Water levels fall at threatened California dam as 200K evacuated Lake Oroville is one of California's largest manmade lakes.

 -- Water levels are continuing to fall at the Oroville Dam, where officials ordered nearly 200,000 people to evacuate to safer ground over fears that the country's tallest dam would release uncontrolled floodwaters downstream. Spillage from the dam has stopped as a result of the lower water levels, The Associated Press said.

The objective is to lower the lake level by 50 feet in order to prepare for the future inflow of water, Butte County Sheriff Kory Honea said at a news conference this afternoon.

Drivers jammed highways overnight and this morning as thousands of residents scrambled to flee the area. Hundreds of cars lined Highway 99, creating bumper-to-bumper traffic as people headed north, the AP reported.

Evacuations remain in effect this afternoon, Honea said, calling it a "dynamic situation." No date or time has been set to lift evacuation orders, he added. All 5,780 inmates from the Butte County Jail were evacuated to the Santa Rita Jail in Alameda County, where they will remain until it is safe to return, the Butte County Sheriff's Office said.

Raj Gill, who manages a Shell gas station in the region, was torn between evacuating and staying open to serve a steady stream of customers.

"You can't even move," he told the AP. "I'm trying to get out of here too. I'm worried about the flooding. I've seen the pictures. That's a lot of water."

Lake Oroville, which is 75 miles north of Sacramento and about 25 miles southeast of Chico, is one of California's largest manmade lakes, and it swelled after a month of heavy rains that battered the region.

Water began topping the emergency spillway on the 770-foot-tall dam Saturday, causing erosion damage that could lead to a surge of water being released, officials said.

Officials from California's Department of Water Resources said they planned to use helicopters to drop rocks to fill in the gouge in the spillway.

Residents of Oroville, Gridley, Live Oak, Marysville, Wheatland, Yuba City, Plumas Lake and Olivehurst were ordered to evacuate immediately, authorities said.

Butte County Sheriff Kory Honea said officials made the decision to nearly double the volume of water being released from the dam to stop erosion at the top of the auxiliary spillway.

"Hopefully, that will release pressure on the emergency spillway and they'll find a repair to prevent a complete failure," Honea said Sunday. "[The] situation is dynamic and could change anytime."

California Gov. Jerry Brown issued an emergency order in response to the situation.

"The state is directing all necessary personnel and resources to deal with this very serious situation," he said in a statement Sunday.

The state's National Guard said it would provide eight helicopters to assist in reconstructing the emergency spillway.

The helicopters, along with two airplanes, will be available today for search and rescue near the Oroville Dam, California National Guard Adjunct Gen. David Baldwin said at a news conference Sunday.

He added that the California National Guard would be ready to deploy if needed.

The Mercury News, a local newspaper, reported that three environmental groups — the Friends of the River, the Sierra Club and the South Yuba Citizens League — filed a motion with the federal government in 2005 as part of the Oroville Dam's relicensing process, urging federal officials to bolster the dam's emergency spillway.

In that motion, the groups warned that the dam did not meet modern safety standards, The Mercury News reported.

The Federal Energy Regulatory Commission rejected their request after state agencies determined the refurbishment an unnecessary expense, the paper said.

ABC News reached out to the FERC for a comment about the 2005 motion but did not immediately receive a response.