Soil erosion令到河的面積減小? Why?

When there is soil erosion in the river, usually it occurs in the upper course or the middle course of the river, then the soil, pebbles or rocks will be washed to the river.

This will result in the increase in transportation in the river.

And in the lower course, where the river velocity is lower (not river discharge), deposition will be the major procedure. So, the soil and rocks will deposit in the lower course of the river, thus, the wetted area of the river will decrease (Since the soil occupy spaces in the river, resulting the decrease in area for water to flow.)