What is the Oroville Dam, and what will happen if the spillway fails?

At least 188,000 people living downstream of California's Oroville Dam were ordered to evacuate late Sunday over concern that a damaged emergency spillway could cause severe flooding.

Here's everything you need to know about the dam and why it will be devastating to the state if the spillway fails. 

What is the Oroville Dam? 

The Oroville Dam is the tallest dam in the USA and is a critical piece of the state’s water system.

Oroville Dam is in the foothills of the Sierra Nevada east of the Sacramento Valley about 150 miles northeast of San Francisco. The dam, which was completed in 1968, has risen rapidly this winter as the Feather River and its tributaries have gushed down swollen after storms that brought heavy rain and snow. The storms, which came after more than five years of severe drought, have led officials to release water from various dams across Northern California.

How did the damage happen? 

A gaping hole appeared in the dam’s main spillway last week, and that prompted state officials to begin releases over the emergency spillway for the first time in the reservoir’s history.

Though the dam itself remains intact, erosion damage to the emergency spillway raises the potential of that spillway failing and unleashing an uncontrolled torrent of floodwaters.

Trying to head off a disaster, state officials increased the flow down the dam’s main spillway Sunday night. Their efforts to deal with the crisis could determine not only whether entire towns are inundated but also whether the state’s second-largest reservoir emerges with manageable damage or something much worse.

Why does the dam matter to the state?

Lake Oroville is the largest reservoir in the State Water Project, a network of canals and pumping stations that move water from Northern California to the Central Valley and Southern California. It’s one of the key reservoirs in the system that stores water for the dry spring and summer months.

“It would be a massive blow to the state’s water system if they lose Oroville,” said Peter Gleick, a water researcher at the Oakland-based Pacific Institute. “The question is, will it erode away the emergency spillway? Will there be a big uncontrolled release of water? Or will they be able to draw the lake down enough to prevent that?”

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 release more and more water, leading to a “cascading failure.”

Gleick said that if a major flood were to occur, dealing with a water supply problem would become “a secondary issue.”

How much will damage cost? 

Repairing the main spillway could cost $100 million to $200 million, William Croyle, the Department of Water Resource's acting director, said Saturday.  The damage costs mount with the additional erosion damage to the emergency spillway.

How long will it take to fix the dam? 

Even a small failure of the emergency spillway will mean some expensive repairs, which may take months, according to Roger Bales, an engineering professor and director of the Sierra Nevada Research Institute at the University of California-Merced.

“If repairs take longer than just this summer, which seems likely,” Bales said, “the reservoir may need to be operated at a lower capacity until those repairs are done.”