Why does bedload decrease in size downstream? Watch
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Bedload size decreases downstream because as the river moves down stream it's velocity increases. As the velocity increases, it allows the river to pick up heavier and larger sediment, this sediment will bump into each other and into the river bank causing bits to break off (attrition and abrasion). By the time the river's reached it's lower course, the sediment that was once large will have been broken into smaller pieces of sediment and become more smooth and rounded from rubbing against other sediment.
The river will drop most of it's bedload when it reaches it's mouth (usually the sea) because of the sudden change in velocity, from high velocity to low velocity.
Hope this helps
Velocity increases between the source and mouth of the river, the bedload size and shape is large at the source and get smaller towards the mouth. The cross sectional area increases towards the mouth. Discharge is the amount of water