Tahoe's huge, natural collection of water dwarfs even the biggest human-made reservoirs in California. The water that evaporates from the lake every 24 hours could supply the city of Los Angeles for a day. That's why the federal government set up the Lake Tahoe Basin Management Unit, rather than include it in a national forest. The government clears up all kinds of misconceptions about the lake. Perhaps the biggest misunderstanding involves the lake's origins.
Some people mistakenly believe the lake was formed by the collapse of a volcanic crater.
Actually, plate tectonics had more to do with it in the beginning. The basin was formed 5 million to 10 million years ago when the Sierra crest began to lift on the west and the Carson Range did likewise to the east. Volcanic eruptions entered the picture later. About 2 million years ago, an andesite lava flow from the north shore of Mount Pluto formed a barrier across the basin's northeastern outlet. Eventually, as the lake rose, the Truckee River was able to cut through these flows and flow around the volcanics to the lowlands of Nevada.