Terms in this set (40)

In May 1983, three years after Lake Powell had first filled, an unusually long-lasting winter over the 108,335-square-mile (280,590 km2) Colorado River basin above the Glen Canyon Dam ended with a sudden influx of warm weather. Rain and snowmelt joined together to produce a combined inflow of over 111,500 cubic feet (3,160 m3) per second,[7] however, the Bureau of Reclamation predicts that the probable maximum flood at Glen Canyon is 697,000 cubic feet (19,700 m3) per second, almost 7 times that total, [8] and the average annual peak flow prior to 1963 was 93,400 cubic feet (2,640 m3) per second.[9] The Glen Canyon Dam has two tunnel spillways, capable of bypassing 276,000 cu ft/s (7,815 m3/s).

In the Colorado River floods of 1983, the Glen Canyon Dam exhibited poorly designed and cavitating spillways, strong structural vibrations and as a result, an inability to bypass enough floodwater in the event of a major flood. After the 1983 flood, however, the spillways were redesigned to reduce or eliminate the cavitation damage seen in the 1983 flood by the use of air slots. These spillway revisions were tested successfully in 1984.[18] This revision basically eliminates the possibility of the dam being undermined via erosion of the spillways, and reduces the danger of the dam being over topped - the only reason the lake rose as high as it did was due to the flashboards placed atop the closed spillway gates.[18] If the reservoir rises over the dam crest it would overtop the dam and gradually erode it. This would be dangerous not only because the dam power plant is located at its base, but because the dam is built within porous, erosion-prone Navajo Sandstone; an overtopping would likely lead to erosion of the dam abutments and a subsequent failure.[11] In 1990, the Bureau of Reclamation prepared a study for a Glen Canyon Dam failure, predicting that the resulting flood would scour the bottom few hundred feet of the Grand Canyon, overtop Hoover Dam, and cause severe damage all along the lower Colorado River.[
John D. Lee, for who Lee's Ferry is now named, came to the crossing in 1870 with the goal of setting up a permanent ferry service for Mormon settlers heading south to Arizona.[10] In 1857, Lee had partaken in the Mountain Meadows Massacre, in which a group of Mormons and Native Americans attacked a passing non-Mormon wagon train(Fancher, a guy who named his lead ox ?Brighham Young cuz he liked to whip him), killing about 120 people. The ill-conceived attack was the result of feuding between the Mormons, who wished to retain their independent state in Utah, and the U.S. federal government, which was trying to assert authority by force over the area. Lee was part of a group called Sons of Dan whose MO was dressing up like Piaute indians and killing. Lee talked Fancher's group into giving up their guns in exchange for halting hostilities. The Lee and his 50 indian Paiutes slaaughtered everyone. Years after the massacre, the LDS church sent Lee to the remote Colorado River crossing to shield him from the law as part of an attempted cover-up.[19]
Lee arrived in September with two of his 19 wives and some of his 57 children, and created a small settlement named Lonely Dell. The ferry was formally established in January 1873, with the launching of the Colorado, the first of many boats that would ply the treacherous and fluctuating river at this point.[17]. Four years later, Lee was finally accosted by the U.S. government and tried for his role in the massacre. Found guilty, he was executed by firing squad at Mountain Meadows on March 28, 1877 - the sole scapegoat for the more than fifty Mormon and Native American men that had participated in the massacre.11 yrs later the church reinstated him posthumously.
There are approximately 1,737 known species of vascular plants, 167 species of fungi, 64 species of moss and 195 species of lichen found in Grand Canyon National Park.[50] This variety is largely due to the 8,000 foot elevation change from the Colorado River up to the highest point on the North Rim.[50] Grand Canyon boasts a dozen endemic plants (known only within the Park's boundaries) while only ten percent of the Park's flora is exotic.[50] Sixty-three plants found here have been given special status by the U.S. Fish and Wildlife Service.[50]
The Mojave Desert influences the western sections of the canyon, Sonoran Desert vegetation covers the eastern sections, and ponderosa and pinyon pine forests grow on both rims.[51]
Natural seeps and springs percolating out of the canyon walls are home to 11% of all the plant species found in the Grand Canyon.[51] The Canyon itself can act as a connection between the east and the west by providing corridors of appropriate habitat along its length.[51] The canyon can also be a genetic barrier to some species, like the Tassel-eared squirrel.[51]
The aspect, or direction a slope faces, also plays a major role in adding diversity to the Grand Canyon. North-facing slopes receive about one-third the normal amount of sunlight, so plants growing there are similar to plants found at higher elevations, or in more northern latitudes.[51] The south-facing slopes receive the full amount of sunlight and are covered in vegetation typical of the Sonoran Desert.[51]
The Grand Canyon (Hopi: Ongtupqa; Yavapai: Wi:kaʼi:la) is a steep-sided canyon carved by the Colorado River in the United States in the state of Arizona. It is contained within and managed by Grand Canyon National Park, the Hualapai Tribal Nation, and the Havasupai Tribe. President Theodore Roosevelt was a major proponent of preservation of the Grand Canyon area, and visited it on numerous occasions to hunt and enjoy the scenery. It is considered one of the Seven Natural Wonders of the World.[1]
The Grand Canyon is 277 miles (446 km) long, up to 18 miles (29 km) wide and attains a depth of over a mile (6,000 feet or 1,800 metres).[2] Nearly two billion years of the Earth's geological history has been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted.[3] While the specific geologic processes and timing that formed the Grand Canyon are the subject of debate by geologists,[4] recent evidence suggests the Colorado River established its course through the canyon at least 17 million years ago.[5][6] Since that time, the Colorado River continued to erode and form the canyon to its present-day configuration.[7]
For thousands of years, the area has been continuously inhabited by Native Americans who built settlements within the canyon and its many caves. The Pueblo people considered the Grand Canyon ("Ongtupqa" in Hopi language) a holy site and made pilgrimages to it.[8] The first European known to have viewed the Grand Canyon was García López de Cárdenas from Spain, who arrived in 1540.[9]
The Grand Canyon is a huge fissure in the Colorado Plateau that exposes uplifted Proterozoic and Paleozoic strata, and is also one of the 19 distinct physiographic sections of the Colorado Plateau province. It is not the deepest canyon in the world (Kali Gandaki Gorge in Nepal is far deeper), nor the widest (Capertee Valley in Australia is about 0.6 mi/1 km wider and longer than Grand Canyon); however, the Grand Canyon is known for its visually overwhelming size and its intricate and colorful landscape. Geologically it is significant because of the thick sequence of ancient rocks that is beautifully preserved and exposed in the walls of the canyon. These rock layers record much of the early geologic history of the North American continent.

A map of the Grand Canyon and surrounding areas, c. 1908.
Uplift associated with mountain formation later moved these sediments thousands of feet upward and created the Colorado Plateau. The higher elevation has also resulted in greater precipitation in the Colorado River drainage area, but not enough to change the Grand Canyon area from being semi-arid. The uplift of the Colorado Plateau is uneven, and the Kaibab Plateau that Grand Canyon bisects is over a thousand feet higher at the North Rim (about 1,000 ft or 300 m) than at the South Rim. Almost all runoff from the North Rim (which also gets more rain and snow) flows toward the Grand Canyon, while much of the runoff on the plateau behind the South Rim flows away from the canyon (following the general tilt). The result is deeper and longer tributary washes and canyons on the north side and shorter and steeper side canyons on the south side.
Temperatures on the North Rim are generally lower than the South Rim because of the greater elevation (averaging 8,000 ft/2,438 m above sea level).[10] Heavy rains are common on both rims during the summer months. Access to the North Rim via the primary route leading to the canyon (State Route 67) is limited during the winter season due to road closures.[11]
The Grand Canyon is part of the Colorado River basin which has developed over the past 40 million years. A recent study places the origins of the canyon beginning about 17 million years ago. Previous estimates had placed the age of the canyon at 5 to 6 million years.[12] The study, which was published in the journal Science in 2008, used uranium-lead dating to analyze calcite deposits found on the walls of nine caves throughout the canyon.[13] There is a substantial amount of controversy because this research suggests such a substantial departure from prior widely supported scientific consensus.[14] In December 2012, a study published in the journal Science claimed new tests had suggested the Grand Canyon could be as old as 70 million years.[15] However, this study has been criticized as "[an] attempt to push the interpretation of their new data to their limits without consideration of the whole range of other geologic data sets."[12]
The result of all this erosion is one of the most complete geologic columns on the planet.
The major geologic exposures in the Grand Canyon range in age from the 2 billion year old Vishnu Schist at the bottom of the Inner Gorge to the 230 million year old Kaibab Limestone on the Rim. There is a gap of about one billion years between the stratum that is about 500 million years old and the lower level, which is about 1.5 billion years old. This large unconformity indicates a period of erosion between two periods of deposition.
Many of the formations were deposited in warm shallow seas, near-shore environments (such as beaches), and swamps as the seashore repeatedly advanced and retreated over the edge of a proto-North America. Major exceptions include the Permian Coconino Sandstone, which contains abundant geological evidence of aeolian sand dune deposition. Several parts of the Supai Group also were deposited in non-marine environments.
The great depth of the Grand Canyon and especially the height of its strata (most of which formed below sea level) can be attributed to 5,000 to 10,000 feet (1500 to 3000 m) of uplift of the Colorado Plateau, starting about 65 million years ago (during the Laramide Orogeny). This uplift has steepened the stream gradient of the Colorado River and its tributaries, which in turn has increased their speed and thus their ability to cut through rock (see the elevation summary of the Colorado River for present conditions).
Weather conditions during the ice ages also increased the amount of water in the Colorado River drainage system. The ancestral Colorado River responded by cutting its channel faster and deeper.
The base level and course of the Colorado River (or its ancestral equivalent) changed 5.3 million years ago when the Gulf of California opened and lowered the river's base level (its lowest point). This increased the rate of erosion and cut nearly all of the Grand Canyon's current depth by 1.2 million years ago. The terraced walls of the canyon were created by differential erosion.[16]
Between three million and 100,000 years ago, volcanic activity deposited ash and lava over the area which at times completely obstructed the river. These volcanic rocks are the youngest in the canyon.