Damage to the spillway was first noticed on February 7 and set off a series of actions by officials concerned that damage to an emergency spillway could dump large amounts of water into the Feather River, which runs through downtown Oroville. It spread across the concrete-lined channel, and on Saturday the emergency spillway was used for the first time since the dam was completed in 1968.
In the main spillway, which is lined, or paved, erosion has caused a hole nearly the size of a football field and at least 40 feet deep to form in the lower part of the channel.
An immediate evacuation from the lower levels of Oroville and other areas have been issued due to a "hazardous situation" developing with the dam's 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.
Heavy rains and snow that struck California this winter have resulted in the dam's reservoir being filled up to the point of overspill. The dam itself is actually sound, but the facility's two spillways created to release some of that pressure have been damaged - in fact the main spillway has a large hole in it, leaking into the watershed below.
As people begin returning to their homes around Lake Oroville and downstream along the Feather River, forecasters are predicting the next rounds of wet weather, projected to move in Thursday, won't be as strong as the storms that led to problems in Oroville. The goal is to lower lake levels by 50 feet in anticipation of the rain, which could come as soon as Wednesday.
Still, the mandatory evacuation order remained in place Monday for Butte, Sutter and Yuba counties.
Officials are also releasing water from the dam in a bid to reduce the water levels as much as possible ahead of rainfall.
Helicopters cary huge bags full of rock to stabilize the emergency spillway next to the Oroville Dam on Monday Feb. 13, 2017, in Oroville, Ca.
In response to this developing situation, DWR is increasing water releases to 100,000 cubic feet per second.
Reportedly, large population residing in areas being evacuated is that of Indian origin Americans.
After advising that the spillway was predicted to fail imminently, Butte County Sheriff Korey Honea said at an evening press conference that erosion was not progressing as rapidly as had been feared and the amount of water flowing over the spillway had dropped quickly.
Though about 188,000 people were displaced by the evacuation orders, Brown's letter to Trump asks to prioritize those who need help most.
The earthfill dam is just upstream and east of Oroville, a city of more than 16,000 people.
Evacuation orders remain in effect for towns below the spillway, including Oroville, Gridley, Live Oak, Marysville, Yuba City, Olivehearst, Linda, and Wheatland.