How and why does bedload change in size and shape downstream of a river?

Question

I'm currently doing a geography project, and i could use some help with this hypothesis...

Answer 1

Wikipedia:

The term Bed load describes particles in a flowing fluid (usually a river) that are transported along the bed. This is in opposition to suspended load and wash load which are carried entirely in suspension.

Bedload moves by a variety of methods, including rolling, sliding, traction, and saltation.

Generally, bed load downstream will be smaller and more rounded than bed load upstream (a process known as downstream fining). This is due in part to attrition and abrasion which cause the stones to bump against each other and against the river channel, thus removing its rough texture and making it smaller. However, selective transport of sediments also plays a role in relation to downstream fining: larger-than average particles are more easily entrained than small-than average particles, since larger particles are protruding from the bed whereas small particles are shielded/ hidden by larger particles thus requiring a higher lift and drag force to entrain.

Answer 2

The bedload becomes smaller when the river is flowing downstream, and the rocks also become smaller, smoother and rounder due to attrition and abrasion (rubbing against/ banging into eachother.)

(The reason the flow slows down is because the energy of the flow decreases)

Hope that helps

Answer 3

When the velocity of a river begins to fall, it has less energy and so no longer has the competence or capacity to carry all its load. So, starting with the largest particles, material begins to be deposited.

Deposition occurs when:

Discharge is reduced following a period of low Precipitation;

Velocity is lessened on entering the sea a lake (resulting in a delta);

Shallower water occurs on the inside of a meander (resulting in a point bar;

The load is suddenly increased, (caused by debris from a landslide)

The river overflows its banks so that the velocity outside the channel is reduced (resulting in a flood plain).

As the river loses energy, the following changes are likely:

The heaviest or bedload material is deposited first. It is for this reason that the channels of mountainous streams are often filled with large boulders large boulders increasing the size of the wetted perimeter.

Gravel, sand and silt transported either travel as bedload or in suspension — will be carried further, to be deposited over the flood plain or in the channel of the river as it nears its mouth.

The finest particles of silt and clay, which are carried in suspension, may be deposited where the river meets the sea — either to infill an estuary or to form a delta.