Rivers are a powerful erosive force. Over time, they wear down even the tallest mountains and carry away the physical evidence that the mountains once stood. Powered by the force of gravity, the world's rivers deliver about 20 billion tons of rock and soil to the oceans each year. Depending on the coarseness and volume of sediments they carry and the speed at which they move, rivers erode their channels at different rates.
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.
Debris flows result when flash flooding from intense rainfall or rapid snowmelt moves loose rock and boulders down canyon walls and side channels. Debris flows are particularly likely to occur in desert conditions, where the hard-baked soil and scarcity of deep-rooted vegetation do little to absorb water or hold rocks in place during heavy rains. Piles of loose soil and rock that collect in a channel are picked up by the water and carried quickly downstream. The action of the debris tears away at the banks and bed of the river, thus widening the surrounding canyon. The faster the water moves the more debris it carries and the more erosive the debris flow becomes. Debris flows, which may resemble a river of concrete more than a river of water, can carry car-sized boulders and travel a mile or more, depending on the terrain. When they reach more level ground and the flow stops, thick deposits form over a broad area. The size of the deposits downstream reflects the extent to which erosion has occurred upstream.
Of 36 Colorado River tributaries recently studied, all had characteristic debris-flow deposits. Nearly 60 percent showed evidence of debris-flow activity during the last 25 years. Composed of 10 to 40 percent sand by weight, debris flows in the Grand Canyon transport much of the sediment that builds beaches along the river. Large boulders transported by debris flows may also create or change its rapids.