Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds, those with no outlets to the ocean, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California. It is noted for both its arid climate and the basin and range topography that varies from the North American low point at Badwater Basin in Death Valley to the highest point of the contiguous United States, less than away at the summit of Mount Whitney. The region spans several physiographic divisions, biomes, ecoregions, and deserts.
Definition
The term "Great Basin" is applied to hydrographic, biological, floristic, physiographic, topographic, and ethnographic geographic areas. The name was originally coined by John C. Frémont, who, based on information gleaned from Joseph R. Walker as well as his own travels, recognized the hydrographic nature of the landform as "having no connection to the ocean". The hydrographic definition is the most commonly used, and is the only one with a definitive border. The other definitions yield not only different geographical boundaries of "Great Basin" regions but regional borders that vary from source to source.The Great Basin Desert is defined by plant and animal communities, and, according to the National Park Service, its boundaries approximate the hydrographic Great Basin but exclude the southern "panhandle".
The Great Basin Province was defined by botanist Armen Takhtajan to extend well beyond the boundaries of the hydrographically defined Great Basin: it includes the Snake River Plain, the Colorado Plateau, the Uinta Basin, and parts of Arizona north of the Mogollon Rim.
The Great Basin physiographic section is a geographic division of the Basin and Range Province defined by Nevin Fenneman in 1931. The United States Geological Survey adapted Fenneman's scheme in their Physiographic division of the United States. The "section" is somewhat larger than the hydrographic definition.
The Great Basin culture area, or indigenous peoples of the Great Basin, is a cultural classification of indigenous peoples of the Americas and a cultural region located between the Rocky Mountains and the Sierra Nevada. The culture area covers approximately, or just less than twice the area of the hydrographic Great Basin.
Hydrology
The hydrographic Great Basin is a area that once drained internally. All precipitation in the region evaporated, sank underground or flowed into lakes. As observed by Fremont, creeks, streams, or rivers find no outlet to either the Gulf of Mexico or the Pacific Ocean. The region is bounded by the Wasatch Mountains to the east, the Sierra Nevada and Cascade Ranges to the west, and the Snake River Basin to the north. The south rim is less distinct. The Great Basin includes most of Nevada, half of Utah, substantial portions of Oregon and California, and small areas of Idaho, Wyoming, and Baja California, Mexico. The term "Great Basin" is slightly misleading; the region comprises many small basins. The Great Salt Lake, Pyramid Lake, and the Humboldt Sink are a few of the "drains" in the Great Basin. The Salton Sink is another closed basin within the Great Basin.The Great Basin Divide separates the Great Basin from the watersheds draining to the Pacific Ocean. The southernmost portion of the Great Basin is the watershed area of the Laguna Salada. The Great Basin's longest and largest river is the Bear River of, and the largest single watershed is the Humboldt River drainage of roughly. Most Great Basin precipitation is snow, and the precipitation that neither evaporates nor is extracted for human use will sink into groundwater aquifers, while evaporation of collected water occurs from geographic sinks. Lake Tahoe, North America's largest alpine lake, is part of the Great Basin's central Lahontan subregion.
Ecology
The hydrographic Great Basin contains multiple deserts and ecoregions, each with its own distinctive set of flora and fauna. The ecological boundaries and divisions in the Great Basin are unclear.The Great Basin overlaps four different deserts: portions of the hot Mojave and Colorado Deserts to the south, and the cold Great Basin and Oregon High Deserts in the north. The deserts can be distinguished by their plants: the Joshua tree and creosote bush occur in the hot deserts, while the cold deserts have neither. The cold deserts are generally higher than the hot and have more even spread of precipitation throughout the year.
The climate and flora of the Great Basin are strongly dependent on elevation; as the elevation increases, the temperature decreases and precipitation increases. Because of this, forests can occur at higher elevations. Utah juniper/single-leaf pinyon and mountain mahogany form open pinyon-juniper woodland on the slopes of most ranges. Stands of limber pine and Great Basin bristlecone pine can be found in some of the higher ranges. In riparian areas with dependable water cottonwoods and quaking aspen groves exist.
Because the forest ecosystem is distinct from a typical desert, some authorities, such as the World Wildlife Fund, separate the mountains of the Great Basin desert into their own ecoregion: the Great Basin montane forests. Many rare and endemic species occur in this ecoregion, because the individual mountain ranges are isolated from each other. During the Last Glacial Period, the Great Basin was wetter. As it dried during the Holocene epoch, some species retreated to the higher isolated mountains and have high genetic diversity.
Other authorities divide the Great Basin into different ecoregions, depending on their own criteria. Armen Takhtajan defined the "Great Basin floristic province". The U.S. Environmental Protection Agency divides the Great Basin into three ecoregions roughly according to latitude: the Northern Basin and Range ecoregion, the Central Basin and Range ecoregion, and the Mojave Basin and Range ecoregion.
Fauna
Great Basin wildlife includes pronghorn, mule deer, mountain lion, and lagomorphs such as black-tailed jackrabbit and desert cottontail and the coyotes that prey on them. Packrats, kangaroo rats and other small rodents are also common, and are predominantly nocturnal. Elk and bighorn sheep are present but uncommon. Small lizards such as the Great Basin fence lizard, longnose leopard lizard and horned lizard are common, especially in lower elevations. Rattlesnakes and gopher snakes are also present. The Inyo Mountains salamander is endangered. Shorebirds such as phalaropes and curlews can be found in wet areas. American white pelicans are common at Pyramid Lake. Golden eagles are also very common in the Great Basin. Mourning dove, western meadowlark, black-billed magpie, and common raven are other common bird species.Two endangered species of fish are found in Pyramid Lake: the Cui-ui sucker fish and the Lahontan cutthroat trout.
Large invertebrates include tarantulas and Mormon crickets. Exotic species, including chukar, grey partridge, and Himalayan snowcock, have been successfully introduced to the Great Basin, although the latter has only thrived in the Ruby Mountains. Cheatgrass, an invasive species which was unintentionally introduced, forms a critical portion of their diets. Feral horses and feral burros are highly reproductive, and ecosystem-controversial, alien species. Most of the Great Basin is open range and domestic cattle and sheep are widespread.
Geology
The Great Basin includes valleys, basins, lakes and mountain ranges of the Basin and Range Province.The basin and range topography is the result of extension and thinning of the lithosphere, which is composed of crust and upper mantle. Extensional environments like the Basin and Range are characterized by listric normal faulting, or faults that level out with depth. Opposing normal faults link at depth producing a horst and graben geometry, where horst refers to the upthrown fault block and graben to the down dropped fault block.
Sediment build-up over thousands of years filled the down-faulted basins between ranges and created relatively flat lacustrine plains from Pleistocene lake beds of the Great Basin. For example, after forming about 32,000 years ago, Lake Bonneville overflowed about 14,500 years ago in the Bonneville Flood through Red Rock Pass and lowered to the "Provo Lake" level. Lake Lahontan, Lake Manly, and Lake Mojave were similar Pleistocene lakes.