Water pours down Oroville Dam spillway as reservoir rises following big storms

It’s a sight that usually means California is having a good winter and water supplies are healthy.

This week, operators at Oroville Dam, the tallest dam in the United States, which holds back California’s second-largest reservoir, opened the spillway gates and began releasing billions of gallons of water down the massive concrete spillway into the Feather River below.

The reason? It’s not to waste water. But to prevent potential floods.

Like many reservoirs in California, the 10-mile long reservoir in Butte County which is a linchpin of the state’s water supply, has been filling fast over the past three weeks due to a series of major storms. During wet winters, dam operators often let water out of reservoirs to preserve  space to capture more water so they can release it in an orderly way. If they let reservoirs fill to the very top too early in the winter and spill on their own, the volumes of water that come down spillways and outlet pipes can cause flooding to homes and businesses downstream.

“It’s a balancing act,” said Jeffrey Mount, a professor emeritus at UC Davis and senior fellow at the Public Policy Institute of California’s water center. “A dam engineer’s life is like a soldier’s life. It’s 99% boredom and 1% terror. But vast majority of winters it’s pretty straightforward stuff.”

In the three weeks between Dec. 16 and Wednesday, the watershed around Oroville Reservoir has received 12 inches of rain. They hills are green. The ground is saturated. The reservoir level has risen 75 feet since then. And the massive lake, which was built by former California Gov. Pat Brown in 1967 as part of the State Water Project, has gone from 51% full to 75% full.

The water from Oroville goes to cities and farms across the state, from the San Jose to San Diego. The reservoir is currently at 136% of its historical average level for this time of year.

How much water has flowed in over the past three weeks?

Oroville has gone from holding from 1,740,668 acre feet on Dec. 16 to 2,578,222 acre feet today.

That’s an increase of 837,554 acre feet. Put another way, Oroville Reservoir now holds 273 billion gallons more water than it did three weeks ago — enough to fill 400,000 Olympic swimming pools, nearly 10 times what Santa Clara County’s largest reservoir, Anderson, near Morgan Hill, holds when full, and enough water for 4.2 million people in a year.

At 6 a.m. on Monday, engineers at the state Department of Water Resources opened the gates on Oroville’s enormous spillway, which is as wide as 15 lanes of freeway.

“To maintain this storage space, DWR must increase releases from Lake Oroville,” the department said in a terse statement, citing regulations from the U.S. Army Corps of Engineers.

Dam operators increased releases nearly tenfold from 1,856 cubic feet per second on Christmas Day to 16,135 on Monday.

But the reservoir is continuing to rise about 3 feet per day because twice as much water is still coming in than is going out.

“When a raindrop hits the ground, it takes it a while to get to the reservoir,” Mount said. “A huge portion of the water goes into the soil, and into the river slowly, before it comes into the reservoir. You always see a peak runoff within a day of the storm. But you still have high flows for days after a storm event.”

After two dry days, DWR reduced flows out of the reservoir Wednesday to 12,842 cfs, although 29,051 cfs continued to flow in. With at least 10 days of sunny weather forecast, releases are likely to be reduced in the days ahead.

Oroville, like Shasta Lake near Redding, the state’s largest reservoir, is a barometer of droughts and deluges.

In good years, they fill up, water is released and the spillways and rivers downstream come alive. Water is plentiful. In droughts, they remain low and only small amounts of water are released to comply with state and federal laws to keep fish and wildlife alive downstream.

Over the past 20 years, Oroville has filled to the top 11 times: In 2003, 2005, 2006, 2011, 2012, 2016, 2017, 2019, 2023, 2024 and 2025. Those were the wet years. In the dry years, it offered a symbol of shortage and aridity, its cracked mud shoreline looking like a giant bathtub ring.

In August and September of 2022, Oroville Reservoir reached its lowest point ever, just 22% full. For the first time since it opened in 1967, its power plant shut down because there wasn’t enough water to spin the turbines and generate electricity. But massive storms in early 2023 ended the drought.

Oroville also made national news in 2017. That winter the concrete spillway famously collapsed in one section during massive storms, prompting the evacuation of 188,000 people as water raged uncontrolled into the river below and concerns grew that part of the dam might fail.

A $1 billion construction project rebuilt it and upgraded the dam a year later.

Although this winter is off to a good start, water experts around the state are hoping for more storms in February and March to continue to boost reservoir levels and the Sierra Nevada snowpack, which provides 30% of the states water.