Grand Canyon National Park started out as a forest reserve in 1893, but in 1919 it was formally re-establised as a National Park. Situated in the Northern reaches of Arizona, the park centers around the Grand Canyon itself - a great chasm cut deep into the earth through which the Colorado River runs. The National Park includes the entire river's course between the Glen Canyon and Lake Mead National Recreation Areas.
In 1975 the area of the park was almost doubled with the inclusion of the 1932-proclaimed Grand Canyon National Monument, and 1969-proclaimed Marble Canyon National Monument. Portions of the Glen Canyon and Lake Mead National Recreation Areas were also included, and the park's area today is now 4,927 kmē (492,666 ha). At the 'top' end of the Canyon lies the Glen Canyon Dam - one of the many hydro-electric dams making use of the Colorado River's ample supply of water. At the other end is the majestic Hoover Dam - the largest hydro-electric dam in the world.
The central attraction of the national park is the Grand Canyon, an immense crack in the earth's crust that runs from East to West along Arizona's Northern border and then South down the Western border, following the course of the Colorado River. The maximum canyon width within the park is 29 km, and is on average 1500 m deep. The two rims are of slightly uneven height, with the Northern rim being an average of 365 m higher than the Southern, and during the winter is closed due to heavy snowfalls.
Because of the extensive variations of elevation throughout the canyon, four seperate and distinct climate areas are created. The colder Northern rim is heavily forested with fir, pine, aspen and spruce trees, in contrast to the juniper and piņon spread thinly over the warmer Southern Rim. It takes a very durable animal to live in such harsh environments, but as usual, nature has provided wildlife - antelope, cougar and mountain sheep to name a few. Prehistoric tribes used to inhabit the areas within and surrounding the abyss - ruins of their cliff dwellings can still be seen today; mute evidence of their once-great civilizations. To the South of the park, the somewhat more modern Hayasupai have their reservation. In addition to the paved roads around the rims, numerous trails descend into the canyon. Only one of these - the Kaibab Trail - traverses the gorge from one rim to the other.
Like any spectacular location, the Grand Canyon sees a fair share of visitors, but with tourism comes ecological upset. It isn't only tourism that takes it's toll - the hydro-electric dams change the water flow, which dramatically changes the eco-system of the canyon. After the 1963 construction of Glen Canyon Dam, such changes were noticed, and in the 1990s the regional government took action, instituting a new water-management plan in 1996. Using the dam's flood-gates, a controlled release can simulate the natural spring floods that occured in the past. A year later the airspace over the canyon was restricted for small aircraft and helicopters with the aim of reducing water, air, and noise pollution.
The Hoover Dam is one of the world's major engineering achievements. The largest hydro-electric dam in the world, the immense concrete arc towers 221 m high, and is 379 m long at the crest. Like all hydro-electric dams, a large reservoir is formed by the water build-up - in this case, one of the largest artificially-created bodies of water. Lake Mead boasts an area of 694 kmē, with an 885 km shore-line (see Picture Gallery).
Part of the Boulder Canyon project, the dam's construction was begun in 1931, and took five years to complete. Originally named in memory of President Herbert Hoover, it was known for a while as the Boulder Dam until its initial name was restored in 1947. The massive hydro-electric generators the vast structure houses provide electricity to Arizona, Nevada and Southern California, capable of supplying 1.5 million kW of power.