Rejuvenation occurs when there is either a fall inn sea level relative to the level of the land or a rise pf the land relative to the sea. This enables a river to renew its capacity to erode as its potential energy is increased. The river adjusts to its new hase level, at first in its lower reaches and then progressively inland. In doing sp, a number of landforms may be created: knick points, waterfalls and rapids, river terraces and incised meanders.
A knick point is a sudden break or irregualrity in the gradient along the long profile of a river. Some knick points are shaply defined, for example waterfalls, whereas others are barely noticeable. Although a number of factors can cause such features to occur, they are most commonly attributed to rejuvenation.
When a river is rejuvenated, adjustment to the new base level starts at the sea and gradually works its way up the river’s course. The river gains renewed cutting power (in the form of vertical erosion)., which encourages it to adjust its long profie. In this sense the knick point is where the old long profile joins the new. The knick point recedes upstream at a rate which is dependent on the resistance of the rocks, and may linger at a relatively hard outcrop. It can be difficul to determine whether a waterfall occurs due to variability in rock type or rejuvenation.
A river terrace is remnant of a former floodplain which has been left at a higher level after rejuvenation of the river. Where a river renews its donwcutting, it sinks its new channel into the former flooodplianm leaving the old floodplain above the lovel of the present river. There terraces are cut back as the new valley is widened by lateral erosion. If renewed rejuvenation takes place, the process is repeated ad a new pair of terraces is formed beneath the original ones. The River Thames has created terraces in its lower course by several stages of rejuvenation. Terraces provide useful shelter from floods in a lower-course river valley, and natural routeways for roads and railways. The built-up areas of Oxford and London are mainly located along the terraces of the River Thames.
If a rejuvenated river occupies a valley with well-developed meadners, renewed energy results in them becoming incised o deepnd. Incised streams and rivers have cut deeply into the landscape in many parts of the British Isles. The nature of the landforms created is largely a result of the rate at which vertical ersion has taken place. when incision is slow and lateral erosion is occurring, an ingrown meander may be produced. The valley becomes asymmetrical, with steep cliffs on the outer bends and more gentle slip-off slopes on the inner bends. With rapid incision, where downcutting or vertical erosion dominates, the valley is more symmetrical, with steep sides and a gorge-like appearance, These are described as entrenched meanders.