Great Basin Desert

Great_Basin_Desert_3The sprawling Great Basin Desert gives North America a cold, interior rain shadow desert to rival Asia’s Gobi Desert. Haunted by the harsh winters of its 6,000–7,000-foot (2,000–2,333 m) elevation, screened from rain by the towering Sierra Nevadas and California to the west and the Rocky  Mountains to the east, the Great Basin’s endless mountain chains, massive salt flats, fossil lakebeds, and rolling expanses of sagebrush present  a harsh, dramatic, often monochromatic landscape that challenges the plants, animals, and people who venture into its soul-stirring expanse. The Great Basin Desert encompasses more than 190,000 square miles (492,000 sq km), filling the space between the Wasatch Mountains n the east, the Columbia Plateau on the north, the Mojave Desert to the southwest, and the massive uplift of the Mogollon Rim to the south in Arizona. Many experts also include as a division of the Great Basin Desert the lower-elevation Painted Desert and Monument Valley in Arizona. This neatly divides the Great Basin Desert into two major habitats—the rolling, mountain-punctuated, sagebrush-dominated expanses to the north and east and the dramatic, colorful, layered expanses of the Painted Desert and Monument Valley and portions of the Grand Canyon at the southern edge. Great Basin: Terrible Thirst and Endless Sagebrush The same forces that stretched, pulled, and uplifted the entire American Southwest also created the landscape of the Great Basin Desert, a portion of the larger Basin and Range Province, which extends from California’s Sierra Nevadas down into Mexico and Texas and up to Nevada and Utah. One old survey map described the mostly north-south running mountain chains of this area as giant caterpillars, crawling haphazardly northward. The mountains rise to about 9,000 feet from the mostly flat, low-lying,  down-dropped blocks of intervening desert, which lie at an elevation of Utah, Arizona, Nevada 1,000 to 6,000 feet (333 to 2,000 m). Most of the mountain ranges are about 60 to 120 miles (96–192 km) long and perhaps 3 to 15 miles (5–24 km) wide. Ecologist Ann Zwinger wrote a vivid account of the appearance of this vast desert in her book The Mysterious Lands. “The Great Basin Desert has a past-finished aspect, as if all that could be done to it has been done, and now it is old and tired and worn out, grizzled and gutted, faded and weather beaten. Sometimes the land has a worn, velvet look, tucked with arroyos, pleated with mountains, a landscape seemingly without seasons or eternally half past autumn, a landscape left out to dry, forgotten, tattered with rain, wrinkled with sun, and yet, in a peculiar sense I cannot explain, always vital and never forlorn.” Cut off from the wet storms generated over the ocean by ranges of mountains, the Great Basin is a rain shadow desert. Most areas get just This reflective arm of Lake Powell represents one of the most massive, vital, and controversial engineering projects in human history, the construction of a chain of reservoirs along the Colorado River that now provides irrigation and drinking water to people in seven states and has made possible the development of major cities in the midst of the desert. However, environmentalists insist much of the water is lost through seepage and evaporation or wasted on thirsty, government-subsidized plants like cotton. (Peter Aleshire) 6 to 12 inches (152–304 mm) of rain annually. Worse yet, 60 percent of that moisture comes during the cold, frost-prone, snow-dusted winters, when it can do plants and animals the least good. Moreover, the region forms a great, thirsty bowl in which almost all of the rivers and streams start in scattered, rugged mountaintops and run down to deep low points, with no exterior drainage. Therefore, the streams tend to gush during the short and unreliable wet months to collect in low-lying depressions, which once harbored great ice age lakes, but which have now turned into salt flats, made sterile by the salt and minerals left behind by the evaporation of first those giant ice age lakes and now by
these fleeting, seasonal lakes. (The salt flats can be seen in the color insert on page C-8.) Most of the drainage ends up in the Humboldt Sink, which includes the saltier than seawater Great Salt Lake, an incongruous shimmer that is but a pathetic remnant of an ice age lake that once constituted a virtual inland sea. This whole stretched, austere linear landscape was created in the tugging and pulling that took place as the western half of North America was altered in the titanic jostling of plate tectonics. The long, slowmotion collision between the North American Plate and the Pacific Plate forced the edge of the North American Plate down beneath the larger, denser edge of the Pacific Plate. The leading edge of the North American Plate melted at 50 to 100 miles (80–160 km) beneath the surface, and some of the molten rock escaped back to the surface along fractures in the overlying rock, creating chains of volcanoes like the present-day Cascade Range  along the Pacific Northwest. Meanwhile, the conveyor belt movement of the Pacific Plate smashed a series of massive islands and fragments of other continents onto the western edge of the Pacific Plate, adding California and the towering Sierra Nevadas o the western edge of the Great Basin. This towering mountain range blocked moisture from the ocean and set the stage for the current landscape and climate. At some point, a shift in the currents in the deep mantle driving the two crustal plates together shifted, transforming the head-on collision into a massive system of transverse faults that include the infamous San Andreas. This opened up the Gulf of California, put pressure on the San Andreas Fault, and created the north-south mountain chains of North American deserts. Pressure from below pushed up blocks of the crust along fault lines while dropping the intervening  valleys. The landscape remains in turmoil. In many places, fresh faults cut across lava  flows dating back 30 million years. Earthquakes often jiggle the mountain ranges and  create fresh fault lines, although not nearly as frequently as to the west in California along the San Andreas Fault. Cataclysm Leaves Wealt h of Minerals That complex geological history has left the region with a wealth of minerals, many of them formed when the rocks in these desert ranges were deep beneath the ocean along the crack in the Earth between two crustal plates. The weak crust along such gigantic fissures lets molten rock from the deep Earth escape to the surface, as it does now along undersea ridges like the  Mid-Atlantic Ridge, which is recognized as the greatest mountain chain on Earth. As  magma and superheated water percolates through such fractures in the crust, the heat and pressure spur chemical reactions that cause a variety of minerals to form in the fissures. So the gold, copper, silver, iron, lithium, beryllium, molybdenum, and barite that have drawn miners to the Great Basin Desert for more than a century all bear witness to the complex and violent geological history of the rocks that comprise its mountains. The colorful, barren, swirled sands of the Painted Desert mark the boundary between the  reat Basin Desert and the warmer, wetter, more diverse Sonoran Desert. Composed of layers of fossilized sea bottom and dune deposits, the easily eroded, mineral-rich Painted Desert harbors treasures like the fossil-rich formations of the Petrified Forest
National Monument. (Peter Aleshire) For most of its history, the Great Basin Desert was a grassland graced by vast lakes that supported thriving populations of hunters and gathers,
stalking great herds of antelope, camels, and even mammoths. Some 12,000 years ago during the last Ice Age, places like the now-desolate Carson Sink harbored huge lakes. Flush with more than 18 inches of rainfall a year during that long cold snap in the planet’s climate, the once year-round Humboldt River flowed into Lake Lahontan, which flooded most of central and northwestern Nevada. It covered 9,000 square miles (2,330 sq km) and had a high-water mark of 4,378 feet (1,459 m) above sea level. As the climate warmed and the river withered, it became a fitful and capricious drainage system that mostly vanished into thirsty sands and salt flats. Mark Twain observed that “one of the pleasantest and most invigorating exercises one can contrive is to run and jump across the Humboldt River till he is overheated, and then drink it dry.” The once-mighty Lake Bonneville in Utah formed an even more impressive body of water during those wet, lush times. Trapped in an interior basin, the lake gradually filled up, reaching a high-water mark roughly 14,000 years ago. At that point, it rose above a natural dam of rock at Red Rock Pass in southern Idaho and rushed down to the Snake River. It spawned a flood of biblical proportions. The lake fell 10 feet (3.3 m) in four days. In six weeks, the lake level fell 350 feet (117 m), causing a massive flood rushing down into the Snake River, whose traces remain vivid and jumbled all along that path. The lake stabilized at about 4,800 feet (1,600 m) above sea level for the next 6,000 years, before climate shift gradually dried it up.
The Great Salt Lake is another remnant of the Ice Age. As it evaporated, it left behind salts and minerals. That resulted in the Bonneville Salt Flats, a place so hard packed and flat with crystallized salts that rocket-powered race cars now use it to set land speed records. The Great Basin didn’t actually become a desert until after the ancient lake vanished and the climate warmed, which means the plants and animals have perfected their elaborate adaptations to a desert environment in the past 6,000 to 10,000 years.