If any single geographical feature symbolizes the United States in the minds of world travelers, it is Arizona's Grand Canyon.This 1,904sq-mi national park, located entirely in northwestern Arizona, extends from Lees Ferry, in Glen Canyon National Recreation Area, west to the Grand Wash Cliffs, in Lake Mead National Recreation Area. Waters from seven western states-Arizona, Utah, New Mexico, Colorado and Wyoming, plus Nevada and California below the Grand Canyon-drain into the mighty Colorado River, 1,450mi long from its source in Colorado's Rocky Mountains to Mexico's Gulf of California.The Grand Canyon was carved over eons by the powerful waters of the Colorado River, forcibly aided by wind and water erosion. Today a giant swath of the earth's geological history is visible in the colorfully striated layers of rock that reach down more than a mile below the canyon's rim.This is not the world's deepest canyon, but the Grand Canyon of the Colorado River is known throughout the world for its immense and labyrinthine rock landscape.At dawn and dusk, in particular, the low-angle sun highlights the vividly colored canyon walls. Bands of green, blue, purple, pink, red, orange, gold, yellow and white define a succession of exposed ancient rock layers.Today, about 5 million tourists enter the park each year. Their impact on the environment has led the National Park Service to contemplate dramatic action to protect the Canyon for future generations.