13.5: The Great Basin and the Basin and Range

The Great Basin is the largest area of interior drainage in North America, meaning there is no outlet to the ocean and all precipitation remains in the basin or is evaporated. It covers western Utah, most of Nevada, and extends into eastern California, southern Oregon, and southern Idaho. Streams in the Great Basin gather runoff and groundwater discharge and deliver it to lakes and playas within the basin. A subregion within the Great Basin is the Basin and Range which extends from the Wasatch Front in Utah, west across Nevada to the Sierra Nevada Mountains of California. The basins and ranges referred to in the name are horsts and grabens, formed by normal fault blocks from the crustal extension, as discussed in the chapters on Plate Tectonics and Crustal Deformation and Earthquakes. The lithosphere of the entire area has stretched by a factor of about 2, meaning from end to end, the distance has doubled over the past 30 million years or so. Valleys without outlets form individual basins, each of which is filled with alluvial sediments leading into playa depositional environments. During the recent Ice Age, the climate was more humid and while glaciers were forming in some of the mountains, pluvial lakes formed covering large areas. During the Ice Age, valleys in much of western Utah and eastern Nevada were covered by Lake Bonneville. As the climate became arid after the Ice Age, Lake Bonneville dried leaving the Great Salt Lake in Utah as a remnant.

Figure \(\PageIndex{1}\): Map of the Great Basin.

The desert of the Basin and Range extends from about 35°N to near 40°N and results from a rain shadow effect created by westerly winds from the Pacific rising and cooling over the Sierras, becoming depleted of moisture by precipitation on the western side. The result is relatively dry air descending across Nevada and western Utah. A journey from the Wasatch Front southwest to the Pacific Ocean will show stages of desert landscape evolution from the fault block mountains of Utah with sharp peaks and alluvial fans at the mouths of canyons, through landscapes in Southern Nevada with bajadas along the mountain fronts, to the landscapes in the Mojave Desert of California with subdued inselbergs sticking up through a sea of coalesced bajadas. These landscapes illustrate the evolutionary stages of desert landscape development.

Figure \(\PageIndex{2}\): Aerial view of several parallel mountain ranges in and intervening basins in the Basin and Range, between the Last Chance Range and Eureka Valley (in Death Valley National Park in California) and the Sierra Nevada Mountains. The Eureka Sand Dunes can be seen in the lower left part of the photo. (By Marli Miller; via Geologic Time Pics.)

Desertification

When previously arable land suitable for agriculture transforms into desert, this process is called desertification. Plants and humus-rich soil promote groundwater infiltration and water retention. When an area becomes more arid due to changing environmental conditions, the plants and soil become less effective in retaining water, creating a positive feedback loop of desertification. This self-reinforcing loop spirals into increasingly arid conditions and further enlarges the desert regions.

Desertification may be caused by human activities, such as unsustainable crop cultivation practices, overgrazing by livestock, overuse of groundwater, and global climate change. Human-caused desertification is a serious worldwide problem [22]. The world map figure shows what areas are most vulnerable to desertification. Note the red and orange areas in the western and midwestern regions of the United States, which also cover large areas of arable land used for raising food crops and animals. The creation of the Dust Bowl in the 1930s is a classic example of a high-vulnerability region impacted by human–caused desertification. As demonstrated in the Dust Bowl, conflicts may arise between agricultural practices and conservation measures. Mitigating desertification while allowing farmers to make a survivable living requires public and individual education to create community support and understanding of sustainable agriculture alternatives.

Figure \(\PageIndex{3}\): World map showing desertification vulnerability.

Reference

22. Geist, H. The causes and progression of desertification. (Ashgate Aldershot, 2005).