Valley - Geography.

« 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 thoseof 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 andre-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 cutterraces 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 ofthe 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, glacierscan 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’ shapecross-section compared with the ‘V’ shape characteristic of mountainous river valleys.

Most of the mountainous areas of North America and northern Europe have glacialvalleys that formed during the last Ice Age.

Glaciers flowed down river valleys in these regions, leaving steepened valley sides.

Yosemite Valley in California is anexample 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 ofglacial 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 thenbecome flooded when the ice melts, creating fjords.

Norway has several examples of fjords along its coast, including the Sognafjorden and Hardangerfjorden , whichextend 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 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 deepelongated 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 archesare 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 thatanticlines 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 thefolding of the crust compressed the rocks in the syncline, making them resistant to erosion.

The Zagros Mountains in Iran provide examples of synclinal valleys formedin 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 rivererosion 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 theRhône River in southern France cut deep canyons, which were then submerged when sea level rose again.

Most submarine canyons, however, are carved by the actionof turbidity currents .

These are currents of dense, sediment-laden water that periodically cascade down the canyons, scouring as they go. Contributed By:Michael A.

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