identify the false statement. streams cause erosion by

identify the false statement. streams cause erosion by

identify the false statement. streams cause erosion by

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Streams

stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel. Thus, streams may vary in width from a few centimeters to several tens of kilometers. Streams are important for several reasons:

  • Streams carry most of the water that goes from the land to the sea, and thus are an important part of the water cycle.
  • Streams carry billions of tons of sediment to lower elevations, and thus are one of the main transporting mediums in the production of sedimentary rocks.
  • Streams carry dissolved ions, the products of chemical weathering, into the oceans and thus make the sea salty.
  • Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. Much of the surface landscape is controlled by stream erosion, evident to anyone looking out of an airplane window.
  • Streams are a major source of water, waste disposal, and transportation for the world’s human population. Most population centers are located next to streams.
  • When stream channels fill with water the excess flows onto the the land as a flood.  Floods are a common natural disaster.

 

 

 

 

 

identify the false statement. streams cause erosion by
identify the false statement. streams cause erosion by

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Erosion by Streams

Streams erode because they have the ability to pick up rock fragments and transport them to a new location. The size of the fragments that can be transported depends on the velocity of the stream and whether the flow is laminar or turbulent. Turbulent flow can keep fragments in suspension longer than laminar flow.

 

Streams can also erode by undercutting their banks resulting in mass-wasting processes like slumps or slides.

When the undercut material falls into the stream, the fragments can be transported away by the stream.

Streams can cut deeper into their channels if the region is uplifted or if there is a local change in base level. As they cut deeper into their channels the stream removes the material that once made up the channel bottom and sides.

Although slow, as rocks move along the stream bottom and collide with one another, abrasion of the rocks occurs, making smaller fragments that can then be transported by the stream.

Finally, because some rocks and minerals are easily dissolved in water, dissolution also occurs, resulting in dissolved ions being transported by the stream.

 

Sediment Transport and Deposition

The rock particles and dissolved ions carried by the stream are the called the stream’s load. Stream load is divided into three categories.

  • Suspended Load – particles that are carried along with the water in the main part of the streams. The size of these particles depends on their density and the velocity of the stream. Higher velocity currents in the stream can carry larger and denser particles.

 

  • Bed Load – coarser and denser particles that remain on the bed of the stream most of the time but move by a process of saltation (jumping) as a result of collisions between particles, and turbulent eddies. Note that sediment can move between bed load and suspended load as the velocity of the stream changes.
identify the false statement. streams cause erosion by
identify the false statement. streams cause erosion by

  • Dissolved Load – ions that have been introduced into the water by chemical weathering of rocks. This load is invisible because the ions are dissolved in the water. The dissolved load consists mainly of HCO32 (bicarbonate ions), Ca+2, SO4-2, Cl, Na+2, Mg+2, and K+. These ions are eventually carried to the oceans and give the oceans their salty character. Streams that have a deep underground source generally have higher dissolved load than those whose source is on the Earth’s surface.

The maximum size of particles that can be carried as suspended load by the stream is called stream competence.   The maximum load carried by the stream is called stream capacity.  Both competence and capacity increase with increasing discharge. At high discharge boulder and cobble size material can move with the stream and are therefore transported.   At low discharge the larger fragments become stranded and only the smaller, sand, silt, and clay sized fragments move.

When flow velocity decreases the competence is reduced and sediment drops out. Sediment grain sizes are sorted by the water. Sands are removed from gravels; muds from both. Gravels settle in channels. Sands drop out in near channel environments. Silts and clays drape floodplains away from channel.

 

Changes Downstream

As one moves along a stream in the downstream direction:

  • Discharge increases, as noted above, because water is added to the stream from tributary streams and groundwater.
  • As discharge increases, the width, depth, and average velocity of the stream increase.
  • The gradient of the stream, however, will decrease.
identify the false statement. streams cause erosion by
identify the false statement. streams cause erosion by

It may seem to be counter to your observations that velocity increases in the downstream direction,

since when one observes a mountain stream near the headwaters where the gradient is high,

it appears to have a higher velocity than a stream flowing along a gentle gradient.

But, the water in the mountain stream is likely flowing in a turbulent manner,

due to the large boulders and cobbles which make up the streambed.

If the flow is turbulent, then it takes longer for the water to travel the same linear distance, and thus the average velocity is lower.

Also as one moves in the downstream direction,

 

  • The size of particles that make up the bed load of the stream tends to decrease. Even though the velocity of the stream increases downstream, the bed load particle size decreases mainly because the larger particles are left in the bed load at higher elevations and abrasion of particles tends to reduce their size.
  • The composition of the particles in the bed load tends to change along the stream as different bedrock is eroded and added to the stream’s load.
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