Valley

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.

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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.

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