Fluctuations in Stream Discharge

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  • 0:02 What is Stream Discharge
  • 2:01 Example Problem
  • 2:39 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 will be able to explain what stream discharge is, as well as when and how it varies (or doesn't vary) over the course of a river. A short quiz will follow.

What is Stream Discharge?

Stream discharge is the amount (volume) of water carried by a stream past a point per second. It is measured in meters cubed per second or cubic feet per second.

There are several factors that affect stream discharge. The velocity of the water affects it; faster water means more passes per second so more discharge. The width and depth of the river also affects it; a larger river at the same speed will have higher discharge.

You can calculate stream discharge using this equation: the discharge equals the velocity in meters per second multiplied by the width of the stream in meters multiplied by the depth of the stream in meters. Though this only works if either the stream is square-shaped (not likely!) or you use the average width and average depth. The bed of the river might not be the same depth all the way across, after all.

How Does Stream Discharge Change?

Generally, stream discharge doesn't change from one part of the river to the next. The same amount of water has to flow all the way down the river, otherwise you would get gaps in the river flow at one location and have the water build up at another location. The water all has to go somewhere, so it needs to keep flowing at the same rate. That also means that smaller parts of the river must flow faster and larger parts of the river must flow slower. That way, the same amount of liquid flows by each second, no matter where you are.

But there are still ways that the discharge of a river can change. Discharge changes whenever you add or take away water. In a really hot area, some discharge will be lost through evaporation. And when it rains, the river's discharge will increase, too. Last of all, the river's discharge will increase when another river joins with it to make one large river. If the river's discharge increases suddenly due to heavy rain, it can become too much and cause the river banks to flood, creating a flood plain.

When discharge increases, usually all the parts of the equation increase together. The velocity of the river increases, the width of the river increases, and the depth of the river increases. The latter two happen due to increased erosion.

Let's go through an example of how to use the discharge equation.

Example: Calculating Stream Discharge

Let's say you're doing a geographic survey at a river in a valley near to where you live. You measure the speed of the water using a float, a stopwatch, and a tape measure, and it turns out that the water is flowing at 12 meters per second. You also measure the average width and depth of the river. The average width is 8 meters and the average depth is 4 meters. What is the stream's discharge?

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