Question

why are the river plains thickly

Answer 1

You mean why are river plains thickly populated..They are thickly populated due to following reasons---
1) Plains are flat, so it is easy for transportation of goods, for settlements and even for agricultural activities.
2) Plains are fertile unlike the mountain areas where there is constant denudation of soil.
3) Rivers are at their best in plains, they provide water for domestic, industrial and agricultural activities.

Answer 2

THE PLAINS OF RIVER IS THICKLY BECAUSE WHEN RIVER FLOWS WITH VELOCITY IT CARRY SOME AMOUNT OF SAND OR SOIL DUE TO THIS SOME AMOUNT OF SOIL ACCUMULATED NEAR THE PLAINS SO THE PLAINS ARE THICKLY...