Almost all the landscape of Earth is the result of erosion. Only small areas are constructional, including volcanoes, river plains and deltas, coral reefs, and the surfaces of glaciers and ice sheets. Mountain ranges are built by forces within the Earth, but individual mountains are only the remnants left behind by the erosion of the valleys that separate them. Although hills and mountains are visually dominant, valleys are the active, evolving part of a landscape.
Rivers do most of the erosional work of landscape shaping. Rivers have been credited with 85 to 90 percent of the total sediment transported to the sea. (A grain of sand may require 1,000 years to be carried through a large river network to the sea.) This contrasts with about 7 percent transported by glaciers, 1 to 2 percent by groundwater and ocean waves, and less than 1 percent each by wind and volcanoes. Presumably, these volumes of sediment transport are proportional to the relative volumes of landscape eroded by the various agents. Therefore, any general consideration of landscape development must deal primarily with the work of rivers.
River networks are multiple-branching systems, beginning with tiny rivulets flowing downhill during rainstorms that join into rills and gullies and eventually into creeks and streams that continue to flow even when the rain has stopped. These are fed by water that has soaked into the soil and that then slowly reemerges from streambanks to maintain what is called the base flow of the stream. Small tributary streams eventually merge into progressively larger ones and finally into a trunk stream that enters the sea. The entire branching network covers a drainage basin , also known as a watershed. Drainage basins of small streams are nested within those of these trunk systems and are the basic units of landscapes.
In their headwater regions, river networks are primarily erosional. They acquire soil and weathered rock debris from hillslopes and valley walls, ranging in grain size from fine mud, sand, and gravel to huge boulders. Each sediment size can be moved by an appropriate river-current velocity. As larger pieces of rock roll and tumble in a river, they are gradually reduced in size until they can be moved more easily.
During heavy runoff intervals, the discharge of water in rivers increases by factors of one thousand to ten thousand, or even more. When the river channel can no longer accommodate all the water, some of it overflows onto the adjacent floodplain . For many rivers, overbank flooding is nearly an annual event related to seasonal precipitation or snowmelt.
Most erosion by rivers is accomplished during the brief intervals of high discharge and flooding. At these times, rivers flow not only faster, deeper, and wider, but also much muddier. Because of the great increase in turbulence in a deep, fast-flowing stream, it can carry one hundred to one thousand times more sediment than it can at low-water stages. Thus, if a river at flood has one thousand times as much water, and that water is one thousand times muddier than at low flow, then one million times more sediment is being moved. Such numbers are typical of actual measured values.
Rivers erode their channels by:
As with sediment transport, most riverbed erosion occurs during brief times of high discharge. By far, however, the most sediment in a river comes from the hillslopes on the valley sides rather than by direct river action. In that sense, rivers act more like passive gutters or storm drains that are forced to carry water and sediment that are delivered to them from their drainage basins rather than actively producing their own.
Yet, unless a river has eroded at least a narrow slot in the landscape, there will be no adjacent hillslopes from which soil and rock can be delivered to the river channel by gravity. Thus, hillslope and river development are closely integrated during landscape evolution. Any abrupt change in any part of the system, whether by a landslide, climate change, or human intervention, will propagate uphill across the landscape as well as downstream through the drainage network. Landscape evolution involves a constant reaction among many processes that tend toward a balanced state.
On a new landscape such as might be created by a volcanic eruption, the uplift of the ocean floor, or the melting of glacier cover, the initial river runoff follows chance irregularities downhill toward the sea. Lakes form in closed basins, and then overflow to continue the downhill trend. This kind of initial drainage network is called a consequent system because it is a direct consequence of the preexisting landscape. Generally, though, the drainage network becomes better organized as basins are filled in and shortcuts are established by erosion. As they evolve, drainage networks become well adjusted to their climate region and the rock types over which they flow.
Rivers gradually change their courses to follow belts of the most easily eroded rocks, avoiding resistant rocks that form highlands. River networks that show a high degree of adjustment to rock structure are called subsequent systems because whatever their original patterns may have been, they have subsequently evolved in response to structural control. Typically, subsequent drainage networks consist of either long, parallel tributaries eroded along belts of weak rock, or of angular "zigzag" valleys following fracture patterns in the rocks.
Where a river crosses rocks resistant to erosion, it usually cuts a steep, narrow gorge or canyon. Its gradient is steeper because it erodes the underlying rocks less rapidly. Channel segments can become rapids or waterfalls as they lag behind the more rapid erosion farther downstream. These channel segments then become the local base level for erosion farther upstream because the valley upstream cannot be lowered any faster than the resistant rock can be eroded. It is common to find a beautiful broad, open mountain valley eroded in soft rocks upstream from a steep, narrow gorge cut through resistant rocks. The gorges form natural sites for dams by which the upstream valleys are flooded as reservoirs.
If a river manages to erode through or around a resistant rock obstacle, the base level of the upstream region is suddenly lowered. As the river then entrenches the valley floor, sequences of river terraces may be left behind on the valley sides, recording the progressive downcutting. Other causes of river terraces include such events as landslides that can dam a gorge until the upstream valley fills with sediment. When the dam is broken, the river quickly entrenches the valley fill, again leaving behind terraces. Many river valleys in formerly glaciated regions have terraces that were built when temporary dams created by glaciers blocked main valleys and flooded tributary valleys.
Arthur L. Bloom
Bloom, Arthur L. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms, 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1998.
Ritter, Dale F., R. Craig Kochel, and Jerry R. Miller. Process Geomorphology, 4th ed.New York: McGraw-Hill, 2002.