Why water levels remain low at one major California reservoir, even after rain (2023)

• California reservoir levels: Charts show water supply across the state

Trinity may be filling slower than other reservoirs because the northernmost part of the state has received less rainfall relative to other parts of the state, according to Jeffrey Mount, senior fellow with the Public Policy Institute of California Water Policy Center.

Additionally, Trinity “is heavily dependent on snowpack; versus Shasta, which is mostly dependent on rainfall to fill,” Mary Lee Knecht, Bureau of Reclamation Region 10 public affairs officer, wrote in an email to the Chronicle. Much of the snowfall may not melt and flow into the reservoir until late spring or summer, according to the Trinity River Restoration Program.

According to a closely watched map from the U.S. Drought Monitor, the northernmost parts of the state continue to remain in “moderate drought” in counties such as Siskiyou, Modoc, Lassen, Shasta and Trinity, with interior parts of Northern California also remaining “abnormally dry.”

Gov. Gavin Newsom rolled back some drought restrictions last week but said he didn’t revoke his drought emergency proclamation because of persistent dryness in certain parts of the state.

“It is incumbent upon us to recognize that the conditions have radically changed throughout the state, but not enough in places like Klamath and around the Colorado River Basin to call for the end of the drought in California,” Newsom said at a news conference last month.

The parts of the state that remain dry are also usually arid areas: The northeast corner of California is known to be a rain shadow, whereas the southeast — where drought also persists — is desert, Mount said. The southeast corner of California also gets much of its precipitation from summertime monsoon rains instead of winter storms, Mount said.

Inyo, San Bernardino, Imperial and Riverside counties remain in “moderate drought,” with parts of Inyo and San Bernardino counties in “severe drought,” according to the U.S. Drought Monitor.

Most of the atmospheric rivers that hit California since December have been concentrated in the Bay Area and Central Coast, with some also hitting the Los Angeles region, Mount said, referring to a map of atmospheric rivers from the UC San Diego Institution of Oceanography.

Few, however, have been directed at the Klamath Basin, with many landing just north or south of the region or brushing against it, Mount said. Trinity Lake is part of the Klamath Basin, with Trinity River being the largest tributary of the Klamath River.

“For what you might call the luck of the draw, just enough of those atmospheric rivers shifted a couple hundred miles to the south this year rather than plowing into their normal location, which would be in the Trinity, Klamath watershed and Shasta and upper Sacramento. So we’ve got one of those years where we turned our normal gradient of precipitation — dry in the south, wet in the north — and flipped it so that our far north was not particularly wet,” Mount said.

But even areas of California that on paper are out of drought will still feel the effects of long-term water supply problems. The Drought Monitor can be “notoriously unreliable” for California because it doesn’t take into account groundwater conditions — which have been slow to recover despite winter’s deluges — and the fact that California transports water across the state to meet local needs, according to Mount.

Even if “the drought is largely over, water scarcity is enduring,” said Jay Lund, vice director of the UC Davis Center for Watershed Sciences. That remains true in the San Joaquin and Sacramento valleys, where it may take years for groundwater to recover from the overpumping during drought years.

It also remains true in the Klamath Basin, where contentious debates about water use between agriculture and ecosystem preservation, which the drought exacerbated, won’t be alleviated soon, Lund said.

“Tribes want to see lots of releases of water for salmon. Farmers are really seeing very little water from the projects because … of tremendous changes that are going to be occurring with the removal of some of the hydropower dams. It’s just a lively place for water conflicts,” Lund said.

To the south, in San Bernardino County, Heather Dyer, CEO of the San Bernardino Valley Municipal Water District, is not ready to declare the end of the drought either. The water district’s service area has received 56.9 inches this wet season, above the historical average of 31.1 inches, she said.

Still, when looking at the pattern of rainfall over the past 20 years, this year’s above-average total hasn’t made up for the cumulative loss of precipitation the region has suffered since 1997, Dyer said. The district’s cumulative departure from the mean since then has trended downward, with wet years like this only marginally shifting the marker upward, she said.

“To me, being in a drought is basically the cumulative amount of rain over time, and what that means to our ecosystems and our water systems,” Dyer said. “It’s going to take more than one wet year to get out of that hole.”

In anticipation of future dry years, the San Bernardino Valley Municipal Water District is starting construction on a new stormwater capture project in April, which Dyer estimates will take 18 months to complete.

“I wish we had that in place. I wish that many water agencies had those types of systems in place,” she said. “I feel like this has given me a new sense of urgency that we need to be building infrastructure for the future, and this year is exactly why we need to do that.”

Reach Claire Hao: claire.hao@sfchronicle.com; Twitter: @clairehao_