Answer:As the sediment-laden river flows downstream, the process that takes place as the river slows down at the end of the steep canyon is "C. deposition."
Step-by-step explanation:
When the river slows down due to the widening and shallowing of the canyon, it loses energy, causing the sediment it carries to settle and deposit on the riverbed and banks. This process of deposition occurs as the river's reduced velocity can no longer support the transport of the sediment, leading to the accumulation of the deposited material.
Therefore, as the river slows down, the sediment is deposited due to the decrease in the river's energy, resulting in the geological process of deposition.