Rejuvenation is the process by which a river adjusts to a new base level. Base level changes occur when the volume of water at the mouth of the river decreases, commonly by euastatic or isostatic sea level change or tectonic movement. This is marked by an irregularity in the long profile known as a knick point. The lowered base level renews the river's capacity for vertical erosion, therefore the concave proflie can become re-establised as the irregularity is eroded by downards erosion. The energy that causes this type of erosion is known as Gravitational Potential Energy (GPE).