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 downcutting stream channels. Meandering streams often become entrenched
An entrenched river is a river that is confined to a canyon or gorge, usually with a relatively narrow width and little or no flood plain, and often with meanders worn into the landscape...
, 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
The Nile is a major north-flowing river in North Africa, generally regarded as the longest river in the world. It is long. It runs through the ten countries of Sudan, South Sudan, Burundi, Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda and Egypt.The Nile has two major...
, which was rejuvenated when the Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
dried up in the late Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
. 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
Aswan , formerly spelled Assuan, is a city in the south of Egypt, the capital of the Aswan Governorate.It stands on the east bank of the Nile at the first cataract and is a busy market and tourist centre...
and 8000 feet below sea level at Cairo
Cairo , is the capital of Egypt and the largest city in the Arab world and Africa, and the 16th largest metropolitan area in the world. Nicknamed "The City of a Thousand Minarets" for its preponderance of Islamic architecture, Cairo has long been a centre of the region's political and cultural life...
. After the Mediterranean re-flooded, those gorges gradually filled with silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
Rejuvenation may result from causes which are dynamic, eustatic or isostatic in nature. All of these cause the river suddenly to erode its bed vertically (downcutting) faster as it gains gravitational potential energy. That causes effects such as meander
A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...
s cut down as gorges, steps where the river suddenly starts flowing faster, and fluvial terraces derived from old floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
A region may 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. 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 sheets.
Eustatic rejuvenation rejuvenates the mouth of the stream. Regrading of a stream toward a new base level will proceed 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.
- decrease in load
- increase in runoff because of increased rainfall
- 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.
Either way, rejuvenation results in a "knickpoint", as it appears on a river's long profile, which often turns out to be rapids or a waterfall, such as Seljalandsfoss
Seljalandsfoss is one of the most famous waterfalls of Iceland. It is very picturesque and therefore its photo can be found in many books and calendars...
in Southern Iceland, where isostatic (dynamic) uplift has occurred as a result of both construction and deglaciation.
For an example for rejuvenation due to rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing, see .