River valleys

The valleys in which streams flow are produced by those streams through long-term erosion and deposition. The landforms produced by fluvial action are quite diverse, ranging from spectacular canyons to wide, gently sloping valleys. The patterns formed by stream networks are complex and generally reflect the bedrock geology and terrain characteristics.

Formation of River Valleys

River valleys consist of valley bottoms and the adjacent valley sides. Between valleys are undissected uplands known as interfluves. Valley floors may be quite narrow, as in the case of the Black Canyon of the Gunnison River, or wide, as in the case of the Huang He or the Brahmaputra. Similarly, valley sides may have very gentle rolling slopes, or they may be nearly sheer, as in the case of the Arkansas River's Royal Gorge. In many areas, the interfluves are simply divides between adjacent valleys, but on tablelands such as the Colorado Plateau, they may be tens of kilometers wide in places.

River channels and river valleys are products of the streams that flow through them. As a stream erodes a channel for itself in newly uplifted terrain, it eventually carves a valley whose form is determined by the erosive power of the stream, by the structural integrity of the rock and debris of the valley walls, by the length of time that the stream has been operating on its surroundings, and by past environmental conditions. These past environmental conditions are attested by stream channel and valley profiles that have not entirely erased landforms produced during the most recent episodes of glaciation and climate change. For example, the valley of the Mississippi River is formed in the complex deposits of a much larger, more heavily laden glacial meltwater stream that existed only 15,000 years ago. River channels and valleys may be called palimpsests, a term originally used to describe parchment manuscripts that had been partly scraped clean and then reused. Previous landscape elements are seen on the fluvial (river-carved or river-deposited) landscape, just as old words show through on a recycled piece of parchment.

In many parts of the world, streams are found flowing through valleys that appear to have been formed by a far larger stream. The valley width, amplitude of meanders, and caliber of coarse sediment are proportional to far larger stream courses. Such streams are said to be underfit, and the valleys are largely remnant features from times of wetter and cooler climates that accompanied glaciation, or the streams were glacial meltwater channels during deglaciation.

The fact that stream channels and valleys are not chance features on the landscape was first noted by British geologist John Playfair in 1802. Playfair suggested that instead of being isolated features on the landscape, streams are part of well-integrated networks. More importantly, these networks are finely adjusted to the landscape and one another so that tributaries almost always join the main trunk stream at the same level as that stream. Streams that must plunge over waterfalls to join a larger stream are quite rare. Such discordant streams are found primarily in recently glaciated terrain where the stream-valley system has not yet become fully adjusted, such as in Yosemite Valley. This remarkable consistency of stream accordance is the strongest evidence indicating that streams carve their own valleys.

Streams develop into network patterns that are strongly influenced by bedrock structure. Where the bedrock is relatively uniform and without strong joints and faults, a dendritic pattern of drainage develops. In this pattern, a stream system is branched like a tree. On inclined plains such as the Atlantic coastal plain, the stream pattern is often parallel, with major streams flowing directly down the topographic slope to the sea. Inclined mountain systems such as the Sierra Nevada also produce parallel drainage. Where structural folding of the terrain has produced linear ridge and valley topography, main trunk streams occupy the linear valleys and are quite long, with short, steep tributaries feeding them off the flanks of the hills or mountains. This type of pattern has developed in areas such as the Great Valley of Virginia and is known as trellis drainage. Volcanoes often produce a radial pattern of drainage. Mount Egmont in New Zealand is often cited as an example.

Base Level

The depth of a river valley is a function of the height of the land above base level, the length of time that has passed since the stream began to erode, the resistance of the bedrock to erosion, the load of sediment that the stream is carrying, and the spacing of adjacent streams. “Base level” refers to the theoretical lower limit to which a stream can cut. The ultimate base level for all streams is sea level, but the local base level is significantly higher for many streams. Streams require a minimum slope to transport their sediment to the sea; this limits the depth to which a stream may cut. The upper section of the Huang He near Tibet flows at elevations of more than 3,000 meters, but the river must still flow more than 4,000 kilometers to the sea. To carry its heavy load of sediment over that distance, the river must maintain its channel at a rather high elevation.

Valleys that are deep and relatively narrow are called canyons. Especially narrow valleys are called gorges. Streams require time to erode great canyons, although the rate of cutting can be quite rapid compared with most geologic processes. The downwearing of interfluves is far slower, so young (recently uplifted) landscapes produce the deepest canyons. As downcutting by the main stream ceases, upland weathering and erosion lower the local relief. The deepest, narrowest canyons are eroded in strong, homogeneous rocks such as granite and quartzite. The Royal Gorge of the Arkansas River and the Colca Gorge of Peru are excellent examples; both are cut in recently uplifted masses of resistant rock formations. The Colca Gorge is far narrower than the Grand Canyon and is nearly twice as deep at more than 3,000 meters.

As a mountain mass or tableland is uplifted, streams often develop that flow directly down the initial slope of the land. Such streams are called consequent streams because their course is a direct consequence of the terrain slope. Because streams seek the path of least resistance, their courses often follow the outcrop pattern of weak rocks such as shale, producing what is known as a subsequent stream pattern. In many cases, however, streams seem to ignore the terrain and structural slope of the land entirely, flowing through mountains of quite resistant rock. An excellent example is the Black Canyon of the Gunnison River in Colorado, where the river carves a deep canyon through a high plateau, with its channel cut in resistant gneisses and igneous intrusives. What makes this so surprising is that much lower terrain, underlain by thick sequences of weak shale, lies only 3 kilometers west of the gorge's head, which would seem to provide a much easier path to the sea. The reason for this course—and many other anomalous stream courses—is that the course of the Gunnison was established before the plateau's uplift. As the land rose beneath the river, it maintained its position, carving an ever-deepening gorge in the rocks as they rose. Far older and more extensive is the anomalous course of the New River, which cuts directly across the structure of the Appalachians, flowing a great distance to the Ohio River rather than the shorter, more direct route to the Atlantic. Once again, this river was established before the uplift of the land in which it is now entrenched—in this case, the ancient Appalachians. Thus, the river is somewhat ironically named, considering its course is older than the Appalachians.

At any given time, a stream may either erode or aggrade (build up) its bed. This vertical change in stream profile is determined by many factors internal to the stream and outside environmental factors. Over geological time, streams tend to erode their beds deeper and deeper. Over shorter time intervals, however, a stream may reach a state of equilibrium between its erosive energy and the load of sediment that the stream is carrying. A delicate balance is maintained within the stream channel. For example, if stream energy is increased by an increase in flood volume or frequency, the stream is likely to respond by eroding its bed. Conversely, if stream power remains constant and the sediment load significantly increases, the stream will respond by aggrading. This channel aggradation increases the channel slope, giving the stream more energy to transport the sediment load, and a new equilibrium is reached.

In the twenty-first century, the study of river valleys continues to reveal key insights about the planet and its structures. The discovery of ancient river valleys beneath the Antarctic ice sheet has allowed insight into the continent's geological history and climate. Discoveries concerning river valleys can also provide insight into the people who developed civilizations among them. For example, the discovery of an ancient network of cities along the Amazon River has radically changed the perceptions of the ancient civilizations that grew near the Amazon River valleys. 

Principal Terms

aggradation: the process by which a stream elevates its bed through the deposition of sediment

base level: the theoretical vertical limit below which streams cannot cut their beds

fluvial: of or related to streams and their actions

prokaryotic: an upland area between valleys

stream equilibrium: a state in which a stream's erosive energy is balanced by its sediment load such that it is neither eroding nor building up its channel

underfit stream: a stream that is significantly smaller in proportion than the valley through which it flows

valley: part of the earth's surface where stream systems are established; it includes streams and adjacent slopes

Bibliography

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