What does 'river rejuvenation' refer to?

River rejuvenation refers to the process where a river gains energy, typically as a result of a change in base level—such as tectonic uplift or a lowering of sea level. When a river's base level drops, it effectively increases the river’s gradient, giving it enhanced capacity for erosion. This energetic state allows the river to cut deeper into its channel and erode the surrounding landscape more effectively, which can lead to the formation of features like knickpoints or entrenched meanders.

This definition highlights how rejuvenation affects a river's dynamics, enabling it to reshape its valley and enhance its ability to transport sediments. Other choices do not accurately capture this essential process: losing energy results in decreased erosion rather than rejuvenation, seasonal temperature changes do not involve energy changes in the context of erosion, and the establishment of new tributaries does not inherently relate to the river’s energy dynamics or its erosive power.