Valley - geography.

« 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 themiddle 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 anequilibrium between the rates that sediments are being deposited and eroded.

Rivers are very dynamic systems that respond instantly to changes that affect the equilibriumbetween deposition and erosion.

For example, a mudslide may momentarily disrupt the equilibrium by depositing extra sediment into a river, or a thunderstorm mayincrease 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, whichall 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 headwatersto 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 lowerstretches 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 ableto 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 asparticles 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 theriver 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 inriver 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 theMississippi 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 terracesinto 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 theMississippi 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 canbe 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 valleysthat 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 aglacial 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 valleyswhere 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 thenbecome flooded when the ice melts, creating fjords.

Norway has several examples of fjords along its coast, including the Sognafjorden and Hardangerfjorden , which extendmore 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 agraben .

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 arecalled 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 erodefaster 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 crustcompressed the rocks in the syncline, making them resistant to erosion.

The Zagros Mountains in Iran provide examples of synclinal valleys formed in a young mountainbelt, and the Appalachian Mountains in the United States provide examples of anticlinal valleys formed by erosion in an old mountain belt.. »