Denser air have more molecules of air receiving heat energy from the sun. When the air molecules are so thin like that in outer space the temperature drops to very low values. The Himalayan region are the mountain region with higher elevations. The Terai region have lower elevations thus have denser air molecules.
A construction of the river formed by the floating ice that is accumulated in the rover by the northwards moving direction of the upstream rivers, this significantly reduces the flow of the river and causes the ice dam formation.