How the lake was formed |

How the lake was formed

Two thousand years may sound like a lot, but not to Lake Tahoe.

The 23-mile-long, 12-mile-wide Tahoe – a unique lake because of its depth, elevation and clear water – has existed for many millennia.

The stage was set for Tahoe’s creation millions of years ago. Fault movements around the Sierra Nevada created the mountains around Tahoe and the graben – or trough – where the lake is today. It’s unknown how long ago these faults shifted, says Rich Schweickert of the University of Nevada, Reno, but the most recent movement happened within the last 3.5 million years.

There was no lake, however. What’s now at the bottom of Tahoe was a valley which may have appeared a lot like Carson Valley to the east.

However, thanks to a bunch of volcanic eruptions about 2 million years ago, a lava dam was created northwest of what is now Tahoe City.

There was nowhere for water to escape, and a lake, which could have been as much as 800 feet higher than it is now, was formed. Over thousands of years, the lake’s only outlet should have slowly eroded its headwaters, steadily lowering the lake to a level far below where it is today.

However, during the Pleistocene age between 1 million and 100,000 years ago, huge glaciers blocked the outlet, again raising the lake’s level.

It’s known there were a series of massive flood events during that time, which carried giant chunks of ice and boulders as big as 40 feet in diameter down the Upper Truckee River, beyond where Sparks is now.

Tahoe now is unique because lakes at such high elevation usually are fairly shallow. Tahoe is nearly 1,600 feet deep at an elevation of 6,225 feet.

“Looking around Nevada and Utah, the big lakes you see usually fill flat valleys,” said Schweickert, chairman of the UNR Department of Geological Sciences. “Most mountain lakes – for example, up in the high Sierra – are small. Tahoe is special because of a combination of the faults and the volcanic eruptions that plugged the outlet.”

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