River erosion and desposition... I am so confused!


    Rep:
    Desperately need to know this for AS exam on Thursday! It's something I've struggled with all year, and I'm sure all the books contradict themselves...

    So, when does a river erode the most? Is it in the upper course? I've learned that a river erodes when it has enough energy to overcome friction, and a river has more energy in the lower course because the discharge is higher. So why do the books say that erosion is dominant in the upper course?

    Also, if a river has more energy in the lower course, why does it carry smaller material? If competence increases with velocity, and velocity is higher, then surely it should carry larger load in the lower course?

    I'd be so grateful for any help, I'm starting to stress a tad about this Thanks!

    Rep:
    No, a river will switch from vertical to lateral/horizontal erosion as it travels into the lower course - Both 'areas' have erosion. BTW the river has more energy in the upper course because of potential and kinetic energy and such but it is lost through friction with beds/banks and high calibre debris.

    The material in the river has been reduced by attrition and such so the calibre of the load is reduced by the time the river makes it to the lower course/

    Rep:
    Okay, great But where are erosion and deposition dominant (upper or lower)?

    Rep:
    A river is more efficient and so has greater energy in the lower course. This is because the potential energy has been converted to kinetic energy so the velocity is increased. Also, due to this, there will be greater abrasion, so the jagged rock banks will be eroded and smoothed, leading to reduced friction so velocity if further increased, hence increasing the efficiency of a river.

    Rep:
    Haven't they shown you the Hjulstrom chart? It encapsulates the central idea that makes sense of it all rather than having to memorise a lot of stuff.

    The size of particles a river can transport depends on velocity and the velocity depends on the gradient.

    High velocity means net erosion and low velocity means net deposition.
    It's all makes logical sense if you think in terms of the velocity and the size of particles that can be transported at that velocity (imo)

    Rep:
    (Original post by coolcric321)
    A river is more efficient and so has greater energy in the lower course. This is because the potential energy has been converted to kinetic energy so the velocity is increased. Also, due to this, there will be greater abrasion, so the jagged rock banks will be eroded and smoothed, leading to reduced friction so velocity if further increased, hence increasing the efficiency of a river.
    you're thinking in the right terms but rivers flow with a greater velocity in the upper course where the gradient is steeper... They can transport pebble sized particles here. Lower course rivers have lower velocity and can't even transport fine sand... Which is why you get silted up estuaries, coastal deltas etc. It's why port authorities have to dredge out a channel in the river bed to keep the docks open.

    Rep:
    (Original post by Joinedup)
    you're thinking in the right terms but rivers flow with a greater velocity in the upper course where the gradient is steeper... They can transport pebble sized particles here. Lower course rivers have lower velocity and can't even transport fine sand... Which is why you get silted up estuaries, coastal deltas etc. It's why port authorities have to dredge out a channel in the river bed to keep the docks open.
    no you're wrong, rivers have a higher velocity in the lower course because there is less friction

    Rep:
    This is the problem, different people and different books say different things. Which is right?

    Rep:
    Velocity INCREASES as a river becomes more efficient in the lower course

    Rep:
    Ah yes, this confused me a lot when I revised for Unit 1. It's funny how people above assert themselves when they have the theory right, but don't quite grasp the overall trend. I'm by no means but I have the advantage of asking my teacher.

    A river's velocity GENERALLY increases further downstream. The assertion made by the Bradshaw model that "velocity increases downstream" is correct but not in all cases. The three factors that means velocity increases is that:
    1. there is less friction as material has been rounded by attrition
    2. a greater hydraulic radius which means the channel is more efficient
    3. a river has a higher discharge so it naturally travels faster

    However the gradient decreases downstream so the river has less gravitiational potential energy. This may mean some rivers actually slow down downstream. As a result of the higher energy available upstream because of the steeper gradient, erosion (in general) is considered dominant upstream. While the river does generally get faster, it has less energy downstream so it can't transport as large as load when it was upstream, which is why deposition is dominant downstream.

    Hope that helps slightly, I'm not an expert by any means. Unit 1 exam on Thursday which I'm struggling with!

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: May 22, 2012

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