State's Declaration of 'Worst drought on record' is grossly misleading
Reno, Lake Tahoe and the Sierra Nevada are no strangers to drought, the most famous being the Medieval megadrought lasting from 800 to 1250 A.D. when annual precipitation was less than 60 percent of normal. The Reno-Tahoe region is now about 65 percent of annual normal precipitation for the year, which doesn’t seem like much, but imagine if this were the “norm” each and every year for the next 200 years.
Research by scientists at the University of Nevada, Reno and their partners at Scripps Institution of Oceanography in San Diego indicates that there are other instances of such long-lasting, severe droughts in the western United States throughout history. Their recent paper, a culmination of a comprehensive high-tech assessment of Fallen Leaf Lake – a small moraine-bound lake at the south end of the Lake Tahoe Basin – reports that stands of pre-Medieval trees in the lake suggest the region experienced severe drought at least every 650 to 1,150 years during the mid- and late-Holocene period.
“Using an arsenal of cutting edge sonar tools, remotely operated vehicles (ROVs), and a manned submersible, we’ve obtained potentially the most accurate record thus far on the instances of 200-year-long droughts in the Sierra,” Graham Kent, director of the Nevada Seismological Laboratory said. “The record from Fallen Leaf Lake confirms what was expected and is likely the most accurate record, in terms of precipitation, than obtained previously from a variety of methods throughout the Sierra.”
Kent is part of the University of Nevada, Reno and Scripps research team that traced the megadroughts and dry spells of the region using tree-ring analysis, shoreline records and sediment deposition in Fallen Leaf Lake. Using side-scan and multibeam sonar technology developed to map underwater earthquake fault lines such as the West Tahoe fault beneath Fallen Leaf Lake, the team also imaged standing trees up to 130 feet beneath the lake surface as well as submerged ancient shoreline structure and development. The trees matured while the lake level was 130 to 200 feet below its modern elevation and were not deposited by a landslide as was suspected.
The team, led by John Kleppe, University of Nevada, Reno engineering professor emeritus, published a paper on this research and is presenting its findings in seminars and workshops.
“The lake is like a ‘canary in a coal mine’ for the Sierra, telling the story of precipitation very clearly,” Kent said. “Fallen Leaf Lake elevations change rapidly due to its unique ratio between catchment basin and lake surface of about 8 to 1. With analysis of the standing trees submerged in the lake, sediment cores and our sonar scanning of ancient shorelines, we can more accurately and easily trace the precipitation history of the region.”
Water balance calculations and analysis of tree-ring samples undertaken by Kleppe, Kent and Scripps scientists Danny Brothers and Neal Driscoll, along with Professor Franco Biondi of the University’s College of Science, suggest annual precipitation was less than 60 percent of normal from the late 10th century to the early 13thcentury. Their research was documented in a scientific paper, Duration and severity of Medieval drought in the Lake Tahoe Basin, published in the Quaternary Science Reviews in November 2011.
Tree-ring records and submerged paleoshoreline geomorphology suggest a Medieval low-lake level of Fallen Leaf Lake lasted more than 220 years. More than 80 trees were found lying on the lake floor at various elevations above the paleoshoreline.
“Although the ancient cycle of megadroughts seems to occur every 650 to 1150 years and the last one was 750 years ago, it is uncertain when the next megadrought will occur. With climate change upon us, it will be interesting to see how carbon dioxide loading in the atmosphere will affect this cycle,” Kent said.
Professor Paula Noble, in the University’s College of Science’s Department of Geological Sciences and Engineering, is expanding this research to include the fine-scale study of climate change through out the Holocene (about 12,000 years) using recently collected 40-foot-long sediment cores in Fallen Leaf Lake.
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Photo Cutline: Graham Kent and University of Nevada, Reno researchers were joined by Scripps research team, spending many days on Fallen Leaf Lake to gather sonar and side-scan radar data to study earthquake faults and paleoshorelines. Photo by Jean Dixon, University of Nevada, Reno.
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LAKE TAHOE, Calif. (KGO) -- Scientists in the High Sierra have discovered a previously unknown forest, and there's a twist. The ancient trees in this forest are under water. They are standing at the bottom of Fallen Leaf Lake, just south of Lake Tahoe. New research shows both the forest and the lake hold critical clues to climate change.
Fallen Leaf Lake is already known for its spectacular beauty. Now it turns out the lake also has a dramatic secret.
"What I like to call a ghost forest," says Prof. Graham Kent of the University of Nevada, describing trees up to 100 feet tall, as high as a 10-story building, but covered by water. "We have old wood from a thousand years ago, 2,000, 3,000 down there."
Scientists in a small submarine took photos showing just a few of the hundred or more trees on the bottom of the lake. They're using state of the art sonar to map the mysterious forest and try to figure out why it's there.
"Side scan sonar technology is exactly the equipment that's used to find sunken ships," says Kent.
A high-tech image shows the lake has seen big changes in the water level over many years. A sonar device shows slices of the ground under the lake bottom.
Researchers say all this evidence confirms that a thousand years ago there was a prolonged drought in the Sierra. It lasted about 200 years -- long enough for huge trees to grow where Fallen Leaf Lake now sits. The precipitation is believed to have been just 60 percent of normal. That is similar to the devastating midwest "Dust Bowl" in the 1930s. Scientists believe it may happen again.
"So take the great Dust Bowl and extend it from 10 years to 200 and some years," explains Kent. "And just wonder how the economies of California and Nevada are going to be affected by it."
Kent says the long Sierra drought happened naturally a thousand years ago. But he and other top researchers believe human-caused global warming might bring on another severe drought even faster in the future. If that happens, Fallen Leaf Lake could be the proverbial canary in the coal mine.
"This is the lake that's going to start feeling the effects of the next drought, whenever that happens, much more than any other lake in the area," he says.
The research indicates the lake level might drop fast, as much as 50 to 100 feet in just a couple of decades, and that could bring huge change for thousands of vacationers who flock to the lake every summer.
Scientists plan more research to make more accurate predictions. But now at least they know Fallen Leaf Lake and its ghostly forest is the place to find answers.
"You don't get many chances to get a perfect record," says Kent.
The forest at the bottom of the lake was actually discovered by another professor who happened to be fishing on the lake. His lure got stuck in the top one of those 100-foot tall trees.
Written and produced by Jennifer Olney.