Concept:The speed of a river depends mainly on the gradient or slope of its channel.
Explanation:In the upper course, the river flows down a steep slope, so it moves fast and has much energy.
As the river reaches the lower course, the land becomes nearly flat.
The gradient, which is the rate of descent of the river bed, is greatly reduced.
Because the slope is gentle, the water loses speed and flows slowly.
Also, materials eroded from the upper course are deposited in the lower course, further reducing the slope.
Thick vegetation does not normally block the river channel itself, so it is not the main reason.
Human activities may affect rivers locally, but they do not cause the general slow flow at the lower course.
The volume of water actually increases downstream, yet a larger volume does not make the river slow down.
Answer:The flow is slow because the gradient is greatly reduced, so the correct option is C.