Grand Canyon National Park, in Arizona, is a dramatic example of how rivers can change a landscape. All rivers flow in a downhill direction on scooped-out paths called channels. About nine million years ago, the land where Grand Canyon National Park is today a relatively flat area called the Colorado Plateau. The Colorado River used to flow across the plateau. Gradually, certain processes deep inside the earth caused the Colorado Plateau to start rising. The rising land made the channel of the Colorado River steeper, causing the river to flow faster. As the land rose higher, the river flowed even faster, cutting its channel deeper and deeper, until the channel reached its present depth of about 1 mile (1.6 km) in Grand Canyon National Park. Scientists estimate that the river cut the canyon at an average rate of 6 inches (16.5 cm) every thousand years. From measurements and samples of river water taken in the Grand Canyon, scientists calculate that the Colorado River moves about half a million tons of sediment past any given point each day!

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