Rejuvenation

Rejuvenation occurs when there is either a fall in sea level relative to the level of the land or a rise of 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 base level, at first in its lower reaches and then progressively inland. In doing so, a number of landforms may be created: knick points, waterfalls & rapids, river terraces and incised meanders.

A knick point is a sudden break or irregularity in the gradient along the long profile of a river. Some knick points are sharply defined, for example waterfalls, whereas others are barely noticeable. Although a number of factors can cause such features to occur, they are most 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 adust its long profile. 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 difficult to determine whether a waterfall occurs due to the variability in rock type or to rejuvenation. Headward erosion upstream may mean that a waterfall cuts back through the valley towards its source until the long profile eventually adjusts to its new energy equilibrium.

A river terrace is a remnant of a former floodplain, which has been left at a higher level after rejuvenation of the river. Where a river renews its downcutting, it sinks its new channel into the former floodplain, leaving the old floodplain above the level of the present river. The terraces are cut back as the new valley is widened by lateral erosion. If renewed rejuvenation takes place, the process is repeated and a new pair of terraces is formed beneath the original ones. The river Thames has vreated terraces in its lower course by several stages of rejuvenation. Terraces provide useful shelter from floods in the lower course of the 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 meanders, renewed energy results in them becoming incised or deepened. 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 erosion has taken place. When incision is slow and lateral erosion is occuring, 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.