As drought recedes across California, flood risk increases
- US News
- March 17, 2023
- No Comment
Though California may end its winter with quenched reservoirs and near record-breaking snow cover, weather forecasters warn the state will face an increased risk of flooding in the coming months as snowmelt from the Sierra Nevada fills rivers and streams.
On Thursday, the National Oceanic and Atmospheric Administration’s flood forecast reported that drought conditions will continue to improve across much of the state, but the potential for flooding will worsen amid heavy snowpack and increased soil moisture.
“Approximately 44% of the United States is at risk of flooding this spring,” said Ed Clark, director of NOAA’s National Water Center. “California’s historic snowpack coupled with spring rains increases the potential for spring flooding.”
However, the severity of these floods remains to be seen and will depend on a variety of weather factors, experts say.
“It’s going to happen, and the question is whether it’s going to happen quickly or slowly,” said UCLA climate scientist Daniel Swain. “Is it a gradual snow melt – in which case the flood problems would tend to be less – or is it faster… in which case we could talk about something in the big flood area.”
Possible triggers for rapid snowmelt could be an early-season heatwave or another series of warm storms, Swain said, both of which could result in “significant snowmelt flooding, particularly in the San Joaquin Valley and some of the Sierra’s eastern slope runoff watersheds.” to the Great Basin and Nevada.”
The NOAA report forecast moderate flooding in the Central Valley and Sierra, and minor flooding throughout most of Northern California and along the coast.
For a state that has endured three years of drought and has only been battered by 11 atmospheric rivers since the beginning of the year, the prospect of clear-sky flooding is a mixed blessing, as some of that water will inevitably seep into severely depleted aquifers.
But for communities that have suffered deaths and displacement as a result of flooding, the prognosis is alarming. Those communities include Pajaro in Monterey County, which was flooded after a levee breached late Friday night.
The floods displaced hundreds of people in the mostly migrant city with no clear timetable for their return. It has also raised significant concerns about crop yields in the heavily agricultural region this year.
A similar series of storms in January breached levees along the Cosumnes River near Sacramento, flooding low-lying fields and roads and contributing to the deaths of at least three people.
State and federal officials, who just a few months ago were grappling with dwindling reservoirs, are now strategically clearing dams to make room for incoming rivers.
Water managers “ensure that reservoir releases are coordinated as much as possible — given the amount of snow cover — to minimize the downside impact as much as possible,” says Jeremy Arrich, manager of the Division of Flood Management at the California Department of Water Resources, said during a briefing on Wednesday.
“However, there is a lot of snow lying over these watersheds, and we expect a significant amount of rivers to come through during the snowmelt season.”
Recent storms are already causing minor flooding along the Sacramento, Salinas, Merced and San Joaquin Rivers, and “many rivers and streams along the foothills of the Sierra Nevada, particularly in the San Joaquin Valley, will continue to flow high over the next week.” a combination of heavy rainfall and snowmelt,” the NOAA Outlook said.
The latest round of storms saw a near-record-breaking crest on the San Joaquin River near Patterson, with water rising to less than a foot of its high-water mark as of February 2017, according to the US Drought Monitor.
The Pajaro River at Chittenden reached its highest crest since February 1998, while the nearby Salinas River at Spreckels rose 3.89 feet above flood level, the second highest after the 1995 record. The Nacimiento River surpassed its 1969 flood mark by 1.51 Foot .
Such waves only increase the potential for flooding as melting snow arrives on rivers, streams, and tributaries that could be high.
Snowfall at the UC Berkeley Central Sierra Snow Lab at Donner Pass has topped 650 inches this season, compared to a normal full season of about 360 inches.
“If we have average or below-average temperatures and no appreciable warm atmospheric fluxes with rain, there will still be a lot of snowmelt, but it will be more gradual,” said Andrew Schwartz, principal scientist at the snow laboratory. “If we see long periods of unusually high temperatures, flooding from this large snowpack can become a significant risk.”
Although the storms and associated flooding have proven dangerous and deadly, they have also been a boon to water supplies and other conditions in the state. The latest Drought Monitor update shows that nearly 64% of California is no longer in a drought category, including 45% with no drought at all and 19% as abnormally dry. That’s a notable reversal from just three months ago, when almost the entire state was mired in record-breaking drought.
The drought-free areas include all of the Northwest Coast, much of central California and the Sierra, and portions of southern California, including swaths of Kern and Los Angeles counties.
In the west, the drought trend is improving.
(Paul Duginski/Los Angeles Times)
Additionally, no part of California remains in the two worst categories — extreme or exceptional drought — compared to 36% just three months ago.
The Department of Water Resources increased its allocation of supplies to state agencies to 35% last month, and officials said this week that number could rise.
The bounty has also allowed the state to divert more water to aquifers, including diverting more than 600,000 acre-feet from the San Joaquin River to areas where it can spread and seep into the aquifer below the San Joaquin Valley.
On Wednesday, Southern California’s Metropolitan Water District — the wholesaler that supplies 26 agencies and 19 million people in the area — rescinded its unprecedented water conservation mandate. That mandate, which went into effect last June, saw nearly 7 million people under severe restrictions on water use, including one- and two-day outdoor watering restrictions.
However, officials warned that Southern California’s other main source, the Colorado River, remains at risk despite an improvement in drought and water conditions in the state.
“One of the great challenges for California lies outside of California, in the Colorado River Basin,” said state climatologist Mike Anderson. “The basin has been in a drought for 23 years and is facing some pretty big challenges.”
He added that spring “will experience a very dynamic state as we try to cope with all the water that has appeared here both in January and March, as well as in February when there was a lot of snow”.
In fact, California’s 154 primary inland reservoirs gained 9.9 million acre-feet of water between Nov. 30 and Feb. 28, according to the drought monitor, bringing their total storage — 23.2 million acre-feet — to 96% of the historical average for the US brings time of the year.
But in the Colorado River system, storage capacity as of February 28 was 15.1 million acre-feet, just 46% of average and 29% of capacity.
The remainder of March appears to be potentially wetter, with another atmospheric flow hitting California early next week and possibly another after that. Both may be colder storms that will increase the state’s snow cover.
The forecast comes just days after climatologists declared the end of La Niña, a tropical Pacific climate pattern associated with dry-than-average conditions in the state, particularly in southern California.
The southern El Niño-La Niña oscillation, sometimes referred to as the ENSO, has a major impact on temperature and precipitation patterns in different parts of the world. ENSO-neutral conditions are expected to persist into early summer, with the likelihood of a wetter El Niño pattern developing soon thereafter increasing.
Inspecting the damage in Pajaro on Wednesday, Gov. Gavin Newsom said the winter’s wild run is indicative of extreme weather variability caused by climate change.
“Look back at the last few years in this state – it was fire to ice and no warm bath in between,” he said.