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A 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:
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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.
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.
As one moves along a stream in the downstream direction:
|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,