100 to 200 year dry spells not uncommon
Fallen Leaf Lake, California.
By Summit Voice
SUMMIT COUNTY — With the central Sierra Nevada in drought conditions, researchers with the University of Nevada, Reno and their partners at Scripps Institution of Oceanography in San Diego are warning that the region is no stranger to megadroughts, often lasting several hundred years.
The best-known of those is the Medieval megadrought, between 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.
In a recently published paper, the scientists published findings from their research at Fallen Leaf Lake — a small moraine-bound lake at the south end of the Lake Tahoe Basin. The records show that 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.”
The researchers traced the area’s climate history using tree-ring analysis, shoreline records and sediment deposition in Fallen Leaf Lake. Using side-scan and multibeam sonar technology, 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 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 13th century.
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.
