An emergency spillway at the nation’s tallest dam was in danger of collapsing Sunday which led authorities to call for the evacuation of 188,000 people. The 770 foot tall Oroville dam is located 150 miles north of San Francisco. Fox News reports:

The cities of Oroville, Gridley, Live Oak, Marysville, Wheat land, Yuba City, Plumas Lake and Olivehurst were all under evacuation orders. The order was sent out at around 4 p.m. after engineers discovered a hole that was eroding back toward the top of the spillway.

The erosion at the head of the emergency spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville, the California Department of Water Resources said. Those potential flows could overwhelm the Feather River and other downstream waterways, channels and levees…

“We need to do everything we can to maximize our ability to move water our of this reservoir — not just for the coming storm but for the coming storms,” said Bill Croyle, acting director of department of water resources. “Our planning is both short term and long term.”

This graphic explains the location of the damaged main spillway and the emergency spillway in relation to the main dam:

Incredibly helpful graphic for those of us trying to understand what's happening. #OrovilleDam https://t.co/IppjFhvh1l pic.twitter.com/b90TClucgS

— Tim Grieve (@timgrieve) February 13, 2017

Damage to the main spillway was noticed last week at which point the amount of water flowing out the main spillway was cut to prevent the erosion damage from becoming worse. However, with water from storms still flowing into the lake and less water flowing out because of the damaged spillway, the lake began to fill up. Water began to overtop an emergency spillway and engineers noticed this flow of water was cutting into the land in a way that, if left unchecked, could lead to a catastrophic failure of the emergency spillway. The Sacramento Bee describes the worst case scenario:

At 5 p.m. Sunday, Mike Smith, a spokesman for the California Department of Forestry and Fire Protection, said “as much as 30 vertical feet of the top of the spillway could fail.”

A failure of that magnitude likely would cause billions of gallons of water to cascade down a wooded hillside that sits below the lip of the emergency spillway.

“You look at 30 feet times the area of the reservoir,” said Nicholas Sitar, a civil engineering professor at UC Berkeley. “That is how much water is going to come out. That is a huge volume of water.”

That’s why the evacuation order was issued and the amount of water flowing down the main (already damaged) spillway was doubled in an effort to reduce the amount of water flowing over the emergency spillway. The goal now is to keep water flowing down the main spillway until the level of the lake drops by at least 50 feet. That’s necessary because another big storm is expected to hit the area by Wednesday of this week. If the level of the lake isn’t lowered ahead of that storm, the lake could begin to overflow again, putting the emergency spillway at risk.

Here’s a Fox News report which shows some of the erosion caused by the flow of water down the main spillway:

https://www.youtube.com/watch?v=23AO3QMrJDg