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Q & A: stream flow

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Most recent answer: 10/22/2007
Q:
How does the volume of a stream affect its flow rate?
- Marie
St. Clair Shores, Mi, USA
A:
Let's figure this one out. Usually a stream flows along for a while carrying about the same amount of water. Water doesn't normally keep piling up at on place, overflowing the banks, and it's also not usually evaporating very fast. Unless we look somewhere where there's a tributary joining up or a big rainstorm, new water isn't joining in either.

That means that if you look at two slices across the stream, the same amount of water flows past each slice in each second. The volume of water going through the slice per second is the speed of the flow times the area of the slice. So in places where the cross-sectional area is small (that is, the stream is narrow and shallow) the flow must be fast and where it's large (broad and deep) the flow must be slow.

Of course this whole argument breaks down if water is entering or leaving the stream.

Another disclaimer: real streams often have more complicated structures -- they may have a deep channel in the middle with sandy or rocky shallow flat sections of the bottom near the sides. In this case, the water will flow more quickly in the deep center section than in the shallow parts because of friction with the rocky or sandy bottom. If a stream is making a tight bend, water will tend to flow on the outside of the bend and leave silt on the inside. But the total amount of water flowing across one slice of a stream has to be the same as any other slice unless water is entering or leaving.

Mike

(published on 10/22/2007)

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