page_title: Lake Powell
section_title: Geology
hierarchical_section_title: Lake Powell / Geology
context_page_description: Lake Powell is a man-made reservoir on the Colorado River, straddling the border between Utah and Arizona, United States. Most of Lake Powell, along with Rainbow Bridge National Monument, is located in Utah. It is a major vacation spot visited by approximately two million people every year. It is the second largest man-made reservoir by maximum water capacity in the United States behind Lake Mead, storing 24,322,000 acre feet of water when full. However, due to high water withdrawals for human and agricultural consumption, and because of subsequent droughts in the area, Lake Mead has fallen below Lake Powell in size several times during the 21st century in terms of volume of water, depth and surface area.
  Lake Powell was created by the flooding of Glen Canyon by the Glen Canyon Dam, which also led to the creation of Glen Canyon National Recreation Area, a popular summer destination. The reservoir is named for explorer John Wesley Powell, a one-armed American Civil War veteran who explored the river via three wooden boats in 1869. In 1972, Glen Canyon National Recreation Area was established.
context_section_description: Glen Canyon was carved by differential erosion from the Colorado River over an estimated 5 million years.  The Colorado Plateau, through which the canyon cuts, arose some 11 million years ago.  Within that plateau lie layers of rock from over 300 million years ago to the relatively recent volcanic activity. Pennsylvanian and Permian formations can be seen in Cataract Canyon and San Juan Canyon. The Moenkopi Formation, which dates from 230 million years ago (Triassic Period), and the Chinle Formation are found at Lees Ferry and the Rincon.  Both formations are the result of the ancient inland sea that covered the area.  Once the sea drained, windblown sand invaded the area, creating what is known as Wingate Sandstone.
  The more recent (Jurassic Period) formations include Kayenta Sandstone, which produces the trademark blue-black "desert varnish" that streaks down many walls of the canyons.  Above this is Navajo Sandstone.  Many of the arches, including Rainbow Bridge, lie at this transition point.  This period also includes light yellow Entrada Sandstone, and the dark brown, almost purple Carmel Formation.  These latter two can be seen on the tops of mesas around Wahweap, and the crown of Castle Rock and Tower Butte.  Above these layers lie the sandstone, conglomerate and shale of the Straight Cliffs Formation that underlies the Kaiparowits Plateau and San Rafael Swell to the north of the lake.
  The confluences of the Escalante, Dirty Devil and San Juan rivers with the Colorado lie within Lake Powell.  The slower flow of the San Juan river has produced goosenecks where 5 miles (8.0 km) of river are contained within 1-mile (1.6 km) on a straight line.
caption: Rainbow Bridge
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