The lake once used as a bellwether for California’s devastating drought is now at the center of a major flooding threat.

Nearly 200,000 people have evacuated the area surrounding Northern California’s overflowing Oroville Dam, where officials were working rapidly Monday to remove 50 feet of water and stabilize spillways.

Helicopters arrived to drop sacks of rocks into crevices that formed in the main and emergency spillways of the nation’s tallest dam, though that’s only a temporary solution. It remains to be seen when authorities will be able to properly repair the compromised dam and allow residents to return to the area.

The main spillway ― which operators use to control the flow of water from the manmade Lake Oroville into the Feather River ― began eroding last week following heavy rains that filled the lake to capacity, requiring the use of a second, emergency spillway for the first time since its construction.

By Sunday evening, officials had detected erosion in both spillways as water began pouring over onto the hillside and dragging trees and mud into the Feather River, which runs near downtown Oroville. Officials ordered about 188,000 residents of Yuba, Sutter and Butte counties to evacuate amid concern that the emergency spillway could completely collapse and send a 30-foot-high wall of water toward downstream towns.

Tonight's briefing from state officials on the #OrovilleSpillway is archived on our YouTube channel right here: https://t.co/Mu0oZWfVb8 — Cal OES (@Cal_OES) February 13, 2017

That much water could be disastrous, experts warn.

“You look at 30 feet times the area of the reservoir,” which has a surface of about 25 square miles, said University of California, Berkeley, civil engineering professor Nicholas Sitar. “That is how much water is going to come out. That is a huge volume of water,” he told The Sacramento Bee.

While the emergency spillway has not overflowed and remains functional, Butte County Sheriff Kory Honea stood by his decision to evacuate the area.

“I recognize and absolutely appreciate the frustration people who were evacuated must feel,” Honea said at a press conference Monday afternoon. “It wasn’t a decision I made lightly.”

Most schools were closed Monday morning as residents cleared the area and headed to evacuation centers at churches, fairgrounds, campgrounds and an Air Force base.

California has been battered with three atmospheric river storm systems since the start of the year. With rain expected to return Wednesday and continue for several days, emergency responders are working to drain at least 50 feet of water from the lake to relieve pressure on the spillways.

Josh Edelson/AFP/Getty Images A water utility worker looks on as water is released down a spillway as an emergency measure at the Oroville Dam on Monday.

But with seasonal runoff still flowing in, “it’s like trying to drain the bathtub with the faucet still running,” local PBS affiliate KQED Science Editor Craig Miller said Monday morning on the station’s radio broadcast.

“The good news is they say there’s more water coming out than coming in,” he noted.

A graph on the state’s website shows that water levels have declined more than five feet from Sunday morning to 9 a.m. local time on Monday.

9am- Lake Oroville down ~5.5feet since its peak early Sunday @CA_DWR pic.twitter.com/PIIO2G2SHk — Eileen Javora (@kcrajavora) February 13, 2017

The crisis prompted Gov. Jerry Brown (D) to issue an emergency order diverting resources to Oroville.

“I’ve been in close contact with emergency personnel managing the situation in Oroville throughout the weekend and it’s clear the circumstances are complex and rapidly changing,” he said in a statement on Sunday.

On Friday, Brown asked President Donald Trump to declare a major disaster for the state following “relentless heavy precipitation and high winds that caused flash flooding, debris and mud flows, erosion, power outages, and damage to critical infrastructure.” It would open up assistance from the Federal Emergency Management Agency, but the request had gone unanswered as of Monday afternoon.

Since the Oroville Dam secures water for more than 20 million people and much of the state’s farmland, the emergency has triggered questions about whether the state is equipped to maintain and update its aging infrastructure. An assessment by the American Society of Civil Engineers graded California’s levees and flood control systems an F in 2006 and a D in 2012.