why are there sands in river beb ?

Answer 1

The reason you find sand in a river bed is mostly determined by the distance from the source or head of the river. In the mountains, where many streams begin, large cobbles or gravel is more prevalent because their is not enough energy to lift these particles and move them downstream. As you move further downstream water and other particles grind down gravel and cobbles into sand. The water in the river will transport this sand until there is not enough energy left to keep in entrained within the stream. The sand is then deposited at the base of the river bed. Hope that helps!

Answer 2

The real reason has to do with the mineralogy of sand: quartz. Since quartz has no cleavage and is hard (Mohs 7) it generally tends not to break down easily. Feldspars have good cleavage in three directions, amphiboles and pryoxenes have good cleavage in two directions, micas have perfect cleavage in one direction. Quartz, with no cleavage planes, persists in the geologic environment. It becomes rounded and accumulates in river beds and on seashores.

Answer 3

because of some process of sedimentation and crystalization