Oroville Dam Spillway: What you need to know this Monday

Lake Oroville is one of California's largest man-made lakes, and the 230m high Oroville Dam is the nation's tallest.

Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville.

"It's the first time it's ever taken water", he said of the emergency spillway.

However, officials were apparently warned several times that the Oroville Dam emergency spillway wasn't safe, yet they didn't listen.

Those flows could overwhelm the Feather River and other downstream waterways and levees and flood towns in three counties.

The Oroville reservoir spill forced almost 200,000 people to evacuate and Gov. Rather than risk thousands of lives, the decision was made to order evacuations.

He said the big worry is not that the dam itself would fail, but that a failure of the spillway could cut into the hillside and potentially release more and more water, leading to a "cascading failure". But the damage costs appear to be mounting with the additional erosion damage to the emergency spillway.

Crews used helicopters to drop bags of rocks into the gouged portion of the emergency spillway, in an effort to plug the hole.

Repairing the main spillway could cost between $100 million and $200 million, William Croyle, the Department of Water Resource's acting director said Saturday. Late Sunday, officials noted that water levels had lowered enough that water was no longer spilling over the eroded area. Normally, the overflow would drain into the primary spillway to safely disperse it, but with the erosion, the overflow was directed to the emergency spillway.

After years of drought conditions, the Sierra have seen above-normal precipitation in the form of rain and snow, and reservoirs throughout the state were above normal levels.

As people begin returning to their homes around Lake Oroville and downstream along the Feather River, forecasters are predicting the next rounds of wet weather, projected to move in Thursday, won't be as strong as the storms that led to problems in Oroville. They also hope to drain almost 400 billion gallons of water from the lake to lower water levels 50 feet, a tall order. A Red Cross spokeswoman said more than 500 people showed up at an evacuation center in Chico, California.

Residents of Oroville, a town of 16,000 people 65 miles (105km) north of Sacramento, were told to head north.

Local authorities have blamed the hole on unexpected erosion.

In a July 2006 memo to managers, first reported by The Mercury News, a senior civil engineer with the agency's San Francisco office wrote that the agency determined dam safety "would not be compromised in the rare event of an emergency spillway discharge".

  • Arturo Norris