Glimpses of the Grand Canyon (3)
Erosive forces continue to shape the Grand Canyon today, millions of years after it began to form. Normal channel erosion alone, however, cannot explain the many kilometers that separate its rims. The strongest explanation holds that the major factor in the widening of the canyon has been activity from tributary drainage systems -- the side streams, rivulets, and gullies outside the main river channel. The greatest erosive force in these tributaries is the fast-moving landslide called a debris flow.