Geology
The Owens River flows through part of the Basin and Range Province of North America's Great Basin. The Owens Valley is a graben or rift valley, a section of land that has dropped down between two parallel faults, while the land on either side has risen. This has resulted in the flat floor and steep, towering walls of the present-day valley. With the Sierra Nevada on the west side and the Inyo Mountains and White Mountains on the east, with the highest peaks of either range rising to over 14,000 feet (4,300 m) and the floor of the valley at a comparatively low 3,000 to 4,000 feet (910 to 1,200 m), the Owens River flows in one of the deepest valleys in the United States.
Further to the north, the Owens River basin encompasses predominantly igneous rocks and vast remnants of past volcanic activity. The upper 30 miles (48 km) of the river run through the Long Valley Caldera, an enormous 20-mile (32 km)-wide crater formed by a volcanic eruption some 760,000 years ago. The eruption's resulting ash cloud covered much of the southwestern United States, including parts of ten U.S. states. Mammoth Mountain, to the southwest (more popularly known as a major ski area) also formed from eruptions related to the Long Valley Caldera. To the north of the Caldera, extending to the Mono Lake area, lie the chain of Mono-Inyo Craters, which range in age from 400,000 to 500 years old.
During the Pleistocene at the end of the last glacial period, melting glaciers in the Sierra Nevada and Inyo/White Mountains fed prodigious amounts of runoff into the Owens River, causing it to expand to many times its current size. The increased volume of the river caused Owens Lake to rise as well, eventually spilling out the south side of the valley into the Mojave Desert. Ancient, now-abandoned river channels suggest that the extended Owens River ran south to China Lake, then east into Searles Lake, north into the Panamint Valley (where it formed Panamint Lake) and finally east into Death Valley and the ancient Lake Manly. This great inland sea was also fed by the Mojave River from the south, the Amargosa River from the east and the Death Valley Wash from the north. During this relatively short time, the Owens River became part of a vast interior drainage system that stretched east to west covering over 8,000 square miles (21,000 km2). During the peak of runoff, water from this massive basin may have even escaped to the Colorado River through a valley leading to the southeast.
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