Sierra Droughts: Tahoe's Drowned Forest
- September 1, 2010
Lake Tahoe is a reservoir controlled by the dam at Tahoe City with water storage equal to six feet and one inch. There is a federal limit on the maximum level of the lake so that it may not exceed 6,229.1 feet in elevation, but when the lake drops to 6,223 feet no more surface water can feed into the Truckee River. During times of drought, Tahoe's low levels affect boaters, wildlife and water management.
Droughts have always impacted the region, some worse than others. During a severe drought in the early 1930s, Tahoe's surface level fell below the rim and exposed tree stumps off the beach of the Tallac Historic Site near South Lake Tahoe. A University of California at Berkeley scientist, Samuel T. Harding, examined the stumps and determined that they had lived for 100 to 150 years before rising water levels submerged them. The Tallac stumps are between 4,846 and 5,527 years old; other stumps have been found deeper in the lake from Emerald Bay and Fallen Leaf Lake to Stateline. They range up to seven feet tall, three and a half feet in diameter and up to 6,300 years old. The existence of the ancient trees suggests that between 4,000 and 5,000 years ago a much drier climate forced Lake Tahoe's water level to fall well below the natural rim for at least 1,500 years.
Eventually wetter and cooler conditions returned and for about the past 4,000 years the climate in the west has stabilized. During this period of relatively warm and wet weather, Tahoe's lake level has stayed relatively high, which has preserved the tree stumps and prevented them from rotting away. One theory suggests that seismic activity may have quickly raised or lowered portions of the lake bottom, which could have affected water levels, but most scientists think that climate change is the more likely culprit. Another indicator that suggests prolonged drought is the lack of archeological evidence of human habitation around the lake more than 3,500 years ago. Researchers believe that Indian artifacts and relics older than that are probably submerged in parts of the lake. Harding also pointed out the sand dunes at Kings Beach could only have formed if Tahoe's water levels were much lower. According to Harding, the dunes are too high to be formed under today's conditions. For wind to build dunes that large, a much wider beach and larger source of sand would be required.
Extreme droughts that occurred thousands of years ago don't seem too threatening today, but there is plenty of evidence that desiccating dry spells are still part of our contemporary climate regime. Over the past 20 years, paleoclimatologist Scott Stine of ÂãÁÄÖ±²¥, East Bay has analyzed tree stumps he located in Mono Lake and the Walker River. His studies using tree ring and radio-carbon methodologies have found two prolonged dry spells that occurred between 1,200 and 600 years ago; one lasted for 220 years and another for 141 years. During those mega-droughts, many lakes and rivers in the Sierra Nevada dried up for decades, a disastrous scenario that makes complaints about closed boat launch facilities seem trivial by comparison. The period of modern settlement in the Sierra Nevada (about the last 150 years) has been one of the wettest intervals of the past 1,000 years. We can only hope that it stays that way.
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