After a bone-dry January, California experienced four atmospheric rivers in the first two weeks of February. This was a welcome development—January is typically the state’s wettest month and crucial for water supply, and the lack of rain and snow was deeply concerning to many across the state.

California relies heavily on atmospheric rivers to build snowpack, fill reservoirs, and recharge groundwater. This water feeds into one of the world’s most elaborate conveyance systems, which moves water hundreds of miles to cities and farms throughout the Central Valley, the Bay Area, and Southern California.

But even during this recent period of abundance, there are constraints on how much can be stored or moved around. Addressing these bottlenecks in the system to take better advantage of wet periods will be essential to cope with a changing climate. But it will not be easy, and all approaches are likely to involve costly and controversial trade-offs.

The February storms illustrate some of the challenges the state faces.

Most of early February’s precipitation fell north of Sacramento, with runoff flowing into reservoirs that were at or above their historic averages thanks to two preceding wet years.

Large reservoirs in the Central Valley are operated to meet water supply and flood control objectives. Rather than storing all the water they can, during the winter reservoir operators are required to maintain enough space in their reservoirs to capture high inflows and reduce the risk of flooding downstream.

When the February storms arrived, the surge of water into the state’s two largest reservoirs—Shasta and Oroville—quickly filled the flood reserve space. Because the winter flood season is far from over, dam operators had no choice but to let the water go to make space for possible future floods.

And they let go a lot of water. Between February 1 and 18, those two reservoirs alone released more than 2 million acre-feet of water into the Sacramento and Feather Rivers to maintain space for future stormwater. That is a year’s supply for six million homes or 700,000 acres of farmland.

These reservoir releases, along with runoff throughout the Sacramento and San Joaquin watersheds, made their way into the Sacramento–San Joaquin Delta, where there is another bottleneck.

In general, water that flows into the Delta goes to three places: farms in the Delta use it; large pumping plants run by the federal Central Valley Project (CVP) and State Water Project (SWP) send it south; or it flows into San Francisco Bay.

The large CVP and SWP pumping plants and their canals have a finite capacity for pumping water from the Delta (roughly 28,000 acre-feet per day). But they rarely pump at full capacity. Maintenance, declining canal capacity, and regulations that maintain water levels near the pumps and protect endangered fish all limit the amount and timing of pumping. And a large amount of water must flow into San Francisco Bay to keep the Delta water fresh enough for use by Delta farms and exporters (see our 2022 policy brief to learn more).

During wet periods like those in early February, salinity and habitat issues are not a major concern because so much freshwater is flowing through the Delta. However, restrictions on pumping rates to protect fish can impact the ability of the projects to export water. Because the projects are located in the southern part of the Delta, the inflow from the San Joaquin River typically dictates how much water can be pumped at this time of the year. Since the storms went mostly to the north, leading to high flows on the Sacramento River, inflows from the San Joaquin were very low, hampering the capacity of the projects to export water.

To illustrate, during February 1–18, more than 5.4 million acre-feet of water flowed into the Delta. Delta farms—which are mostly idle at this time of year–used little of this water. The federal and state projects were able to export 234,000 acre-feet, or roughly 4% of inflow to the Delta.

Regulations to protect fish reduced pumping by roughly 160,000 acre-feet. That unpumped water remained in the Delta and flowed out with the more than 5.1 million acre-feet of uncapturable outflow into San Francisco Bay.

Finally, once water is exported from the Delta, there is a third bottleneck in the system.  As our 2022 report described, in wet years the state and federal projects run out of places to store water south of the Delta. When this happens, the projects must reduce their exports from the Delta. Most of the water pumped from the Delta this time of year goes into San Luis Reservoir, an off-channel reservoir south of the Delta that is shared by the projects. The reservoir will likely fill this winter, hampering efforts to store water.

What does this mean? This very wet early February did not result in large quantities of new stored water. Reservoir operations for flood control, the limited capacity of the federal and state projects, and regulations to protect fish limited storage to about 4% of the runoff. Later this winter, south-of-Delta storage may also become a limiting factor.

Our 2023 policy priorities report highlighted the need to do a much better job of managing wet years to adapt to increasing drought intensity. To their credit, the state and its regional partners are working hard on this, increasing groundwater recharge programs, using forecasts to better operate reservoirs, and planning infrastructure investments to improve storage (we estimate that two of these infrastructure projects—Sites Reservoir and the Delta Conveyance Project—would have more than doubled the amount of water stored, but that is still a small percentage of the 5.1 million acre-feet of uncaptured water). But the early February storms remind all of us that there is a lot of hard, expensive, and sometimes controversial work ahead if the state is going to successfully adapt to changing conditions.

Thanks to Kyle Greenspan for research assistance with data wrangling and figure production.