Rejuvenation (river)

From Wikipedia, the free encyclopedia

A river is said to be rejuvenated when the base level that it is flowing down to, is lowered. This can happen by uplift of land, or by a sea or lake that it is flowing into becoming lower. That makes the river suddenly start eroding its bed vertically (downcutting) 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 terraces derived from old floodplains.

One example 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 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.

A region may be uplifted at any stage. This lowers the base level and streams begin active downward erosion again. Rejuvenated terrains usually have complex landscapes because remnants of older landforms are locally preserved. Parts of floodplains may be preserved as terraces along the downcutting 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.

Rejuvenation may result from causes which are dynamic, eustatic or static in nature.

Dynamic rejuvenation may be caused by the epeirogenic uplift of a land mass. These movements are either associated with neighboring orogenic movements or may be world wide in nature. 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.

Eustatic rejuvenation results from the causes which bring worldwide decrease in sea level, and two types of such rejuvenation are recognized. Diastrophic eustatism is the change in sea level due to variation in capacity of ocean basins, whereas glacio-eustatism is the change in sea level due to withdrawal or return of water into the oceans, occupying the accumulation or melting of successive ice sheet. Eustatic rejuvenation rejuvenates the mouth of stream. Regarding of a stream toward a new base level will precede upvalley. The result may be an interrupted profile with the point of intersection of the old and new base levels.

Three changes may bring static rejuvenation, to the stream.

• 1) decrease in load
• 2) increase in runoff because of increased rainfall
• 3) increase in stream volume through acquisition of new drainage by stream diversion

Rejuvenation due to decrease in load took place during post-glacial times along many valleys that formerly received large quantities of glacial outwash. With change to no glacier conditions stream load decreased and valley deepening ensued