Here's why:

* Erosion: The process of wearing away and transporting rock and soil.

* In valleys, rivers are the primary agents of erosion. As a river flows, it carries sediment and grinds against the valley walls, gradually widening them over time.

* Other erosive forces like wind, ice (glaciers), and rain can also contribute to valley widening.

* Deposition: The process of depositing sediments in a new location.

* While deposition can occur within a valley, it's usually associated with the formation of features like alluvial fans, deltas, or floodplains.

* Deposition doesn't directly contribute to the widening of the valley itself.

Think of it this way: Erosion carves the valley wider, while deposition fills in parts of it.