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SALT LAKE CITY — A new study suggests that the Great Salt Lake was formed by tropical rainfall, not from Pacific storms as was previously thought.

Scientists originally believed the lakes that once covered the American Southwest — to which the Great Salt Lake traces its origin — were formed by Pacific storms that were diverted from the north by a split jet stream that ran eastward across North America.

The storms were thought to have dumped enough precipitation over time to form Lake Bonneville over Utah and Lake Lahontan in Nevada between 20,000 and 14,000 years ago.

A new study by U.S. researchers found that the Pacific storm track hypothesis was incorrect, though, because the Pacific coast did not have its wettest period until thousands of years after the lakes were formed.

The team analyzed ocean sediments and dry western valleys and lake beds collected over 30 years, pollen buried in marine sediments and the results of published climate model experiments to come to the conclusion that the lakes were likely formed from tropical rains from the tropical Pacific and the Gulf of Mexico.

"We think that the extra precipitation may have come in the summer, enhancing the now weak summer monsoon in the desert southwest," said Mitchell Lyle, a professor of oceanography at Texas A&M University and lead author of the study. "But we need more information about what season the storms arrived to strengthen this speculation."

About Lake Bonneville
Lake Bonneville was a prehistoric lake that covered northwestern present-day Utah and extended into parts of present-day Idaho and Nevada.

Formed about 32,000 years ago, it existed until about 14,500 years ago, when much of the lake was released through the Red Rock Pass in Idaho during the Bonneville Flood.

The lake was more than 1,000 feet deep and more than 19,691 miles in area, similar in area to Lake Michigan but much deeper.

The flood, as well as climate change over the following years, caused Lake Bonneville to dry up, but Utah is dotted with its remains: the Great Salt Lake, Utah Lake, Sevier Lake and Rush Lake are all the remnants of Lake Bonneville.

Analysis of physical data, such as the chemistry of lake bed sediments, and use of mathematical models can both be used to confirm the study's results, according to Noah Diffenbaugh, an assistant professor of environmental Earth system science at Stanford University.

The researchers hope the results can help further research in the area of climate change.

"The wet glacial conditions in the Great Basin are an interesting climate problem, in part because it was an important time in the arrival of humans in North America," Diffenbaugh said, adding that it will be helpful to "understand climate in the coming century, when we expect not only further global warming but also changes in water demand in the western U.S. arising from increasing population and urbanization."

Scientists say climate change is steadily reducing the snowpack in California's Sierra Nevada and in the Rocky Mountains, at the same time as western desert population growth is exploding. Understanding past climate change — including that which led to the formation of the Great Salt Lake — could help scientists predict the effects of future climate shifts.

The study involved scientists from Stanford, Texas A&M University, Columbia University, University of California-Santa Cruz, Hokkaido University of Japan, Brown University and the U.S. Geological Survey and appeared this week in the Sept. 28 issue of Science.

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Stephanie Grimes


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