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<br />2 The State of the Colorado River Ecosystem in Grand Canyon
<br />
<br />bill that established Grand Canyon as a national park
<br />on February 26, 1919, in recognition of its exceptional
<br />natural beauty and geologic wonders. Grand Canyon
<br />National Park is also of cultural and spiritual significance
<br />to many of the region's Native Americans and contains
<br />more than 2,600 documented prehistoric ruins, which
<br />span thousands of years and provide an important record
<br />of human adaptation to an arid environment. In addi-
<br />tion to its geologic and cultural significance, the Grand
<br />Canyon ecosystem is home to a diverse array of plants
<br />and animals such as the humpback chub (Gila cypha) and
<br />the southwestern willow l1ycatcher (Empidonax traillii exti-
<br />mus), both of which are species that are federally listed as
<br />endangered. Because of its global significance as a natural
<br />and cultural treasure, Grand Canyon National Park was
<br />inscribed by the United Nations Educational, Scien-
<br />tific and Cultural Organization (UNESCO) as a World
<br />Heritage Site in 1979.
<br />The GCPA (see timeline) directs the Secretary of the
<br />Interior to operate Glen Canyon Dam and exercise other
<br />authorities "in such a manner as to protect, mitigate
<br />adverse impacts to, and improve the values for which
<br />Grand Canyon National Park and Glen Canyon National
<br />Recreation Area were established, including, but not
<br />limited to natural and cultural resources and visitor use"
<br />(GCPA, sec. 1802(a)). As a result, the Glen Canyon Dam
<br />Adaptive Management Program, created by the 1996
<br />Record of Decision (ROD) for the operation of Glen
<br />Canyon Dam, focuses on a study area that encompasses
<br />the Colorado River corridor from Glen Canyon Dam to
<br />the western boundary of Grand Canyon National Park.
<br />The study area includes the approximately 15 river miles
<br />(RM) of river from the dam to Lees Ferry within Glen
<br />Canyon National Recreation Area and the entire 277-
<br />RM river corridor bclow Lees Ferry and within Grand
<br />Canyon National Park. In total, the study area includes
<br />some 293 RM of the Colorado River (fig. 1).
<br />
<br />Administrative History
<br />
<br />The Colorado River is the most important water
<br />resource in the American \Vest, serving as the main
<br />source of drinking water for more than 25 million people
<br />(Water Education Foundation, 2001). The Colorado
<br />River has been extensively engineered to meet the
<br />demands placed upon it (see timeline). There arc 22
<br />major storage reservoirs in the Colorado River Basin
<br />and 8 major out-of-basin diversions (Pontius, 1997).
<br />The two largest storage projects-Hoover and Glen
<br />Canyon Dams-arc located on either end of Grand
<br />
<br />Canyon National Park. Glen Canyon Dam is located
<br />just north of the Grand Canyon National Park bound-
<br />ary, where it creates Lake Powell. At full capacity, Lake
<br />Powell was designed to hold 27 million acre-feet (maD
<br />(>33,000 million m3) of water and is the key storage unit
<br />within the Colorado River Storage Project (CRSP) (U.S.
<br />Department of the Interior, 1970).
<br />Signed into law by President Dwight D. Eisenhower
<br />in 1956, the Colorado River Storage Project Act
<br />authorized four mainstem water-storage units, includ-
<br />ing Glen Canyon Dam. Construction of Glen Canyon
<br />Dam began on September 29, 1956, and the last bucket
<br />of concrete was poured on September 13, 1963 (U.S.
<br />Department of the Interior, 1970). The regulation
<br />of the Colorado River by Glen Canyon Dam began
<br />with the closure of the dam in 1963 and when Lake
<br />Powell began filling. The CRSP reservoirs allow the
<br />upper basin States-Utah, Colorado, Wyoming, and
<br />New Mexico-to store water in wet years and release
<br />water in times of shortages, thereby enabling the upper
<br />basin to meet its obligations under the 1922 Colorado
<br />River Compact while also maximizing future water uses
<br />(Ingram and others, 1991). To repay Federal expendi-
<br />tures for the water-storage units and supplement the costs
<br />of related irrigation units, CRSP dams were equipped
<br />with hydroelectric generators to produce salable power.
<br />Glen Canyon Dam operates eight electric generators,
<br />which produce 78% of the total power generated by the
<br />CRSP (Hughes, 1991). In 2004, Glen Canyon Dam gen-
<br />erated approximately 3.3 million megawatthours (MWh).
<br />The power is sold to approximately 200 wholesale
<br />customers-municipal and county utilities, rural electric
<br />cooperatives, U.S. Government installations, and other
<br />nonprofit organizations-located primarily in six States:
<br />Arizona, Colorado, Utah, .Wyoming, New Mexico, and
<br />Nevada (National Research Council, 1996).
<br />
<br />Natural History
<br />
<br />Before the dam, the Colorado River was a sediment-
<br />rich river that when swelled with snowmelt from the
<br />Rocky Mountains transported large quantities of sedi-
<br />ment during spring and early summer and commonly
<br />produced flood events. Peak discharge typically reached
<br />85,000 cubic feet per second (cfs) at 2-yr intervals and
<br />120,000 cfs at 6-yr intervals during these seasonal flood
<br />events (Topping and others, 2003). By contrast, flows of
<br />less than 3,000 cfs were typical during late summer, fall,
<br />and winter. Prior to the dam, water temperature also
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