At a recent lecture sponsored by the Squaw Valley Institute, Dr. Schweikert, Professor of Geology at the University of Nevada, Reno, discussed the geological structure and evolution of the Tahoe Basin. Of the clearly identified earthquake faults running approximately north-south through the region, two major faults pass through the lake and one major fault is located west of the lake.

It is now believed that McKinney Bay, on the west shore, was probably formed by a giant land collapse and may have generated a tidal wave in the lake as high as 90 feet, that sloshed back and forth from shore to shore for as long as 12 hours.

Although significant earthquakes are a near certainty sometime in the future, the chances of this happening sometime in our lifetimes is remote. No need to hurry out and purchase earthquake insurance!

Although it is commonly believed that Lake Tahoe is of volcanic origin, the Lake Tahoe Basin was actually formed by faulting—fractures in the earth’s crust allowing blocks of land to rise and sink. This occurred over several million years as the Sierra Nevada mountains were rising from a shallow sea. Two principal steep faults evolved, the eastern margin created the Carson Range while the Sierra Nevada mountains rose on the western side. From the “up thrown” fault blocks the highest peaks in the Lake Tahoe Basin originated. Freel Peak (10,891) , Monument Peak (10,067) the present Heavenly Valley Ski Area, Pyramid Peak (9,983) in Desolation Wilderness, and Mt. Tallac (9,735) which has a fault running at its base.

A lake formed near the southern and lowest part of the basin, fed by snow, rain, and draining creeks and rivers. The lake level increased in depth until it found an outlet, then near the present town of Truckee. Several active volcanoes poured lava into the basin, eventually damming the outlet. The waters rose again, several hundred feet higher than the present level. Finally, a new outlet was cut, just east of one volcano, Mt. Pluto, the present location of Northstar Ski Area. Cave Rock on the east shore of Lake Tahoe is the eroded remains of another volcano that once poured lava into the basin. Its “cave" is wave cut, a relic of the ancestral and much bigger Lake Tahoe.

During the Ice Age, glaciers scoured the surrounding landscape to the shape what we see today. These rivers of ice followed pre-existing V-shaped stream canyons, carving them as they moved downward to smooth U-shaped valleys. Emerald Bay, Cascade Lake, Fallen Leaf Lake and the Echo Lakes now fill some of these U-shaped valleys. Moraines, glacial debris left behind, blocked the outlet again, changing it to the present Truckee River outlet at Tahoe City.

Lake Tahoe is about 22 miles long and 12 miles wide with 72 miles of shoreline. The surface area covers 191 square miles. Lake Tahoe is the third deepest lake in North America and the tenth deepest in the world. Its greatest measured depth is 1,645 feet and averages 1,000 feet. (In North America, Crater Lake in Oregon at 1,930 feet and the Great Slave Lake in Canada at 2,010 feet are deeper.) The floor of the basin is near the 4,580 foot elevation, lower than the surface of the Carson Valley to the east! Lake Tahoe’s average surface elevation is 6,225 feet above sea level, making it the highest lake of its size in the United States. Its exact elevation, controlled by a dam in Tahoe City, depends on how much water flows in from the mountains and how much is let out into the Truckee River.

Unlike most bodies of water in North America, Lake Tahoe’s water does not eventually flow into the ocean. The Truckee River, its only outlet, flows east through Reno and into Pyramid Lake in Nevada.

The water temperature near the surface generally cools to 40 F to 50 F (4.4 C to 10 C during February and March, and warms to 65 F to 70 F (18.3 C to 21.1 C) August and September. Below 600-700 foot depths, the water temperature remains a constant 39 F (3.9 C).

The water in Lake Tahoe could cover a flat area the size of California by 14 inches. This is also enough to supply everyone in the United States with 50 gallons of water per day for 5 years. And believe it or not, the amount of water that evaporates from the surface of Lake Tahoe every year could supply a city the size of Los Angeles for 5 years.

The lake is so clear that objects can be seen up to depths of 120 feet. One reason is that 40% of the precipitation falling into the Lake Tahoe Basin, lands directly on the lake. The remaining precipitation drains through the decomposed granitic soils found in marshes and meadows, creating a good filtering system for water. These soils are relatively sterile, therefore water filtered through them enter the lake relatively “pure”. Another contributing factor is the exportation of all sewage from the Lake Tahoe Basin. Although Lake Tahoe is going through a natural aging process (succession), filling up with sediments like any other lake, additional amounts are washing into the lake as slopes are cleared for construction . Each sediment particle carries nutrients which stimulate algae growth that will eventually cloud the famous clarity of the lake. Development of marshes and stream zones, such as the Tahoe Keys, prevents filtration of incoming water from the mountains and allows more sedimentation to occur. However, the National Forest lands of the Lake Tahoe Basin, covering more than two thirds of the basin's watershed, play an important role in assuring that the beauty of the Lake and the high quality of its water resources are safeguarded.