Visualizing Erosion and Deposition in a Meander

Figure 9.15 As the water travels down a meander, the area of maximum velocity changes. As shown in cross-section A, when the meander is straight, the maximum velocity is located near the center. When the meander curves, the maximum velocity shifts to the outside of the curve, as shown in cross-section B. As the meander travels around to cross-section C, the maximum velocity shifts again to the outside of the curve. Erosion occurs around curves in the meander in areas of high velocity. The high velocity of the water carries the sediment downstream and deposits it where the velocity decreases, on the inside of a curve. The area where the erosion occurs is called a cutbank and the area where the deposition occurs is called a point bar.

^iKip11 tn MOlj^ y0 eXp|ore m0re about erosion and deposition, visit glencoe.com.

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Erosion And Deposition

^iKip11 tn MOlj^ y0 eXp|ore m0re about erosion and deposition, visit glencoe.com.

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Mississippi River Deposition

Figure 9.17 The Mississippi River Delta was formed from the deposition of river sediments. The area in the top left of both images is a marshland used for both recreation and business. Since 1973, waters upstream of the Mississippi River have been dammed, reducing the sediment flow. Over the course of 30 years, the area of the marshland has decreased without the sediment from upstream.

Deposition of Sediment

The velocity of a stream determines how much sediment it can transport. Rapidly flowing streams have the energy to transport sediment as large as gravel. When streams lose velocity, they lose some of the energy needed to transport sediment, and deposition of sediment occurs.

Alluvial fans A stream's velocity lessens and its sediment load drops when its gradient abruptly decreases. In dry regions such as the North American Southwest, mountain streams flow intermittently down steep, rocky slopes and then flatten out onto expansive dry lake beds. In areas such as these, a stream's gradient suddenly decreases, causing the stream to drop its sediment at the base of the mountain in a fan-shaped deposit called an alluvial fan. Alluvial fans are sloping depositional features formed at the bases of slopes and are composed mostly of sand and gravel. An example of an alluvial fan is shown in Figure 9.16.

^P Reading Check Describe how an alluvial fan is formed.

Deltas Streams also lose velocity and some of their capacity to carry sediment when they join larger bodies of quiet water. The triangular deposit that forms where a stream enters a large body of water is called a delta, named for the triangle-shaped Greek letter delta (A). Delta deposits usually consist of layers of silt and clay particles. As a delta develops, sediments build up and slow the stream water, sometimes even blocking its movement. Smaller distributary streams then form to carry the stream water through the developing delta. Deltas, such as the Mississippi River Delta, are normally areas where the stream flow changes direction frequently.

Over the course of thousands of years, the Mississippi River Delta has changed frequently. Today, any small change in the drainage channels can result in catastrophic flooding for local communities. To prevent floods, an extensive system of dams and levees is in place to protect people and economic activities. A consequence of flood control is the decrease in the regular deposition of sediment throughout the delta. In the absence of regular deposition throughout the delta, normal processes of coastal erosion have caused the delta to shrink over time, as shown in Figure 9.17.

Mississippi Delta Deposition

Figure 9.17 The Mississippi River Delta was formed from the deposition of river sediments. The area in the top left of both images is a marshland used for both recreation and business. Since 1973, waters upstream of the Mississippi River have been dammed, reducing the sediment flow. Over the course of 30 years, the area of the marshland has decreased without the sediment from upstream.

Deposition Surface Water

236 Chapter 9 • Surface Water

(tl)Michael Andrews/Animals Animals, (bl)USGS, (br)USGS

236 Chapter 9 • Surface Water

(tl)Michael Andrews/Animals Animals, (bl)USGS, (br)USGS

| Rejuvenation o

! During the process of stream formation, 2 downcutting can occur. Downcutting is the ^ wearing away of the streambed and is a major erosional process that influences the stream until it reaches its base level. If the base level drops as a result of geologic processes, the stream undergoes rejuvenation.

Rejuvenation means to make young again. During rejuvenation, a stream actively resumes the process of downcutting toward its base level. This causes an increase in the stream's velocity and the stream's channel once again cuts downward into the existing meanders. Rejuvenation can cause deep-sided canyons to form. A well-known example of rejuvenation is the Grand Canyon, shown in Figure 9.18.

Millions of years ago, the Colorado River was near its base level, like much of the Mississippi River today. Then the land was uplifted compared to the level of the ocean, which caused the base level of the Colorado River to drop. This caused the process of rejuvenation, in which the river began cutting downward into the existing meanders. The result is the 1.6-km-deep canyons, which attract millions of visitors each year from all over the world.

Meanders Canyons

Figure 9.18 Rejuvenation shaped the Grand Canyon when the base level of the Colorado River changed and the river began downcutting into existing meanders.

Figure 9.18 Rejuvenation shaped the Grand Canyon when the base level of the Colorado River changed and the river began downcutting into existing meanders.

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Responses

  • rahel
    What caused colorado river rejuvenation?
    8 years ago
  • Katja Linna
    What does a cutbank look like geology?
    8 years ago
  • regan
    How is a meander formed from would rain cause erosion or deposition?
    6 years ago
  • ulrich kuster
    How velocity affects deposition and erosion within a meander?
    5 years ago
  • milen
    Are meanders caused by erosion and deposition?
    5 years ago
  • fiori
    How can deposition change the direction of a stream?
    5 years ago
  • Michelina
    Is meander caused by erosion or deposition?
    2 years ago

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