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Understanding Graded Streams

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  • 0:01 What Is a Graded Stream?
  • 0:50 Factors That Affect Erosion
  • 1:53 Finding a Balance
  • 3:46 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you should be able to explain what a graded stream is, how streams change over time, and how a balance in erosion and deposition can be reached. A short quiz will follow.

What is a Graded Stream?

A stream is a continuous flow of liquid through a channel; for example, a creek or river. Streams and rivers in the real world are constantly changing. They wear away, or erode, at the banks and start to wind left to right. They erode at their beds and get deeper, or even deposit soil from other parts of the landscape and get shallower. A stream can change direction, change size, and even change how steep the land is.

But theoretically, there could be a point where all these factors balance out, where a river stays pretty much the same because everything is perfectly in balance, or in equilibrium. A river that is perfectly balanced in this way, where its profile isn't changing, is called a graded stream.

Factors That Affect Erosion

There are many factors that affect how much erosion happens around the flow of a river. The speed of the current is a factor: faster currents erode more of the soil and rock and can carry larger particles with them. The size of the stream is also a factor: larger streams can carry more material at any one time. Both of these factors are then, in turn, affected by how steep the land is.

As a river winds back and forth, the water may get pushed against a particular bank of the river more heavily than another, and this causes that side of the river to get worn away and more materials to be deposited on the opposite side. This is what makes a river change direction.

Some parts of the river are erosion-dominant, meaning that more material is being eroded than deposited. This part of the river is gradually getting larger. Other parts of the river are deposition-dominant, meaning that more material is being deposited than eroded. In that case, the river is getting smaller. All of these factors combine together to affect how the river is changing at any given moment.

Finding a Balance

Over time, the river becomes increasingly balanced. The factors that make a river larger and the factors that make it smaller balance out. This is when it becomes a graded stream. Though no river in the real world is really graded - rivers are always changing - if the conditions stay the same for long enough, a river may eventually be stable enough that changes happen slowly.

One of the ways this happens naturally is related to how the river flows. A river passes a certain amount of water every second - a certain number of gallons. This amount of water must be consistent along the river; otherwise, you would get gaps in the river. Because of this, the fastest parts of a river tend to be the smallest, thinnest areas, and the slowest parts of a river are the larger, wider areas. Water flows faster down steeper slopes, and water flows slower down shallow slopes, so when we look at rivers, we notice that the fast, thin parts of the river tend to be over steep land, and the wider, slower parts over flatter land. But why is it like this?

Well, the faster, thinner and steeper parts of the stream erode a lot of material. But the slower, wider, and shallower parts of the stream deposit more material than they erode. The steeper areas will tend to get worn away, making them larger. And the shallow areas will tend to build up, making them smaller. But as they change in size, so does the amount of erosion or deposition.

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