5 Things That Could Happen If California's Oroville Dam Breaks

More than 100,000 people were evacuated from their homes, under threat from the damaged Oroville Dam. And while the danger has waned somewhat — officials downgraded mandatory evacuations to evacuation warnings — residents are not out of the the woods yet.

While engineers are working to repair the dam, more rain is expected this weekend.

"Forecast confidence is increasing that this early next week storm could be the warmest, wettest and pack the strongest winds on this series of storms," said the National Weather Service. The area could get 10 inches of rain, and officials worry that the warmer weather could also melt nearby snow.

This additional water has the potential to put more pressure on the dam's spillway repairs.

"The risk level was reduced that I felt people could return to the area, however they need to maintain vigilance," said Butte County Sheriff Kory Honea in a news conference Wednesday.

So what is the worst case scenario?

California's Oroville Dam Latest: Evacuation Orders Lifted for 200,000 Residents

If the spillway breaks, unstoppable flows of water will be released. The dam itself is unlikely to give way, but a spillway breach could lead to cascading flows and damage. Here's what that could mean:

1. The reservoir could be drained

Once the spillway breaches, the high-pressure flow of water could erode the surrounding soil and hillside holding the reservoir back. Under truly disastrous conditions, this could allow the entire reservoir to flood out into the regions downstream.

This would lead to a "30-foot wall of water coming out of the lake," as Cal-Fire Incident Commander Kevin Lawson put it at a Sunday press conference.

2. Highway 70 corridor could be wiped out

Joe Countryman, of the Central Valley Flood Protection Board, told the Sacramento Bee that if this scenario occurred, the towns closest to the dam would be destroyed. The towns of Oroville, Biggs, Marysville, Live Oak and Yuba City would get hit as the water overwhelms the Feather River.

More than 100,000 people live in this region, who could have their homes destroyed. Farmland would be flooded. Anyone remaining in the most intensely affected areas could be killed.

3. Sacramento could even be affected

Sacramento is much farther downstream, but a dam breach has the potential to even have effects there. It won't be subject to the same level of destruction as that faced by the towns closer to the reservoir.

But it could experience powerful and costly floods, wreaking havoc on homes and businesses.

4. Water shortages

Ethan Elkind, a climate attorney at UC Berkeley, notes on his blog that the draining of the reservoir would have serious implications for California's water supply.

"Long term, the dam is the prime source of water (61%) for the California State Water Project, which supplies water to Southern California and the Bay Area, as well as Central Valley farmers," he writes. "Losing that reservoir would likely put those parts of the state in an extreme water shortage situation for the long term."

5. Energy shortages

He also points out that the dam provides a significant amount of the energy for the state. It produces around 2.2 billion kilowatt hours of energy a year, which accounts for around a third of the state's power needs.

Photo credit: California Department of Water Resources