Rapids and cataracts are formed by Fluvial action.

Fluvial action refers to the processes related to the movement of water in rivers, streams, and other bodies of water. It includes erosion, transportation, and deposition of sediment by the action of flowing water. Rapids and cataracts are specific landforms that are created as a result of fluvial action.

Here is a detailed explanation of how rapids and cataracts are formed by fluvial action:

1. Erosion: Erosion is the first step in the formation of rapids and cataracts. As water flows over uneven surfaces, it exerts a force on the surrounding rocks and sediment. Over time, this force can wear away the rock and create irregularities in the riverbed. The erosive action of the flowing water can be enhanced by the presence of sediment and rocks carried by the river.

2. Differential erosion: In some areas, the rocks may be more resistant to erosion than others. This differential erosion leads to the formation of irregularities and variations in the riverbed. The resistant rocks may form small barriers or obstacles that disrupt the flow of water and create turbulence. These irregularities contribute to the formation of rapids.

3. Velocity and gradient: The velocity of the flowing water plays a crucial role in the formation of rapids and cataracts. When the gradient of the river is steep, and the water flows rapidly, it encounters more resistance from the irregularities in the riverbed. This resistance causes the water to churn, creating turbulent currents and rapids.

4. Deposition: In some cases, the erosion and turbulence caused by the flowing water may lead to the deposition of sediment downstream. This deposition can further contribute to the formation of rapids and cataracts by creating obstacles and irregularities in the riverbed.

In summary, rapids and cataracts are formed by the erosive action of flowing water in rivers and streams. The presence of resistant rocks, variations in the riverbed, and high velocity of the water contribute to the formation of these dynamic and visually striking landforms. Fluvial action is responsible for shaping and reshaping the landscape through erosion, transportation, and deposition of sediment, ultimately leading to the formation of rapids and cataracts.