Describe and Explain what rejuvenation is and the effect rejuvenation has on the river.

Rejuvenation is when a river has to readjust itself and the land around it, usually by increasing the amount of vertical erosion it carries out to a change in the base level. It is caused by various factors, including river capture, isostatic sea level change and eustatic sea level change. Due to it changing its processes, a river undergoing rejuvenation creates many new landforms. A river is said to be rejuvenated when the base level that it is flowing down to is lowered. This can happen for various reasons.

Rejuvenated terrains usually have complex landscapes because remnants of older landforms are locally preserved. Parts of floodplains may be preserved as terraces along the down cutting stream channels. Meandering streams often become entrenched, so a product of older river systems is found with steep, very pronounced V shaped valleys - often seen with younger systems. One example of rejuvenation is the Nile, which was rejuvenated when the Mediterranean dried up in the late Miocene. Its base level dropped from sea level to over 2 miles below sea level. It cut its bed down to several hundred feet below sea level at Aswan and 8000 feet below sea level at Cairo. After the Mediterranean re-flooded, those gorges gradually filled with silt.

Rejuvenation may result from causes, which are dynamic, eustatic or isostatic in nature. All of this cause the river suddenly to erode its bed vertically faster as it gains gravitational potential energy. That causes effects such as meanders cut down as gorges, steps where the river suddenly starts flowing faster, and fluvial terraces derived from old floodplains. River terraces are terraced floodplains found in a rivers lower course. They can be paired – where each step is of equal height to its partner of the opposite band or unpaired. After vertical erosion occurs eroding the river down to readjust itself with a new lower base level, lateral erosion against become present. This forms the terraces, with the lateral erosion acting on the gorge the river has vertically eroded itself into. This creates a step from the original floodplain down to the new one, so the floodplain becomes terraced. If the base level continually drops, then the rejuvenation will repeat itself. This leads to many steps in the floodplain and occurs often. River terraces are therefore formed due to rejuvenation, as the river is readjusting itself to a new base level.

Knick points are another effect caused by rejuvenation on the river. Knick points are a sudden drop in the gradient of a river, where it is readjusting itself to a new base level. They can be clearly seen, such as waterfalls or not so obvious, for example rapids. They are caused by isostatic readjustment, where land rises after having weight removed from on top of it. This is evident in Scotland, where land is rising by 3mm per year. This means the river has to erode vertically, as the base level has remained the same, however the land has risen therefore resulting in a sudden gradient drop.

One further effect on the river caused by rejuvenation is an incised meander. They are menders entrenched in a steep gorge, which is the opposite of normal meanders seen a in a river middle course. Furthermore contrary to ingrown meanders, they have steep slip off slopes, which look similar to river cliffs. This results in the channel being symmetrical. They are caused by large amounts of vertical erosion due to falling sea levels, which is eustatic sea level change, causing the base level to drop, thus in turn causing the river to need to readjust itself.