Drought Could Help Reveal Secrets of Sierra's Origins

Freso State geologist Chris Pluhar and his student Wynter Erickson hope these rocks, normally underwater in Millerton Lake, will reveal clues about the geological history of the Sierra Nevada. (Kerry Klein/KQED)

A few miles northeast of Fresno, Chris Pluhar walks through a field pockmarked with little holes. It’s dry, scrubby and surrounded by rolling brown hills. You’d never know that in a normal year, where he’s standing would be under 20 feet of water. It’s usually a part of Millerton Lake, a major but rapidly shrinking reservoir in the Sierra foothills.

“Rather than being in a lake, which is how it's shown on the map, we're standing in the middle of a grassland,” says Pluhar, who teaches geology at Fresno State University. “We were here about three weeks ago and since then, a few new islands have popped up.”

Right now, Millerton’s filled to about a third of its total capacity. The high water line scars the hillside far above Pluhar’s head, and some boat ramps lead to nowhere.

While most people overlooking Millerton would see only a shriveled reservoir, Pluhar saw an opportunity: he could study rocks revealed for the first time in years.

“This is kind of virgin territory for mapping,” he says. “If you look at the U.S., it's mapped all across the country except for places like this.”

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Pluhar studies tectonics—the process by which geologic plates pull apart and jam together to form ridges and mountains—and he’s interested in what these rocks can tell him about the Sierra Nevada. So he brought along a student to help figure it out.

Wynter Erickson is mapping this area for her senior thesis—a process that involves tracking the boundaries between rock layers and measuring how much they’ve tilted.

“You kind of just get in your own little world while everyone's on their speedboats,” she says. “It's awesome.”

These geologists are piecing together pre-history. Some of the rocks here formed more than 100 million years ago, long before the Sierra began to rise. Pluhar hopes Erickson’s measurements will help answer an important question: when did the mountains grow to their current size? And how quickly?

Fresno State student Wynter Erickson uses a compass to measure how much ancient volcanic rocks underneath Millerton Lake have tilted.
Erickson uses a compass to measure how much ancient volcanic rocks underneath Millerton Lake have tilted. (Kerry Klein/KQED)

“The Sierra Nevada is a highly studied mountain range,” says Pluhar, “and yet us scientists can't agree on some of the most basic things—like did it uplift recently or not?”

The Sierra is a jumble of sediments transported from the deep ocean and piled up on top of the continent, with scars of granite and lava from defunct volcanoes. Its history is complex, and it’s not easy to figure out what happened when.

There’s no telling if this one little part of the foothills will resolve a big, longstanding scientific debate. But one thing is certain: now is the time to investigate.

“We're probably, overall statewide, about half of what we would normally see in reservoir storage” for this time of year, says Jay Lund, director of the Center for Watershed Sciences at UC Davis. “Some of the reservoirs are the lowest that they’ve ever been.”

As worrisome as that is, Lund says it does present some rare opportunities.

“You get to see what used to be in the reservoir before they filled it,” he says, like relics and building foundations from towns submerged long ago. “And there are sometimes old Indian artifacts around the state.

Lund also says drought years are the best time to perform maintenance and repairs on dams.

Pluhar consults a mapping app to find the contact between two rock layers.
Pluhar consults a mapping app to find the contact between two rock layers. (Kerry Klein/KQED)

He’s hopeful that an El Niño winter will top up the state’s thirsty reservoirs. But he’s quick to point out that high water levels wouldn’t mean all of the state’s water worries are over.

“For long droughts, it's groundwater which is really by far the largest reservoir for California,” he says—like the over-pumped aquifer underlying the Central Valley. “It'll take a very very long time, if ever, for those groundwater levels to recover.”

It’s hard to find an upside when farms are going fallow and thousands of people are out of drinking water. But back in the parched basin of Millerton Lake, Wynter Erickson is excited that she’s in the right place at the right time to make something good of the drought.

“I think it’s so cool because if we do get our El Niño year, it's not going to be available to map anymore,” she says. “So I think it's pretty incredible that we get to do this research.”

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By the time she and Pluhar finish their maps, their field site will once again be at the bottom of a lake—at least, one can hope.

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