events leading to that decision:
Raging floodwaters from one of the wettest winters on record rip a huge hole in the dam's main concrete spillway. The California Department of Water Resources activates an emergency plan . Managers at the dam 150 miles (241 kilometers) northeast of San Francisco want to hold back water to assess the damage, and the state receives an uncommon exemption from the Army Corps of Engineers to bypass a rule that required it to release huge amounts of water from the rapidly filling lake behind the dam.
Operators vary water flows down the damaged chute to gauge its stability, at times stopping it completely . It's a tricky calculation, as the lake rises toward the highest level on record. The flows are increased at times to minimize rising lake levels, according to agency emails.
Water managers announce an emergency spillway might be used within days — for the first time in the dam's 50-year history — as lake waters continue to rise. "There is no imminent threat to the public ," they say. Alternating increases and decreases in water flows continue. If water begins flowing over the emergency spillway — essentially an unpaved hillside — operators cannot control it, and it will wash large amounts of soil and debris into the river below the dam.
In an email, the state tells local sheriffs that engineers from several organizations are working to determine the maximum amount of water that can be safely released down the eroding spillway. Operators announce it is less likely the emergency spillway will be needed, after they boost water releases from the lake. However, they later cut releases to prevent erosion near power-line towers. In an email to Butte County Sheriff Kory Honea, the state water agency says hydrology reports show the flow of water, even reduced, will keep the lake from spilling over the emergency spillway. In a public statement, dam managers say there is "no imminent threat to the public or the dam."
The state announces water begins spilling into the emergency spillway at 8 a.m., after the lake reaches its highest level on record. State officials later say the second spillway is "operating as intended." The state water agency's acting director, William Croyle, says in a statement that the flow of water down the emergency spillway is well below what it was designed to handle, and "based on our current situation, there is no threat." In an email to the state that evening, the city administrator of Biggs, about 25 miles downstream from the dam, complains that he has received scant information about the dam. The state agency "has earned a grade of 'F' on its ability to timely and completely communicate during this incident," Administrator Mark Sorensen writes.
An 8 a.m. update from the state gives no sign of trouble. It says flow over the emergency spillway is decreasing, and erosion on the unpaved hillside has been "minimal." Later that afternoon, Honea, the sheriff, learns the emergency spillway is breaking apart. He orders the immediate evacuation of nearly 200,000 people. The state estimates the second spillway could fail within 60 minutes .