How do streams deepen valleys when runoff increases?

In the context of hydrologic sciences, streams deepen their valleys primarily through the process of erosion, which is driven by the energy and sediment-transport capacity of the water. When runoff increases, the stream's discharge and velocity rise, enhancing its ability to incise into the landscape.123

The deepening process occurs through several mechanisms:

• Incision and Scour: Increased discharge provides the power necessary to entrain and transport sediment, allowing the river to cut into its bed. In many cases, this results in the formation of inner channels—narrow, deep, gorge-like features—that are carved into the bedrock as the stream adjusts to its flow.12
• Headcutting: In areas with erodible surficial material, such as ephemeral streams, increased runoff can trigger headcutting. This process involves the formation of an abrupt step or "headcut" in the channel profile that migrates upstream, effectively lowering the channel bed as it moves.4
• Flushing of Stored Sediment: During high-flow events or floods, rivers can flush out large quantities of previously stored alluvium from the valley floor. This removal of sediment allows the channel to widen and deepen, often resulting in sediment yields that far exceed those produced by hillslope erosion.5