What causes abrasion of rock and pebbles in fast flowing streams?

Abrasion of rock and pebbles in fast-flowing streams is primarily caused by the mechanical scraping and grinding action of sediment and larger particles being carried along by the water. The swift current increases the energy of the flow, allowing these particles to collide with and erode the riverbed and bank materials. Additionally, turbulence in the water enhances the effectiveness of this erosion process, leading to the smoothing and rounding of rocks and pebbles over time.