Scientists scan Tahoe area lake for evidence of severe past droughts
Sacramento Bee
Vacationers skim the surface of Fallen Leaf Lake on water skis or kayaks, oblivious that scientists are investigating the depths for signs of natural disasters to come.
Last week, scientists began crisscrossing the lake with a $450,000, state-of-the-art sonar tool that will map the lake bed in unprecedented detail.
They're looking for two things:
• Subsurface seismic faults that could bode earthquakes and even tsunamis for the Tahoe region.
• Ancient trees along the outer edges of the lake bottom that are evidence that the region is subject to recurring 200-year droughts of catastrophic proportions.
Fallen Leaf Lake, near the south end of Lake Tahoe, would probably not be giving up these secrets but for a fishing trip around 25 years ago.
John Kleppe, now an emeritus professor at the University of Nevada, Reno, was fishing with a vibrating lure near his Fallen Leaf vacation home when the lure hit something.
The lake, which goes as deep as 400 feet, was about 100 feet deep where his lure struck an obstacle, but the lure was only 40 feet down.
A diver checked it out.
"He said it was a tree," said Kleppe, an electrical engineer with a curious, multidimensional mind.
Kleppe wasn't content to say, "Huh, there's a tree there." He wanted to know why.
It turned out that the tree, a yellow pine, was rooted to the lake bed and of a size that indicated it was 200 years old when it died – drowned, essentially.
Carbon dating and the tree's well-preserved outer rings told Kleppe it was alive in 1191.
His conclusion, spelled out in a 2005 scientific paper, was that Fallen Leaf – created by glaciation thousands of years ago – was dramatically lower for 200 years in the Middle Ages.
Drought dropped the lake level about 100 feet, allowing tree growth for 200 years. When rains resumed, the trees drowned.
Then Kleppe found more trees in Fallen Leaf that were even older. Some date back to about 1500 B.C. – before the birth of Egypt's boy pharaoh, Tutankhamen.
Kleppe has discovered about 100 trees so far.
He hasn't carbon-dated enough of them yet to prove his hypothesis that California and Nevada undergo multicentury droughts on a regular basis, every 1,000 years or so.
He hasn't developed this theory alone. It's the product of collaboration with dendrochronologists, hydrologists and geophysicists.
"People don't typically work together," he said. "You need to share with a team to understand."
That's why scientists from the Scripps Institution and UNR came to Kleppe's lakeside retreat last week. It provided a research base for trying out the sonar equipment.
"What we're interested in is the earth," said Jeff Babcock, a Scripps geophysicist.
The cutting-edge gear scans wide swaths of the lake bottom at once to measure depth precisely, he said.
"We're going to know exactly what the bottom looks like," Babcock said.
The thinking, said grad student Jillian Maloney, is that scanning the lake bottom will clearly show fault movements.
"Under this lake, it would erode a lot less than on land," Maloney said.
Past studies by the Scripps team have suggested the area may be due for a major earthquake every 2,000 to 3,000 years. The survey will help refine those studies, which suggest the fault might generate a quake of up to magnitude 7.3 and a subsequent tsunami wave as high as 30 feet.
At the same time, the Scripps survey will give Kleppe a full view of what he calls the "bathtub ring," the lake's submerged beachfront that he thinks was exposed by ancient drought.
He and his 14-year-old grandson, Will Schmidt, use a remotely operated submersible camera with a claw to search the bathtub ring offshore from a known ancient Washoe historic site.
Schmidt, a video game player, is adept at controlling the submersible. It isn't as stimulating as video games, he said, but "it's definitely more intense because it's a real thing. You can't just start over."
Kleppe's hope is to find Washoe grinding rocks at levels consistent with the droughts of his theory.
He acknowledges he has encountered skepticism.
Some of that, he says, is because the theory implies we need to prepare better to ensure water for California and Nevada's thirsty populations.
"What this means is we may be facing another multicentennial drought," he said. "We have a record (in the trees), and we need to see what it can tell us."
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