"The Lake Tahoe Basin was formed by geologic block
(normal) faulting about 2 to 3 million years ago. A
geologic block fault is a fracture in the Earth's crust
causing blocks of land to move up or down. Uplifted
blocks created the Carson Range on the east and the
Sierra Nevada on the west. Down-dropped blocks
created the Lake Tahoe Basin in between. Some of the
highest peaks of the Lake Tahoe Basin that formed
during this process were Freel Peak at 10,891 ft (3,320
m), Monument Peak at 10,067 ft (3,068 m) (the present
Heavenly Valley Ski Area), Pyramid Peak at 9,983 ft
(3,043 m) (in the Desolation Wilderness), and Mt.
Tallac at 9,735 ft (2,967 m).

Snow, rain, and streams filled the southern and lowest
part of the basin, forming the ancestral Lake Tahoe.
Modern Lake Tahoe was shaped and landscaped by the
scouring glaciers during the Ice Age (the Great Ice Age
began a million or more years ago). Many streams flow
into Lake Tahoe, but the lake is drained only by the
Truckee River, which flows northeast through Reno
and into Pyramid Lake in Nevada.

Lake Tahoe is the second deepest lake in the United
States and the tenth deepest in the world, with a
maximum depth measured at 1,645 ft (501 m), average
depth of 1,000 ft (305 m). footer image. Crater Lake in
Oregon is the deepest lake (1,949 ft or 594 m) in the
United States. Please Note that the depth of Lake
Tahoe changes every day as the lake level changes. The
deepest measurement from the 1998 bathymetric survey
was 1,637 ft (499 m) deep. The depth of Lake Tahoe
depends on the height it is measured from; some
measurements use sea level as a base reference, others
use different points of reference. The appropriate
reference, or datum, for Lake Tahoe's depth is still
being debated. Therefore the measured depth of a lake
is only preliminary data and may change.

Lake Tahoe is about 22 mi (35 km) long and l2 mi (19
km) wide and has 72 mi (116 km) of shoreline and a
surface area of 191 mi2 (495 km2). The floor of the Lake
Tahoe Basin is at an elevation of about 4,580 ft (1,396
m), which is lower than the surface of the Carson Valley
to the east! With an average surface elevation of 6,225 ft
(1,897 m) above sea level, Lake Tahoe is the highest
lake of its size in the United States.

The water temperature near the surface generally cools
to 40 to 50oF (4.5 to 10oC) during February and March
and warms to 65 to 70oF (18 to 21oC) during August
and September. Below a depth of 600 to 700 ft (183 to
213 m), the water temperature remains a constant 39oF (4.0oC).
Lake Tahoe has a water clarity of about 100 ft (30 m)
deep. Factors contributing to its clarity include the
following:* 40 percent of the precipitation that falls into the
Lake Tahoe Basin lands directly on the lake.* Remaining precipitation
drains through granitic soils, which are relatively sterile and create
a good filtering system."