Stream Development

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I ma NK Streams erode paths through sediment and rock, forming V-shaped stream valleys.

Real-World Reading Link When was the last time you saw water flow uphill? Water in all rivers travels downslope to the lowest point. This allows geologists to predict the path of the river based on the features of an area.

Supply of Water

Stream formation relies on an adequate water supply. Precipitation provides water for the beginnings of stream formation. Streams can also be fed by underground deposits of water. As a stream develops, it changes width and size, and shapes the land over which it flows.

Stream channels The region where water first accumulates to supply a stream is called the headwaters. It is common for a stream's headwaters to be high in the mountains. Falling precipitation accumulates in small gullies at these higher elevations and forms briskly moving streams. As surface water begins its flow, its path might not be well defined. In time, the moving water carves a narrow pathway into the sediment or rock called the stream channel. The channel widens and deepens as more water accumulates and cuts into Earth's surface. Stream banks hold the moving water within them.

When small streams erode away the rock or soil at the head of a stream, it is known as headward erosion. These streams move swiftly over rough terrain and often form waterfalls and rapids as they flow over steep inclines. Sometimes, a stream erodes the high area separating two drainage basins and joins another stream, and then draws water away from the other stream. This process is called stream capture, shown in Figure 9.11.

Figure 9.11 The headward erosion of Stream A cuts into Stream B and draws away from its water into one stream.

Stream Development
a mm:

Maximum energy for downward erosion

Figure 9.12 The height of a stream above its base level determines how much downcutting energy the stream will have.

Minimum energy for downward erosion

Sea level

Base level of streams

Base level of streams

Formation of Stream Valleys

The driving force of a stream is the force of gravity on water. This means that the energy of a stream comes from the movement of water down a slope called a stream gradient. When the gradient of a stream is steep, water in the stream moves downhill rapidly, cutting steep valleys. The gradient of the stream depends on its base level, which is the elevation at which it enters another stream or body of water. The lowest base level possible for any stream is sea level, the point at which the stream enters the ocean, as shown in Figure 9.12.

Far from its base level, a stream actively erodes a path through the sediment or rock, and a V-shaped channel develops. V-shaped channels have steep sides and sometimes form canyons or gorges. The Yellowstone River in Wyoming flows through an impressive example of this type of narrow, deep gorge carved by a stream. Figure 9.13 shows the classic V-shaped valley. As a stream approaches its base level, it has less energy for downward erosion. Instead, streams that are near their base level tend to erode at the sides of the stream channel, and over time result in broader valleys with gentle slopes, as shown in Figure 9.13.

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Figure 9.13 A V-shaped valley is formed by the downcutting of a stream. A wide, broad valley is a result of stream erosion over a long period of time.

Identify which river is closer to its base level.

Figure 9.13 A V-shaped valley is formed by the downcutting of a stream. A wide, broad valley is a result of stream erosion over a long period of time.

Identify which river is closer to its base level.

Stream Development Animations

Section 2 • Stream Development 233

(l)Mike Norton/Animals Animals, (r)Tom Bean/CORBIS

Section 2 • Stream Development 233

(l)Mike Norton/Animals Animals, (r)Tom Bean/CORBIS

Vocabulary

Science usage v. Common usage

Meander

Science usage: a bend or curve in a stream channel caused by moving water

Common usage: a winding path or course

Meanders As stream channels develop into broader valleys, the volume of water and sediment that they are able to carry increases. In addition, a stream's gradient decreases as it nears its base level, and the channel gets wider as a result. The decrease in gradient causes an increase in the volume of water the stream channel can carry. Sometimes, the water begins to erode the sides of the channel in such a way that the overall path of the stream starts to bend or wind. A bend or curve in a stream channel caused by moving water is called a meander, shown in Figure 9.14.

Water in the straight parts of a stream flows at different velocities, depending on its location in the channel. In a straight length of a stream, water in the center of the channel flows at the maximum velocity. Water along the bottom and sides of the channel flows more slowly because it experiences friction as it moves against the land.

In contrast, the water moving along the outside of a meander curve experiences the greatest velocity within the meander. The water that flows along this outside part of the curve continues to erode away the sides of the streambed, thus making the meander larger. Along the inside of the meander, the water moves more slowly and deposition is dominant. These differences in the velocity within meanders cause the meanders to become more accentuated over time. This process is illustrated in Figure 9.15.

Oxbow lakes Stream meanders continue to develop and become larger and wider over time. After enough winding, however, it is common for a stream to cut off a meander and once again flow along a straighter path. The stream then deposits material along the adjoining meander and eventually blocks off its water supply, as shown in Figure 9.14. The blocked-off meander becomes an oxbow lake, which eventually dries up.

As a stream approaches a larger body of water or its endpoint, the ocean, the streambed's gradient flattens out and its channel becomes very wide. The area of the stream that leads into the ocean or another large body of water is called the mouth.

Figure 9.14 As the path of the stream bends and winds, it creates meanders and eventually oxbow lakes.

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Interactive Figure To see an animation of meander formation, visit glencoe.com

Figure 9.14 As the path of the stream bends and winds, it creates meanders and eventually oxbow lakes.

Interactive Figure To see an animation of meander formation, visit glencoe.com

Meander Formation Animation

234 Chapter 9 • Surface Water

S.J. Krasemann/Peter Arnold, Inc.

234 Chapter 9 • Surface Water

S.J. Krasemann/Peter Arnold, Inc.

□ NATIONAL GEOGRAPHIC

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Responses

  • TOMBA MUGWORT
    How does streams erode paths thru sediments and rocks forming stream valleys?
    8 years ago
  • roosa
    What holds a stream a moving water within in narrow pathway?
    8 years ago
  • Mewael
    What streams flow swiftly over rough terrain and often form what?
    8 years ago
  • YEMANE
    What is the process of stream development over time?
    8 years ago

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