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 Butte County Sheriff's Department said an eroding hole was found in the auxiliary spillway on Sunday afternoon. The department said after working with the California Department of Water Resources (DWR), evacuations were ordered.
"We had to make a critical and difficult decision to initiate evacuation of the Oroville area," the department said.
The DWR increased the water flowing out of the dam after it reached capacity in early February. To try shore up the water and prevent anymore on-site erosions on the emergency spillway, the DWR plans to drop rocks or boulders.
A California water official says 100,000 cubic feet of water per second continued to flow down the Orville Dam's main spillway.
Department of Water Resources Acting Director Bill Croyle said at a news conference late Sunday that the plan is to continue withdrawing that amount of water for as long as possible.
Croyle says a key goal is to reduce the dam's level ahead of upcoming storms forecast to reach the area Wednesday.
Water levels at Lake Orville rose so high that an emergency spillway was used Saturday for the first time in almost 50 years. Officials noticed erosion on the emergency spillway on Sunday.
Croyle says officials have been unable to access the erosion scar but will be able to analyze the damage better now that water is below its level.
According to the National Weather Service, the areas impacted are Oroville, Palermo, Gridley, Thermalito, South Oroville, Oroville Dam, Oroville East and Wyandotte.
Residents evacuated can shelter at the Silver Dollar Fairgrounds in Chico.
The City of Marysville and Yuba City has also been encouraged to evacuate immediately.
A Northern California sheriff says evacuation orders affecting 188,000 people will stand until there is more information on the condition of the nation's tallest dam's emergency spillway.
For evacuation information, residents in Butte County can call 211 from any landline or cell phone. Yuba or Sutter County residents can call 1-866-916-3566.
With the emergency evacuation orders in place due to the spillways, there are local schools that will be closed on Monday.
The list includes: Yuba College's Marysville Campus, Sutter County Campus, and Beale Campus, Butte College.
This is a developing story. Refresh for more details as they come in.