What is rejuvenation?
Over a long time, a river forms a generally smooth profile (see below). In terms of transporting water and sediment it is the most efficient profile to have. Rivers that have this form of profile are in balance with the environment (equilibrium). Typically a river will always try to form a concave shaped profile.
From time to time something can occur that changes and de-stabilises this equilibrium. This causes the river to vertically erode its channel to re-establish the its long, smooth concave profile. This renewed period is known as rejuvenation.
How does a river become rejuvenated?
River rejuvenation involves a renewed period of vertical erosion to achieve a new and lower base level. The base level is the height or altitude to which the river flows before it either joins another river or reaches the sea. If base level falls, a step or ‘kink’ called a knick point is formed in the river’s long profile.
The fall in base level means the river now has more energy to available. This is used to actively erode the irregularity. So that the river can once again, over time achieve a state of equilibrium by creating a long, smooth profile. Over time the knick point will retreat upstream like a waterfall.
Rejuvenation can be caused by a number of different factors. These are:
- Tectonic changes – Earthquakes can cause uplift along fault lines to create fault scarps several metres high. If this crosses a river, a waterfall will form and the river will begin eroding vertically at this point.
- Fall in sea level caused by eustatic changes – during a glacial period, water is stored on land as ice so the sea level changes
- Fall in sea level caused by isostatic changes – after a period of glaciation the land will start to rise in response to the loss of the weight of the ice. This is called isostatic recovery.
- River capture – over time rivers cut backwards at their source. This is headward erosion. Occasionally, as a river cuts back it can break into an adjacent valley and capture the tributaries of nearby rivers. There is usually a height difference at the point of capture and a waterfall is formed. This is a knick-point.
Landforms created by rejuvenation
A knick point is a sharp break of slope in the smooth, concave long profile of a river. It is usually marked by the presence of a waterfall (or a series of rapids). At this point vertical erosion associated with rejuvenation is at its greatest. The knick point retreats upstream over time.
Meanders are sweeping bends in rivers. They are usually found along the lower course of a river where lateral erosion is greater than vertical. However, when rejuvenation occurs vertical erosion begins to dominate the lateral erosion that usually occurs in a meander (with erosion on the outside bend). This results in a steep sided meander that is cut into the floodplain. It may become entrenched (symmetrical steep sides on both sides of the river) and form a winding gorge like that of the Grand Canyon.
River terraces are old floodplains left perched above the current floodplain. Following rejuvenation the river will cut down into its channel and will gradually form a new floodplain. The old one is left high and dry. You can see these being formed in the river model below.
Settlements are frequently built on river terraces because they represented land safe from flooding. Much of London is constructed on a river terraces.