River rejuvenation

In geomorphology a river is said to be rejuvenated when it is eroding the landscape in response to a lowering of its base level. The process is often a result of a sudden fall in sea level or the rise of land. The disturbance enables a rise in the river's potential energy, increasing its riverbed erosion rate. The erosion occurs as a result of the river adjusting to its new base level. River rejuvenation can lead to a number of changes in landscape. These include the formation of waterfalls and rapids, knick points, river terraces and incised meanders. Rejuvenated terrains usually have complex landscapes because remnants of older landforms are locally preserved. Parts of floodplains may be preserved as terraces alongside the down-cutting stream channels. Meandering streams may become entrenched, so a product of older river systems is found with steep, very pronounced "V" shaped valleys - often seen with younger systems. One ancient example of rejuvenation is the Nile, which was rejuvenated when the Mediterranean Sea dried up in the late Miocene. Its base level dropped from sea level to over two 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 these cause the river to erode its bed vertically (downcutting) faster as it gains gravitational potential energy. That causes effects such as incised meanders, steps where the river suddenly starts flowing faster, and fluvial terraces derived from old floodplains. A region can be uplifted at any stage. This lowers the base level and streams begin active downward erosion again. Dynamic rejuvenation may be caused by the epeirogenic uplift of a land mass. Warping or faulting of a drainage basin will steepen the stream gradient followed by the downcutting. The effect of seaward tilting can be felt immediately only when the direction of that stream is parallel to the direction of tilting.