Though the Sierra Batholith formed back in the Mesozoic Era, this massive pluton of rock did not rise to create the Sierra-Nevada Range until the late Tertiary Period, about 3-4 million years ago. As it rose, the forces of erosion began to sculpt its summits and fault lines developed along its margins. Along the northeast edge of the batholith, a block of crust (known as a graben) dropped between parallel faults, with the Sierra rising to its west and the Carson Range uplifted to its east; the resulting basin is now known as Tahoe.
Streams from the surrounding highlands gradually filled the basin, the margin of which was continually molded by uplift, faulting, landslides, volcanic debris and glacial erosion; the latter was especially significant during the Pleistocene Epoch as glacial valleys formed along the west edge of the basin, glacial moraines altered drainage patterns and glacial meltwater added significant depth to the basin's lake. As one might expect, the surface level of Lake Tahoe, which now sits at 6225 feet, rose and fell significantly over the past 2 million years.
Today, the Truckee River is the only outlet from Lake Tahoe, flowing northeastward into the Great Basin, where it feeds Pyramid Lake. Lake Tahoe itself, with a maximum depth of 1645 feet and a surface area of 191 square miles, is the largest alpine lake in North America and the third deepest lake on the Continent (behind Great Slave Lake and Crater Lake). Known for its scenic setting and for the clarity of its water, Lake Tahoe sits above two fault lines, making the area prone to earthquakes and a potential tsunami; like all lakes, mountains, plains and canyons, it is a beautiful but temporary feature of our planet's landscape!
