What is the dominant activity in valley deepening?

The dominant activity in valley deepening is downcutting. This process occurs when a river or stream erodes downward into its bed, which often happens when the flow of water is swift or the geological materials are particularly susceptible to erosion. Downcutting is critical in shaping the landscape, as it deepens the valley over time by removing material from the bottom rather than only the sides, which would be more indicative of lateral erosion.

Understanding downcutting is essential because it fundamentally alters the hydrology and morphology of a river system. Factors such as sediment load, water volume, and variations in bedrock impact how effectively a river can engage in downcutting. This makes it a primary mechanism in the formation of features such as gorges and canyons, distinguishing it from other processes like meandering, which primarily contributes to lateral or horizontal changes in a river's shape, or flooding, which affects water level and flow without directly leading to valley deepening.