Valley - Geography. I INTRODUCTION Valley, area of low-lying land flanked by higher ground. Valleys usually contain a stream or river flowing along the valley floor. Most valleys are connected to other valleys downstream, which ultimately lead down to the coast. The sides of large valleys in low-lying areas are usually gently sloping with an average slope of just a few degrees. In mountainous regions, however, valleys are typically deep and narrow, and the sides have slopes of 35° or more. Every valley is separated from adjacent valleys by a ridge called a drainage divide. Rain falling on opposite sides of a drainage divide flows in opposite directions toward the bottoms of the adjacent valleys. The area bounded by a drainage divide is called a drainage basin, or, in the United States, a watershed, and it represents all of the land area drained by a valley. One of the broadest and longest valleys in the world is the Mississippi River Valley, which crosses the United States from north to south. The Mississippi River winds down the center of this valley, and is joined at intervals by other major rivers flowing in their own valleys, such as the Missouri, Ohio, and Tennessee rivers. The deepest valley in the world is a section of the Indus River Valley in Kashmir. As it passes through the western end of the Himalayas, the difference in height between the valley bottom and the top of the drainage divide is about 7000 m (about 23,000 ft). Some valleys are totally enclosed by higher terrain, and rivers or streams within them may terminate in a lake. Examples of valleys that are wholly surrounded by higher ground and do not open to the ocean include Death Valley in California and the Jordan River Valley in the Middle East. II RIVER VALLEYS The major agent in forming and shaping most river valleys is the river that runs through it. Rivers shape their valleys by eroding and depositing sediments. The structure of the underlying rocks also plays a role, especially in determining the location of waterfalls and rapids. A River Valley Formation Many early geologists gave little thought to how valleys formed. They assumed valleys formed simply as an accidental result of the uneven formation of the surrounding mountains and hills and that rivers simply flowed through valleys once the valleys had formed. However, this view was challenged at the end of the 18th century by James Hutton, a Scottish scientist. Hutton, who is considered by many to be the father of modern geology, argued that, through erosion, rivers produced the valleys through which they flowed. While erosion by rivers is the main valley-forming process, other processes, such as movement of the earth's crust and glaciers, also play a role in some cases. The rate at which a river deepens its valley depends on several factors. One factor is how fast the water flows down the river channel. This will generally reach a maximum where the volume of water flowing through the river is large and the slope of the river channel is steep. Another factor is the resistance of the material through which the river channel is cutting. At the same time that a river channel cuts down into its valley floor, erosion carries soil and sediment down the valley slopes toward the channel. If a river can easily remove all the material being supplied from the slopes and from upstream, then it can continue to cut more deeply into its bed and increase the steepness of its sides. However, if material is being supplied to the channel faster than it can be carried away, then the excess material accumulates on the valley floor. Steep-sided valleys are often found in young mountain ranges where the land is still being lifted to create mountains. The steep-sided valleys occur because the uplift tends to increase the channel slope, which in turn causes the river to cut more rapidly into its bed. The Indus River, for instance, maintains its course across the western end of the rapidly uplifting Himalayas by eroding its bed at a rate of up to 1 cm/year (up to 0.4 in/year). Across most of the world, however, uplift is slow or absent. As a result, slopes of most valley floors are low, the erosive power of most rivers is modest, and valley-side slopes tend to be relatively gentle. B Longitudinal Profile A graph of the slope of a river channel at each point along its course is called a longitudinal profile. In most cases, the slope of a river becomes less steep as it flows from its headwaters to the ocean. Slopes as high as 200 m/km (1000 ft/mi) can be found in mountainous terrain, but slopes of about 2 m/km (10 ft/mi) are more typical in the middle section of such valleys. Slopes as low as 2 cm/km (1 in/mi) can be found in rivers close to the ocean. In most rivers there is a complex adjustment between the amount of material supplied to a river channel and the ability of the river to remove it. A graded river is a river in which each section of its longitudinal profile is just steep enough to transport the load of sediment supplied to it and thereby maintain its slope. In such rivers, there is an equilibrium between the rates that sediments are being deposited and eroded. Rivers are very dynamic systems that respond instantly to changes that affect the equilibrium between deposition and erosion. For example, a mudslide may momentarily disrupt the equilibrium by depositing extra sediment into a river, or a thunderstorm may increase the flow of water, which increases erosion. The river responds to such changes with changes in channel depth, in channel slope, or in the speed of the water, which all act quickly to re-establish an equilibrium between deposition and erosion. Through the dynamic interplay of erosion and deposition, most rivers develop a longitudinal profile that generally becomes less steep as the river flows from its headwaters to the sea. There are several reasons why the lower stretches of a river are usually less steep than the upper stretches and these reasons have to do with why the lower stretches of a river can still remove its sediment supply even with a shallower slope. An important factor is that the amount of water flowing in the river increases with each successive tributary that contributes to the flow. As the flow increases, a river is able to transport the same quantity of sediment with a shallower slope. A further factor is the tendency for the size of material being carried by rivers to decrease downstream as particles are weathered and abraded. As the average size of the particles gets smaller, a river is able to transport the smaller particles of sediment with a shallower slope. Occasionally, the slope of a river changes abruptly along its course. Faulting or a transition from hard rock to soft rock along a river course can cause a sharp increase in the river slope. These increases in slope can lead to the formation of rapids or waterfalls, such as the Victoria Falls on the Zambezi River in central Africa. Sharp decreases in river slope can also be caused by faulting. If a river slope decreases abruptly, sediment will tend to be deposited at this point, which may lead to the formation of a fan-shaped accumulation of sediment called an alluvial fan. These features are particularly common where valleys emerge along faulted mountain fronts, such as along the flanks of Death Valley in California. C Floodplains Except in mountainous terrain, rivers are almost always flanked by floodplains. Floodplains are flat wide deposits of alluvium, river-deposited sediment, on either side of the river channel. During floods, a river overflows its banks and spreads out the sediment near the river to form a floodplain. Floodplains of large rivers, such as those of the Mississippi River, can be flat areas tens of kilometers across. River channels migrate back and forth across their floodplains as alluvium is repeatedly eroded and re-deposited a short distance downstream. D Terraces If the erosive power of a river increases, due to an increase in water discharge or slope, then it will cut down into its floodplain and form a new floodplain lower down. Terraces are flat sections of old floodplains that are sometimes left attached to the side of the valley high above the current floodplain. Occasionally, a river can cut terraces into the underlying bedrock of the valley side. E Deltas Many valleys end in a delta, a fan-shaped accumulation of sediment where the river reaches the sea. Deltas form because the river supplies alluvium more rapidly than it can be removed by the action of waves and coastal currents. Notable examples are the delta of the Nile River, on the Mediterranean coast of Egypt and the delta of the Mississippi River on the Gulf of Mexico. III GLACIAL VALLEYS Although most valleys owe their origin to erosion by rivers, other mechanisms can carve valleys in the landscape. In regions cold enough for ice to accumulate, glaciers can be a powerful erosive force capable of excavating spectacular valleys. Such glacial valleys typically have very steep sides and broad flat floors, giving a 'U' shape cross-section compared with the 'V' shape characteristic of mountainous river valleys. Most of the mountainous areas of North America and northern Europe have glacial valleys that formed during the last Ice Age. Glaciers flowed down river valleys in these regions, leaving steepened valley sides. Yosemite Valley in California is an example of a glacial valley with near-vertical valley walls. Glacial valleys include several distinctive features. Bowl-shaped valleys, called cirques, result from glaciers cutting into the high mountain peaks at the upper end of glacial valleys. Hanging valleys form where small tributary valleys join a main valley that has been undercut by the glacier. Outwash plains form at the lower end of glacial valleys where the debris eroded by the glacier and carried downstream by streams is deposited. Glaciers are capable of cutting very deep valleys, in some cases resulting in the valley floor being below sea level. Glacially-deepened valleys near the coast can then become flooded when the ice melts, creating fjords. Norway has several examples of fjords along its coast, including the Sognafjorden and Hardangerfjorden , which extend more than 110 km (70 mi) inland. IV CRUSTAL MOVEMENT VALLEYS Crustal movements can also play a direct role in creating valleys. When a crustal block is down-faulted below the blocks on either side, the valley that forms is called a graben. An example of a graben is the Rhine graben in Germany, through which the Rhine River flows. A large graben, or series of grabens, of regional extent is called a rift valley. The Great Rift Valley in Africa extends across the continent from Ethiopia to Mozambique. Rift valleys also run along the center of the mid-ocean ridges, a chain of underwater mountains that runs along the middle of most oceans and is the site of seafloor spreading (see Plate Tectonics; Mid-Atlantic Ridge). A large part of the southwestern United States consists of alternating down-faulted and uplifted crustal blocks. These produce a basin-and-range landscape composed of deep elongated valleys (basins) separated by mountain barriers (ranges) (see Basin: Basin and Range Region). Valleys can also be produced by folding of the crust. When a section of crust is compressed, it folds up like an accordion into a series of arches and troughs. The arches are called anticlines and the troughs are called synclines (see Anticline and Syncline). Initially, the synclines form valleys, called synclinal valleys. Over the ages, however, the anticlines will tend to erode more than the synclines. Eventually, the anticlines will be lower than the synclines, forming anticlinal valleys. The reason that anticlines erode faster than synclines is that the folding of the crust stretched and cracked the rocks in the anticline, making them susceptible to erosion, whereas the folding of the crust compressed the rocks in the syncline, making them resistant to erosion. The Zagros Mountains in Iran provide examples of synclinal valleys formed in a young mountain belt, and the Appalachian Mountains in the United States provide examples of anticlinal valleys formed by erosion in an old mountain belt. V SUBMARINE VALLEYS Valleys are also found below sea level on the edge of continents. These features are usually called submarine canyons. Some submarine canyons were formed by river erosion when sea level was lower in the past. Between 5 and 6 millions years ago the Mediterranean Sea dried out several times and the Nile River in Egypt and the Rhône River in southern France cut deep canyons, which were then submerged when sea level rose again. Most submarine canyons, however, are carved by the action of turbidity currents. These are currents of dense, sediment-laden water that periodically cascade down the canyons, scouring as they go. Contributed By: Michael A. Summerfield Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
