Lake Tahoe is a large lake situated in a granite graben near the crest of the Sierra Nevada Mountains on the CA - NV border, at 39 N, 120 W. The lake level is approximately 1895 m above MSL.. The lake is roughly oval in shape with a N-S major axis (33 km long, 18 km wide), and has a surface area of 500 square km. The land portion of the watershed has an area of 800 square km. Lake Tahoe is considered a deep lake, with an average depth of 330 m, maximum depth of 501 m, and a total volume of 156 cubic km. The surface layer of Lake Tahoe deepens during the fall and winter. Complete vertical mixing only occurs every few years. Due to its large thermal mass Lake Tahoe does not freeze in winter. The lake is renowned for its high water clarity.