What process can cause the widening of river valleys through the removal of material from the banks?

Lateral erosion is the process that primarily involves the widening of river valleys as it includes the wearing away of the riverbanks. When a river flows, it not only cuts downwards (downcutting) but also moves sideways, slowly eroding the materials that make up the banks. This occurs especially in meandering rivers, where the current is faster on the outside of bends, leading to greater erosion in those areas. This process effectively results in the expansion of the valley floor, giving it a more pronounced shape over time.

While bank erosion is also a relevant process related specifically to the banks of the river, lateral erosion distinctly emphasizes the movement and removal of material across a broader area, leading to significant valley widening. It should be noted that downcutting refers specifically to the vertical erosion that deepens the riverbed rather than affecting the valley width. Mass wasting involves the movement of rock and soil down slopes due to gravity, which is not specifically related to river processes.