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Researchers have solved the mystery of how massive lakes in the Western United States were formed.

Lakes were able to grow large – rivaling the Great Lakes – during the peak of the last Ice Age 21,000 years ago, a period known as the 'Last Glacial Maximum,'

The key, researchers discovered, was that evaporation rates were significantly lower than today.

During the Last Glacial Maximum, large lakes (light blue) covered many of the now dry desert basins of Nevada, Oregon and California. At the height of the LGM, Lake Surprise had a surface area of about 390 square miles, roughly the size of San Francisco Bay.

'It was previously thought that the lakes grew because there was more rain and snowfall during this period of the Earth's history,' said Daniel Ibarra, a graduate student in Stanford's Department of Environmental Earth System Science and the first author of the study.

During the Last Glacial Maximum (LGM), giant lakes covered large sections of California, Nevada, Oregon and Utah, including where Salt Lake City is today.

Earth scientists have long been puzzled by how these ancient lakes, now completely dry, grew so large.

The prevailing theory was there was more rain and snowfall during this time period.

But recent evidence from paleoecology and climate model simulations indicates that precipitation rates were actually relatively low compared to later periods.

To resolve the discrepancy between computer models and the interpretation of geologic evidence, Ibarra collected more than 80 samples of tufa – a limestone created by the evaporation of mineralized water – from different locations around the edges of Lake Surprise, a moderate-sized fossil lake in Surprise Valley, California.

The layered shorelines of California's Lake Surprise date back to the peak of the last Ice Age and provide a record of lake level changes over thousands of years.

The layered shorelines of California's Lake Surprise date back to the peak of the last Ice Age and provide a record of lake level changes over thousands of years.

Their findings showed that at the height of the LGM, Lake Surprise had a surface area of about 390 square miles, roughly the size of San Francisco Bay.

Their analyses revealed that 21,000 years ago, the evaporation rate at Lake Surprise was nearly 40 percent lower than today, with precipitation rates similar to the modern era.

More quantitative studies of past climate could help refine the computer models used by the Intergovernmental Panel on Climate Change (IPCC) to simulate Earth's atmospheric conditions under changing atmospheric conditions, said Kate Maher, assistant professor of geological and environmental sciences, who headed the project.

'The IPCC uses climate models to simulate past and future climate, so knowing that some of the models do a better job of simulating past changes gives us more confidence that we understand the physics involved,' Maher said.

'That can give us more confidence in the models we use to simulate future climate change.'