Bedload refers to the movement of sediment along the bed of a stream. It occurs when the energy carried by the water is able to dislodge and transport the particles on the bed. The energy of water is dependent on several factors, including the slope of the stream, the volume of water flowing, and the resistance encountered by the water as it flows.

In the case of mountain streams, the high gradient or slope of the stream results in a higher velocity of water. The increased velocity means that the water has more energy to dislodge and transport larger and denser sediment particles. This allows for the movement of bedload, which typically consists of coarser materials such as gravels, pebbles, or small boulders.

In lowland streams or rivers, the gradient is generally lower and the velocity of the water is reduced. This results in decreased energy available for transporting sediment. Consequently, bedload movement is less pronounced and the bed of the stream may contain finer materials such as sand, silt, and mud.

Therefore, bed load is primarily found in mountain streams with high gradient due to the increased energy and velocity of the water, which allows for the movement of coarser and heavier sediment particles.