As a river flows downhill carrying sediment, when the river widens and slows down, the sediment deposits.
Deposition is the laying down of sediment carried by wind, flowing water, the sea or ice.
Sediment can be transported as pebbles, sand and mud, or as salts dissolved in water.
When the river flowing downhill widens and slows down, the velocity of thee river drops and the sediments begins to deposit as the energy with which the river is transporting it begins to decline due to drop in the river velocity.
Thus, as a river flows downhill carrying sediment, when the river widens and slows down, the sediment deposits.
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