Question: Marble Canyon, Arizona is often used by people entering the principal river of what portion of the US?
Context: The Marble Canyon Dam, also known as the Redwall Dam, was a proposed dam on the Colorado River in Arizona.  The dam was intended to impound a relatively small reservoir in the central portion of Marble Canyon to develop hydroelectric power.  Plans centered on two sites between miles 30 and 40 in the canyon.  At one point a 38 mi tunnel was proposed to a site just outside Grand Canyon National Park to develop the site's full power generation potential, reducing the Colorado River to a trickle through the park. The following is a list of major rapids and other notable features on the Colorado River through Marble Canyon and Grand Canyon below Glen Canyon Dam, in order of their position downstream of Lee's Ferry.  Ratings are given for how difficult the rapid is to navigate by whitewater raft or other water craft.  The Grand Canyon section of the Colorado River, like several other big-water Western rivers, uses a scale of 1-10 for rapids, 10 being the most difficult.  The International Scale of River Difficulty, which classifies rapids from class I to VI, is more common elsewhere in the US and internationally. Pavilion Mountain is a mountain in the Marble Range in the South Cariboo region of the South-Central Interior of British Columbia, Canada, located northeast of the ranching and First Nations community of Pavilion and to the north of Marble Canyon and immediately south of Kelly Lake, which is the focus of Downing Provincial Park.  The term Pavilion Mountain is also used to refer to the historic ranch and associated rangeland on the "bench" on the mountain's southwestern side, and also to the road which traverses that benchland and the mountain's western shoulder and is the route of the Old Cariboo Road.  The switchback descent from the summit of the road was known as the Rattlesnake Grade and was an infamous stretch of the old wagon road.  From a junction at the road-summit, a road leads east along the spine of the mountain to the peak, which is the site of a microwave relay and former fire lookout.  The mountain's only named subpeak, Mount Carson, at 2005 metres (6578 ft, prominence: 120 m), is southeast of the fire lookout and is named for the original owner of the ranch and was briefly misapplied as the name of Pavilion Mountain.  Robert Carson was one of the first settlers in the region and whose sons later became prominent MLAs and provincial cabinet ministers.  The north wall of Marble Canyon is essentially the southeast buttress of Pavilion Mountain. Marble Canyon Provincial Park is a provincial park in British Columbia, Canada, established in 1956 to protect Marble Canyon, a limestone formation at the south end of the Marble Range.  In 2001 the park was expanded to 355 hectares to include all of Pavilion Lake due to the presence of microbialites, a type of stromatolite and the lake's importance to research into astrobiology and other fields.  The park is also important in the culture of the Tskway'laxw people in whose territory it is located, and concealed in the side canyons of the gorge there are important pictograph sites.  Not included in the park but overlooking Pavilion Lake at its farther end from the main part of the canyon is Chimney Rock, the Secwepemc'tsn name for which, K'lpalekw, means "Coyote's Penis", and is an important spiritual site.  A waterfall into Crown Lake, at the park's campground, is famous among ice-climbers as "Icy BC" and the walls of Marble Canyon are a major draw to rock climbers. All three of the park's lakes are popular with recreational fishermen. The Tsk'weylecw'mc or Tsk'waylaxw'mx are a First Nations people in the Central Fraser Canyon area of the Canadian province of British Columbia.  They are a subgroup of both the St'at'imc and Secwepemc peoples and reside at and around the community of Pavilion, an unincorporated settlement comprising the Pavilion Indian Reserve and adjacent ranches on Highway 99 about 30 kilometres north of Lillooet, near Marble Canyon.  There is another reserves of which is immediately on the other side of Marble Canyon near Upper Hat Creek.  Another non-residential reserve on the opposite side of the Fraser River from Pavilion is Pavilion Indian Reserve 1A, used for hunting in that area.  They historically have close ties and a shared cultural identity with their Secwepemc neighbours, particularly the Bonaparte group, but also with the St'at'imc of Fountain and Lillooet.  The Tsk'weylecw'mc are both St'at'imc Secwepemc ethnically, and have close family ties and shared cultural traditions with the Canyon Shuswap at nearby High Bar, the Llenlleney'ten, who although Secwepemc are partly Tsilhqot'in in identity.  Their particular dialect is a hybrid of St'at'imcets and Secwepemctsin.  Most placenames in the Pavilion area are Secwepemctsin, a legacy times in the recent past, just prior to the Fraser Canyon Gold Rush, when the east bank of the Fraser, including Fountain and opposite Lillooet itself, were part of Secwepemc territory. Marble Canyon is a populated place in Coconino County, Arizona, United States.  Marble Canyon is located on U.S. Route 89A at the Navajo Bridge, 12 mi southwest of Page.  Marble Canyon has a post office with ZIP code 86036.  Marble Canyon is near Lee's Ferry, the former location of a ferry established by John D. Lee, a Mormon settler.  It is often used by people entering the Colorado River for fishing and rafting trips. Navajo Bridge is a pair of steel spandrel arch bridges that cross the Colorado River near Lee's Ferry in northern Arizona.  The newer bridge of the pair carries vehicular traffic on U.S. Route 89A (US 89A) over Marble Canyon between southern Utah and the Arizona Strip, allowing travel into a remote region north of the Colorado River including the North Rim of Grand Canyon National Park. Marble Canyon Airport (IATA: MYH, FAA LID: L41) , is a privately owned, public-use airport located one nm south west of the CBD of Marble Canyon, in Coconino County, Arizona, United States. The Colorado River is one of the principal rivers of the Southwestern United States and northern Mexico (the other being the Rio Grande).  The 1450 mi river drains an expansive, arid watershed that encompasses parts of seven U.S. and two Mexican states.  Starting in the central Rocky Mountains in the U.S., the river flows generally southwest across the Colorado Plateau and through the Grand Canyon before reaching Lake Mead on the Arizona–Nevada border, where it turns south toward the international border.  After entering Mexico, the Colorado approaches the mostly dry Colorado River Delta at the tip of the Gulf of California between Baja California and Sonora. Pediocactus bradyi is a rare species of cactus known by the common names Brady's pincushion cactus, Brady's hedgehog cactus, and Marble Canyon cactus.  It is endemic to Arizona in the USA, where it is restricted to Marble Canyon in Coconino County.  It is limited to a specific type of soil, it has a small distribution, and the species is threatened by a number of human activities.  This has been a federally listed endangered species of the United States since 1979.
Answer: Southwestern United States