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The Sierra range of mountains was formed by shifting tectonic plates during the Oligocene epoch. This caused sinking in the land and created a large bowl-like depression in the earth. About 5 million years ago the volcanic activity caused the bowl to close at the north end. The bowl, thus sealed, began to fill with rainwater and water from melting snow. During the Wisconsin Ice Age, which was about 10,000 years ago, a glacier moved into the area and sent rivers of ice into the newly forming lake. One of these carved out Emerald Bay. As the glacier moved back, its ice melted and caused the level in the lake to rise hundreds of feet. This was forced out through the porous volcanic obstruction at the northwest end. It is at this point that the lake drains into what we now know as the Truckee River. There is now a dam at Tahoe City which regulates the flow of water.
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