Monday morning is being spent assessing the situation and beginning emergency repairs to the auxiliary spillway at Lake Oroville Dam, which Sunday evening appeared in danger of failure in a situation that could send thirty feet of water from Northern California's second largest reservoir deluging towns in Butte, Yuba, and Sutter counties. Contrary to some earlier reports, the problem observed on Sunday around 4 p.m. was "a gash ... found in the hillside directly below the 1,700-foot-wide concrete lip of the emergency spillway," as the Sacramento Bee reports, and "Officials feared all or part of the concrete lip could collapse, leading to an unfathomable, uncontrolled release of water." Since then, a steady release of 100,000 cubic feet per second (cfs) out the also compromised main spillway has brought the level of the lake down several feet from its Sunday afternoon high of 902.6 feet over a foot and a half above the point at which water pours over the lip of the emergency spillway.
In the good news column, state officials monitoring the flow out of the main spillway said Monday morning that despite the increased release of water, no further major damage was apparent on the main spillway, and water releases are expected to continue over the next several days to avert another near catastrophe during the coming week's storms.
In the bad news column, this has felt like a slow-moving disaster with potentially devastating results and daily changes of tune by people in charge, and evacuees have been given no timeline for when they might safely be allowed back to their homes. Also, as of now, the extent or severity of the "crevice" observed in the emergency spillway yesterday remains unclear, as the New York Times notes.
The Sacramento Bee said that work was starting Monday to move oversized sandbags filled with broken up stone that were going to be dropped by helicopter into the damaged emergency spillway, shoring up the hillside there.
Update: Images from the Sacramento Bee in the video below show the new crevice below the concrete portion of the emergency spillway. It's this that needs shoring up because officials fear it threatens the integrity of the 30-foot concrete weir above it.
Meanwhile, Department of Water Resources Acting Director Bill Croyle said in a press briefing that he was hoping to bring the lake level down by 20 to 30 feet in the coming days (the NYT had it at 50 feet) in order to decrease the risk of further use of the emergency spillway due to continued storm runoff into the lake.
Very quick fixes will need to be made to both spillways, however, ahead of what is expected to be a huge year for snowmelt and subsequent runoff from the Sierra Nevada mountains, much of which will pour down into Lake Oroville. As Croyle put it, "Our planning is both short-term and long-term," with a need to continue emptying this reservoir so there is more storage capacity for what's yet to come since it is barely mid-February.
The Lake Oroville Dam is the tallest in the nation at 770 feet, and it remains structurally sound and intact. Issues began with an erosion "pothole" in the concrete-lined main spillway early last week. Caution over how much water to release through it, as that pothole grew into a much huger gash, led to a rise in the lake level that necessitated the spillover on the weir of the auxiliary spillway on Saturday, which had not been used since it was built in 1968. Officials warned that they did not know what subsequent erosion might occur, and per ABC 7, DWR spokesman Kevin Dossey said the emergency spillway was rated to handle 250,000 cfs, "but it began to show weakness Sunday after flows peaked at 12,600 cubic feet per second."
So yes, when it rains after six years of drought, it pours, and this probably is not the end of this crisis. Just as a reminder, you can see footage below of Lake Oroville and its dam at the height of the drought in July 2015, the water level hundreds of feet below what it is now.