OROVILLE, Calif. — Large swaths of Northern California were on high alert for flooding Monday as a powerful new storm dumped large amounts of rain on an already saturated region where levees, dams and other waterways are already under major stress.
Officials said residents should be prepared to evacuate at a moment’s notice and should be packed and ready to head to higher ground.
Many parts of the region — especially in the San Joaquin and Sacramento valleys and Sierra Nevada — are on track to have the wettest winter on record. Numerous dams are close to capacity and streams and rivers are primed for flooding.
“We are strongly advising all residents in interior Northern California to be prepared for flooding,” the National Weather Service said in an advisory late Sunday. “We may see flooding in locations which haven’t been impacted in many years.”
Officials urged residents to put together a “go bag” containing important items such as medications and hard-to-replace documents, as well as to plan for the needs of pets and other animals.
The heaviest rain is expected to hit on Monday and Tuesday. Parts of Northern California are already on track to have the wettest winter ever recorded.
On Sunday, the weather service warned that the San Joaquin River at Vernalis “has reached danger stage. Greater risk for levee problems.” Sandbagging operations were underway there as the storm approached.
Officials also said several other waterways were at major risk of flooding, including the Yolo Bypass, Clear Lake, and the Sacramento, Cosumnes, Mokelumne, Merced and Tuolumne rivers.
Another area of deep concern is the Don Pedro Reservoir near Turlock. Officials said it could exceed capacity this afternoon, forcing the use of a spillway. Officials said the spillway would not be used until 3 p.m. at the earliest and urged farmers and property owners downstream to prepare.
Rains were also pounding coastal areas. Flash flood warnings were issued for parts of Big Sur and communities in the hills above Santa Cruz. Flight delays were hitting San Francisco International Airport.
On Saturday, water stood a foot high in Maxwell, a small rural town in Northern California’s Colusa County. Crews had to evacuate 100 people, some by boat, about 2 a.m. because of flooding,
Maxwell is about 50 miles from Oroville, which for the last week has been the scene of a national drama as both spillways at the Oroville Dam were damaged, sparking fears of a catastrophic flood and forcing the evacuation of more than 100,000 people.
Over the weekend, agency engineers incrementally decreased the flow of water in the spillway to about 55,000 cubic feet per second in order to give construction crews room to begin removing an estimated 150,000 square yards of debris that has accumulated in a pool at the bottom, forcing the closure of a nearby underground hydroelectric plant.
It is all part of the effort to pump enough water out of the lake to absorb runoff from incoming storms and to keep the lake from overflowing.
As of Sunday, the estimated costs of shoring up the dam’s main spillway and adjacent emergency spillway had climbed to more than $10 million, according to a report reviewed by The Times.
The area will be under a flood watch Monday and Tuesday, when the new storm is forecast to dump as much as 10 inches of rain on the Feather River Basin. The National Weather Service also warned that the storm is expected to be packing strong winds that could hurl waves in the reservoir toward the 770-foot-tall dam.
Officials were able to reduce the water levels in the reservoir, and say they are prepared for the new storms.
With reduced flows still surging down Oroville Dam’s main spillway on Sunday morning, the extent of the damage it has sustained was clearly visible.
The long, concrete chute was buckled and riven with deep cracks above and below an immense jagged hole engineers discovered earlier this month. Nearly all the water being pumped out of the reservoir at 70,000 cubic feet per second was slamming into the hole, then cascading out into a fissure carved into an earthen slope next to the nation’s tallest dam.
California Department of Water Resources officials had been pumping water at 100,000 cubic feet per second for several days to absorb storm runoff and prevent the reservoir from overflowing, as it did a week ago. The tremendous force of that torrent and the debris it carried contributed to the growth of the hole.
