By Eliott C. McLaughlin and Paul Vercammen

CNN

OROVILLE, California (CNN) -- A massive crevasse that formed in a spillway at Northern California's Oroville Dam has spurred mass evacuations, with nearby residents fleeing the worst-case specter of a three-story wall of water rushing downstream.

In all, about 188,000 people, mostly in Butte, Sutter and Yuba counties, evacuated from the area, some being given only minutes to gather their things.

"Everyone was running around; it was pure chaos," Oroville resident Maggie Cabral told CNN affiliate KFSN on Sunday. "All of the streets were immediately packed with cars, people in my neighborhood grabbing what they could and running out the door and leaving. I mean, even here in Chico, there's just traffic everywhere."

The area had long been in drought until this year when heavy rain and snow bombarded the state. In Oroville, the average annual rainfall is about 31 inches, but since October, the Feather River, which begins at Lake Oroville, had already seen 25 inches of rain as of Saturday, according to the California Department of Water Resources.

The lake also gets water from the northern Sierra Nevada mountain range, which is experiencing one of its wettest seasons.

This week's weather report supplied a sliver of good news, as no rain is predicted until Wednesday.

As of early Monday, water was not flowing over the spillway, and while officials were cautiously optimistic, those hopes could "be dashed at first light when engineers get a better look at the backup spillway. We know (the floodwater pressure) is tearing up the bottom of the spillway," DWR spokesman Doug Carlson told CNN.

2 spillways, 2 problems

Oroville Dam is the country's tallest, and the emergency spillway that's eroding had never been used in the 48 years that the dam has existed, the DWR says. The closest Lake Oroville came to topping the emergency spillway was in January 1997, when the lake level rose to within a foot of flowing over it.

The Oroville Dam provides flood control for the region. The dam has two spillways to release water out of the lake to prevent overflow. Both have problems.

In the main spillway, which is lined, or paved, erosion has caused a hole almost the side of a football field and at least 40-feet deep to form in the lower part of the channel. It can't be fixed immediately.

"You don't throw a little bit of rock in it," said DWR acting director Bill Croyle.

The emergency spillway, which is an embankment covered with trees, is a last resort and was used for the first time in history on Saturday when the lake topped 901 feet, its capacity, and a light flow of water washed into the spillway.

Around 3 p.m. (6 p.m. ET) Sunday, authorities learned that the emergency spillway was also eroding, Butte County Sheriff Kony Honea said.

DWR and CAL FIRE crews quickly began clearing the brush, trees and rocks to limit the amount of debris washing into the lake's diversion pool and the Feather River. The California Department of Fish and Wildlife began evacuating young salmon and steelhead from the Feather River Hatchery downstream.

The erosion of the emergency spillway is dangerous because "when you start to erode the ground, the dirt and everything else starts to roll off the hill," said Kevin Lawson, a CAL FIRE incident commander. "It starts to undermine itself. If that is not addressed, if that's not mitigated properly, essentially what we're looking at is approximately a 30-foot wall of water."

Chaotic escape

California Gov. Jerry Brown issued a state emergency order to help local authorities.

On Sunday afternoon, Honea's office issued a dire warning, employing the most urgent of language -- "This is NOT a drill" -- in imploring residents to evacuate. At the time, officials said the spillway was in danger of failing any minute.

The evacuation order included "all Yuba County on the valley floor" and the city of Marysville, authorities said.

About 35,000 people from Butte County, 65,000 from Yuba County, 76,000 from Yuba City and 12,000 from Marysville City evacuated, according to CAL FIRE.

The DWR instructed Oroville residents to head north, toward Chico.

Conversely, the Yuba County Office of Emergency Services warned its residents, "Take only routes to the east, south, or west. DO NOT TRAVEL NORTH TOWARD OROVILLE!!!!!"

The Sacramento Fire Department echoed the urgency and warned residents that the spillway failure could have effects, including flash flooding, 75 miles downstream in Sacramento.

Stores closed. Shelters opened. Police manned roadblocks. Evacuees waited in traffic trying to escape low-lying areas. Residents mobbed gas stations on their way out of town. By early Monday, many of the pumps at those gas stations had yellow tape across them, indicating they were out of gas.

Sean Dennis, who lives 30 miles south of Oroville, spoke to CNN from his car after spending 4½ hours in heavy traffic.

"It was pretty scary, just because of how fast everything was developing," he said. "Me and my wife managed to throw as much stuff as we could into garbage bags, whatever we could find. We got both of our cars loaded down pretty well. We're not taking any chances."

Racing Wednesday's rain

The urgency later subsided, with authorities reporting that the situation was not as desperate as previously thought. Still, Honea said he wasn't taking any chances.

"I'm not going to lift the evacuation order until I have a better idea of what that means and what risk that poses," he said late Sunday night.

The sheriff said the DWR had a contingency plan in place to use helicopters to drop bags of rocks into the gouged portion of the emergency spillway, in an effort to plug the hole. About a half-mile from the dam, crews were seen breaking up and bagging rocks before dawn Monday.

After issuing the evacuation orders, authorities noted significant decreases in the water coming over the emergency spillway.

The flow of water on the emergency spillway eventually stopped Sunday night as the DWR withdrew more water from the other spillway.

The main spillway was releasing water at 100,000 cubic feet per second in an effort to reduce the amount of water traveling down the emergency spillway, authorities said.

Normal flows down the main spillway are about 55,000 cubic feet per second. An Olympic swimming pool typically holds about 88,000 cubic feet of water.

"We want to drop that water level before the next storm hits Wednesday," the DWR's Carlson told CNN. "It's supposed to be a colder storm, which is good. Last week the rain storm was warmer. So we had both the rain runoff and the warmer rain melting the snow and that snow melt also flowed into the lake."

CNN's Eliott C. McLaughlin reported and wrote from Atlanta. CNN's Paul Vercammen reported from Oroville. CNN's Madison Park, Sheena Jones, Chuck Johnston and Azadeh Ansari contributed to this report.