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Water runs down the main spillway of the Oroville Dam. A section of the spillway collapsed about halfway down.
William Croyle/California Department of Water Resources via AP
Engineers monitor the main spillway of the Oroville Dam.
AP Photo/Rich Pedroncelli
The Bourquin family of Live Oak, California, settles in at an evacuation shelter earlier this week.
EPA/Peter DaSilva via Newscom
Fearing a Flood
The threat of flooding from a huge dam forces more than 180,000 Californians from their homes.

By Cody Crane

This past Sunday, California officials ordered more than 180,000 people to evacuate (move from a place of danger to a safer location) the area near the Oroville Dam, which is in the town of Oroville. Months of heavy rain in Northern California had raised the water level of the lake behind the dam. After a dam structure that helps drain extra water was damaged, authorities feared a devastating flood might occur.

On Tuesday, authorities changed the evacuation order into an evacuation warning. That allows residents to return home, but the danger has not entirely passed. Repairs are still being made to the dam, and more storms are expected in the area by the end of this week.

RISING WATERS

The Oroville Dam is the tallest dam in the nation. It holds back Lake Oroville. The water level in the lake has been rising since October, when heavy rains and snow began to hit Northern California. The wet weather there has continued for months. In the past week alone, the Oroville area received 6 to 7 inches of rain.

Dam operators attempted to drain some of the rainwater from the lake through the dam’s main spillway. The main spillway is a large concrete chute that carries away extra water from behind the dam. But a section of the concrete spillway caved in, leaving a gaping hole.

That forced the dam’s operators to use the emergency spillway for the first time since the dam opened in 1968. This second spillway is not as large or strong as the main concrete spillway. The emergency spillway is dug right into the soil. As rushing waters spilled over its edges, they began to erode (wear away) the soil of the hillside.

Officials worried that this could cause the emergency spillway to collapse completely. They believed a full collapse could send a massive wall of water up to 30 feet high rushing down the hill, flooding the valley below. That’s why they ordered more than 180,000 people to evacuate.

FROM DRY TO FLOODED

All this wet weather is a big change for California. A terrible drought (period with very little or no rain) has gripped the state since 2012.

In 2014, California Governor Jerry Brown declared a statewide drought emergency, which is still in place. There was so little rain between 2012 and this fall that water levels in Lake Oroville had dropped to 33 percent of capacity.

Brad McKeehan, a construction inspector in Oroville, told The New York Times that the lake “looked like the moon” because it was so dry. “There was no vegetation. It was just rock. It looked like Mars or something,” he said.

But this winter, Northern California has been hit with record-breaking amounts of snow and rain. According to Doug Carlson, a spokesman for the Department of Water Resources, Northern California has received more than twice the normal rainfall since October 1.

While the heavy rains and snows have brought relief to drought-stricken Northern California, they’ve brought danger too. These winter storms have led to flooding and landslides in the area. During landslides, huge masses of mud, rocks, and debris flow downhill at frightening speeds. These disasters killed at least four people and caused tens of millions of dollars in damage.

SAFE, FOR NOW

There are more storms heading to the Oroville area later this week. While some evacuees are returning home, not all of them are. Donald Azevado and his family are staying in an emergency shelter for now. “My plan is to stay here,” Azevado told CBS News. “I’m not trying to risk traffic, being stuck in floods. I’m safe where I’m at,” he said.

Engineers are working through the night to shore up the emergency spillway. Using helicopters and dump trucks, they are trying to patch the earth around it with rocks to prevent further erosion. In total, repairs to the spillways are estimated to cost $100 million and could take months.