Question

why are the river valley densely populated

Answer 1

because it is generally used for cultivation of crop.

Answer 2

Because the land is flat and fertile which is good for farming. Building houses, roadways, railways is much easier compared to hilly reasons. The river flows down the slopes of mountains and erode them. They carry forward the eroded material and deposit their load consisting of stones sand and salt along their courses and in their valleys and it is from these deposits that plains are formed.