Oroville Dam floodgates opened as storms fill massive reservoir

In another sign that the drought is ending across much of California, state water officials opened the floodgates at Oroville Dam on Friday to let water out of the state’s second-largest reservoir to reduce the risk of flooding to downstream communities.

“After three years of drought and low lake elevations, it’s really good to see the lake rising,” said Ted Craddock, deputy director of the state Department of Water Resources.

At noon, water began cascading down the huge concrete spillway for the first time in four years.

On Friday, Oroville reservoir was 75% full — or 115% of its historical average for early March. It has risen 180 feet since Dec. 1, and continued to expand steadily with millions of gallons of water pouring in from recent storms.

Devastating storms in February 2017 caused the spillway at Oroville, whose 770-foot tall dam is the tallest in the United States, to crumble, which prompted emergency officials who feared a potential dam collapse to evacuate 188,000 people downstream.

Investigators later found that the spillway, built in 1967, had corroded rebar and a failed drainage system. Construction crews overseen by a national team of independent dam safety engineers rebuilt the spillway in a $1 billion project by 2018. Friday’s event will mark the second time the new spillway has been used since then. It was used once before, in April 2019.

The new spillway is a colossal chute more than 3,000 feet long and as wide as 15 lanes of freeway. Its concrete is 7 feet thick, and contains 13 million pounds of reinforcing steel.

Dam safety engineers were on site Friday for the opening of the gates atop the spillway, and have been performing regular inspections, Craddock said. The new spillway also has instruments to measure pressure, drainage and other factors. Craddock said it performed well in the 2019 release and he expected it to perform well again.

“It’s a very robust structure,” he said.

Lake Oroville, built on the Feather River about 70 miles north of Sacramento by former Gov. Pat Brown in the 1960s, is the linchpin of the State Water Project, a system of dams, canals and pumps that provide water to 27 million Californians from the Bay Area to Los Angeles. The purpose of the dam is not only to store water, but also to provide flood protection to communities in the Sacramento Valley.

In wet years, dam operators draw down the reservoir in winter when its level gets too high. The purpose is to create enough space in the lake so that it can capture huge inflows of water in atmospheric river storms, or in heat waves that might melt the Sierra snowpack. That extra capacity reduces the risk that deluges will hit an already full reservoir, sending water uncontrollably downstream and leading to major flood damage to homes, farms and businesses.

As the weeks pass and spring nears with less and less likelihood of major storms, dam operators allow reservoirs to fill to the top to maximize water storage.

Craddock said Friday that with the huge Sierra Nevada snowpack this winter — the largest in 30 years — the water being let out now will be replaced.

“We expect to see a full lake when we get out of the rainy season,” he said. “A full reservoir will mean we’ll have additional water supply — more than we’ve had in the last few years of the drought.”

Releases from Oroville were at 1,000 cubic feet per second a week ago, through the dam’s Hyatt Power Plant. They have been increased to 3,500 in recent days, then 7,000 Wednesday, and on Friday were planned to hit 15,000 cfs with the opening of the spillway. The spillway is built to handle nearly 20 times as much water, 270,000 cfs.

Some of the water that will flow from Oroville down the Feather River, into the Sacramento River, and ultimately the Sacramento-San Joaquin River Delta, will be captured in other reservoirs. The massive state and federal Delta pumps were running full throttle Friday, moving Delta water south to San Luis Reservoir near Los Banos and other reservoirs.

State and federal dam operators have begun similar releases for flood safety at other large reservoirs, including Folsom, northeast of Sacramento and Millerton, near Fresno.

“In California, we’re used to the swings between dry weather and wet weather,” Craddock said. “What we’ve been seeing in the last couple of decades is more extreme swings between drought conditions and very wet conditions.”

That makes things more challenging for dam operators who are trying to fill reservoirs, but also reduce flood risk.

“It’s new reality that we’re dealing with,” Craddock said.