Question

Why do you think the
speed of a river decreases
as it enters the plains?​

Answer 1

Answer:

The base level of a river is similar to the sea level as river enters plains. As there is not much difference in height, water speed gets decreased.

Sediments deposited at the banks of the rivers hinders the water movement in river

At plains, rivers move in meanders rather than  straight and this is also another reason to decrease the speed of flow.

Also sea waves and tides are very strong . They come in the opposite direction to river.