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# GCSE Geography Doubt [Rivers] Watch

1. Hi, everyone!

I just have this question that's been bugging me and I haven't been able to ask anyone as it is the holidays.

So we are told that as a river travels downstream, velocity, discharge and momentum all increase. We are also told to believe that a river's velocity and momentum can be thought of as the energy being used by the river at that point. In other words, the river has more energy when it is closer to the mouth than the source.

The doubt I have is as follows: if the river's momentum and velocity is greatest near the mouth, why can the river not transport large amounts of load near the mouth. My GCSE book tells me that the river has greatest energy near the source, hence large loads can be transported near the source, but not near the mouth. Doesn't this seem slightly counter-intuitive? The fact that the river has less energy despite having more momentum is totally beyond me. Can anyone conjure up the knowledge to explain?

Any help would be most appreciated.

Regards,
Ser
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3. (Original post by Serenium)
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The best reason I can think of/remember is that the net energy is spread out over a further area when you think about the relative surface areas of the source and the mouth, so there's less force per unit area to transport sediments. You also have to accept that energy will dissipate into a larger ocean/lake/sea system once it reaches the mouth too.
4. Hey there

Velocity, discharge and momentum all increase with distance from source. Velocity and discharge = energy. More energy = higher bedload.

As a river progresses downstream, it becomes responsible for draining an increasingly larger area (as tributaries join the main channel) and so it's discharge increases. More discharge = more energy available. More energy = a higher bedload

The overall Bedload or capacity of a river increases with distance from source. This is the overall quantity of sediment a river can carry.

The competance also increases with distance from source. This is the ability of the river to transport different sizes of sediment. A high competence means large boulders can be transported. A low competence means only sand & silts can be transported.

At the source, there is a large vertical erosion element and so the stream may have a high capacity for silts and grains (which require little energy in order to become entrained and transported) since these particles are only tiny, it's competence will be low.

Hope this answers your question.
5. (Original post by martynsteel)
The best reason I can think of/remember is that the net energy is spread out over a further area when you think about the relative surface areas of the source and the mouth, so there's less force per unit area to transport sediments. You also have to accept that energy will dissipate into a larger ocean/lake/sea system once it reaches the mouth too.
Thanks, I think I understand this explanation. Thanks for your help!

(Original post by Pegasus2)
Hey there

Velocity, discharge and momentum all increase with distance from source. Velocity and discharge = energy. More energy = higher bedload.

As a river progresses downstream, it becomes responsible for draining an increasingly larger area (as tributaries join the main channel) and so it's discharge increases. More discharge = more energy available. More energy = a higher bedload

The overall Bedload or capacity of a river increases with distance from source. This is the overall quantity of sediment a river can carry.

The competance also increases with distance from source. This is the ability of the river to transport different sizes of sediment. A high competence means large boulders can be transported. A low competence means only sand & silts can be transported.

At the source, there is a large vertical erosion element and so the stream may have a high capacity for silts and grains (which require little energy in order to become entrained and transported) since these particles are only tiny, it's competence will be low.

Hope this answers your question.
I understand the explanation and it makes a lot of sense, but it completely goes against what I've been learning from my revision books. The books are saying the competence is higher upstream, but lower downstream (since the gradient is steeper, hence the energy is greater upstream). I wanted to find out why this is the way it is, as surely velocity, momentum and discharge should amount to greater competence downstream as you have suggested in your post.
6. Couldn't even finish the Geography exam, ran out of time on the last question and missed some easy marks, sucks.

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