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Warmer winters spell more floods for northern California

Trina Kleist
Special to The Union

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FOTO 5. Water flowing down spillway.

Submitted photo by Keane Sommers/Nevada Irrigation District

Scotts Flat Reservoir near Nevada City already was nearly full from wintertime rain when melt from this year’s abundant snow came pouring in. What didn’t fit flowed into Deer Creek and on downstream.

Floods likely will surge more often across northern California, as wintertime temperatures rise in the Sierra Nevada and snow shifts to rain, scientists predict.

When precipitation falls as snow, the water stays longer on the mountain as snowpack, then slowly flows out as snowmelt over the spring and summer. The ground has more time to absorb it, said Nevada Irrigation District Watershed Resources Planner Neysa King.

But when precipitation falls as heavy rain, the ground becomes saturated. Once it’s soaked, the ground can’t absorb more, so rainwater runs off.

And it’s not just water managers who face the challenges posed by this climatic shift.

“If we had had this same winter 30 or 40 years ago, a lot more of that (precipitation) probably would have been snow.”— Daniel Swain, UCLA scientist

Triple-whammy costs $6.3 million in Nevada County

California’s winter of 2017 saw three things happening together, regional and local scientists observed:

Record precipitation.

A continuing shift from snow to rain at middle elevations, as wintertime temperatures rise.

Several warm, tropical storms that dumped rain on top of snow.

As a result, western Nevada County saw businesses and homes flood, trees topple, sinkholes yawn, mountainsides crash over roadways, water-bearing flumes collapse, schools close and traffic jam. Damage to roads and other public infrastructure is estimated at $6 million, local officials said. Additional storm damage to NID infrastructure was estimated at $250,000, said former NID spokeswoman Susan Holt.

Those costs don’t include damage to Pacific Gas & Electric Co. infrastructure. The utility employed 5,000 people to repair 3,700 spans of power line and nearly 900 utility poles just in January 2017, spokeswoman Brandi Merlo wrote. PG&E declined to reveal the cost of its storm repairs over the entire winter, including those made to the South Yuba Canal, a main artery for NID water that collapsed in February 2017.

Lower mountains, more rain, regional floods

The winter of 2017 also showed regional flooding can have local consequences. In February 2017, run-off from wintertime rain damaged two spillways at the Oroville Dam in Butte County and triggered the evacuation of 188,000 people living downriver. At least 1,100 evacuees fled to western Nevada County, officials said.

The influx into Nevada County of thousands of Central Valley flood refugees is one of the top three crisis scenarios planned for by local emergency responders, according to county planning documents.

As warming Sierra temperatures shift snow to rain, such a crisis could come again.

The challenge for a reservoir like Oroville is that the section of the Sierra Nevada that channels run-off toward the lake stands at about 5,000 to 6,000 feet average elevation, said climate scientist Daniel Swain, of the University of California, Los Angeles.

Average snowlines across the Sierra Nevada — historically just below 6,800 feet — already have risen in the past two decades, pushed higher by warmer temperatures, according to the Western Regional Climate Center in Reno.

Other research shows elevations below 8,000 feet see bigger impacts from warming winters.

During the winter of 2017, “many storms had snowlines above that elevation. They didn’t produce snow at those lower elevations because it was too warm to do so,” Swain explained.

“If we had had (that) same winter 30 or 40 years ago, a lot more of that (precipitation) probably would have been snow,” Swain concluded.

Trina Kleist is a Grass Valley freelance writer whose clients include Nevada Irrigation District. She may be contacted at tkleistwrites@gmail.com or 530-575-6132.

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