Because it is a long time water washed from the landslide, collapse fell down.
Especially in the deep mountains and valleys, the rain makes the rocks developed in the mountain cracks slide into the river, and the larger stones can not be washed away by the water. Therefore, there are more large stones in the upper reaches of the river, and the smaller stones form bedload under the hydraulic action, which is rushed into the middle and lower reaches of the river by the upper reaches of the river, so there are more stones and river sand in the middle and lower reaches of the river.