While Department of Water Resources officials had been examining the damage to the emergency spillway since daylight, they have been unable to confirm that the spillway is stable enough to start allowing people to return to their homes.
Drone and helicopter footage over Northern California's imperiled Oroville Dam show the widespread damage and furious flooding that forced more than 200,000 people to evacuate on Sunday.
Operators began using the auxiliary spillway - also known as the emergency spillway - which had not been used since the dam's opening nearly 50 years ago.
Lake Oroville is one of California's largest man-made lakes, and the dam is 335-metres tall.
Butte County Sheriff Kory Honea said engineers with the Department of Water Resources informed him shortly after 6 p.m. that the erosion on the emergency spillway at the Oroville Dam was not advancing as fast as they thought.
Authorities warned a heavily eroded drainage channel at the Oroville Dam, north of San Francisco, was at risk of overflowing after weeks of heavy rain filled the rarely-used structure to capacity.
That means the lake's water levels that were contributing to the erosion of the emergency spillway are starting to come down. Storms are forecast for later this week, and officials said they need to keep draining water from the lake for as long as possible. The structure was expected to give way Sunday evening.
Weeks of heavy rain had left Lake Oroville - the second largest reservoir in California - almost full, leaving rural communities along the Feather river in danger.
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.
At least 130,000 people from nearby towns have been told to evacuate the area in northern California. "There is the prospect that we have to issue another evacuation order if situations change".
Helicopters carry huge bags full of rock to stabilize the emergency spillway next to the Oroville Dam on Monday, Feb. 13, 2017, in Oroville, Ca.
Trying to head off a disaster, state officials increased the flow down the dam's main spillway on Sunday night.
In an effort to mitigate the situation, the California Department of Water Resources is increasing water releases from the main spillway to 100,000 cubic feet per second in an effort to lessen the amount of water traveling down the emergency spillway, the sheriff's office said.
The earthfill dam is just upstream and east of Oroville, a city of more than 16,000 people.
The barrier is 770-feet (235 meters) high, making it 14 meters taller than the more famous Hoover Dam.