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 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 River valleys and Sierra Nevada — are on track to have the wettest winter on record. Numerous reservoirs are close to capacity and streams and rivers are primed for flooding.
One area of deep concern is the Don Pedro Reservoir near Turlock. The reservoir exceeded capacity Monday, forcing the use of a spillway for the first time in more than a decade. Officials said the spillway would be used for up to four days and urged farmers and property owners downstream to prepare.
Water flowing down the spillway could cause flooding along the Tuolumne River. Officials said at a news conference Sunday that the flow would begin at 18,000 cubic feet per second and could increase to up to 30,000 cfs over the next few days.
In 1997, the area was the scene of major flooding — the last time the spillway was used. But officials said at the news conference that significantly less water would be released Sunday than in 1997.
"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.
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.
Rains were also pounding coastal areas. Flash flood warnings were issued for parts of Big Sur, parts of Sonoma County and San Benito County 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 Los Angeles Times.
The area will be under a flood watch through today, 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.