How do rivers carve canyons?

A river carves a canyon through the steady process of erosion. The moving water itself wears at the rock, but even more powerful is the sand, gravel, and boulders the river carries, which grind and scour the riverbed like sandpaper. Over very long periods, especially where the land is slowly rising, the river cuts downward faster than the canyon walls wear back, producing a deep, steep-sided gorge. The Grand Canyon, for example, was carved by the Colorado River over millions of years and reaches depths of around 1,800 metres. Harder and softer rock layers erode at different rates, shaping the canyon walls.