About 2 million years ago, which is after the uplift and tilting of the Sierra Nevada mountain range, the Tahoe basin began forming. A block of rock began falling down relative to the mountains on either side along roughly parallel normal faults (a graben). These faults are on the east and west sides of the lake. The Tahoe block (the mass of rock under the lake) fell about 5,000 feet relative to the mountain ranges on either side of it.
The initial outlet of Lake Tahoe was more northerly toward city of Truckee. The first dam formed 1 to 2 million yeas ago when lava lows from Mt. Pluto blocked the flow. The lake level increased until it overflowed at its current outlet forming the Truckee River.
Then during the glacial episodes, various glaciers flowed down the valleys along the southwestern side of the river. The glaciers from Bear Creek and Squaw Valley dammed the rivers at least during the Tahoe (80,000 or 130,000 years ago) and Donner Lake Glaciations (400,000 to 600,000 years ago). It is likely that the Truckee was blocked during the Hobart Glaciation (older than the Donner Lake Glaciation) as well. When the ice dams blocked the flow of the Truckee, water levels increased up to 600 feet.
The ice dams were not very stable. Once the lake levels reached about 9/10 the height of the ice, the ice begins to float and the ice breaks apart. The resulting flood is called a jökulhlaup. These floods transported boulders as large as 10 feet in diameter down the Truckee to Reno. The ice dams could have reformed multiple times during each glaciation. Evidence of higher lake levels can be found throughout the Lake Tahoe region.
Logging requirements:
Send me a note with :
- The text "GC4DZDF Lake Tahoe Dam Through the Geologic Ages" on the first line.
- The number of people in your group (put in the log as well).
- What is the current level of the lake (this is a daily reading)?
- To what elevation could the current man-made dam bring the Lake elevation to?
- What is the natural outlet elevation?
The following sources were used to generate this cache.
- http://www.fs.usda.gov/detail/ltbmu/learning/?cid=stelprdb5109570
- http://donsnotes.com/tahoe/geology.html
- Pleistocene Glaciation of the Northern Sierra Nevada, North of Lake Tahoe California. Birkeland, Peter W. Journal of Geology Vol 72 No 6 November 1964