Stages in the Development of a River’s Long Profile: Youthful Stage & East African Examples
Explore the stages in the development of a river’s long profile, with a special focus on the youthful stage. Learn how rivers shape landscapes through erosion and examples from East Africa, including the Nile and Tana Rivers.
What is a river profile?
A river's long profile refers to the course a river takes from its source to its mouth, with varying stages of development over time. The youthful stage is the first phase in this profile, characterized by several distinct features driven by powerful erosion. Below is a detailed breakdown of the youthful stage of a river, with relevant examples from East Africa.
1. Steep Gradient of the Valley
- In the youthful stage, rivers flow through areas with a steep gradient, which causes the river to flow rapidly downhill. This steep slope accelerates the erosive power of the river.
Example: The Murchison Falls section of the Victoria Nile in Uganda demonstrates this characteristic, with a steep gradient as the river flows from Lake Victoria towards the Albert Nile.
2. Fast Flowing River
- Rivers in their youthful stage have fast-flowing waters that are capable of carrying large volumes of water and sediments, eroding the landscape aggressively.
Example: The Kagera River on the border of Uganda and Tanzania is another example where fast-flowing waters shape the landscape, especially in its upper course.
3. Narrow, V-Shaped Valley Cross-Profile
- The valley formed in the youthful stage is typically narrow and V-shaped, as vertical erosion predominates, cutting deep into the riverbed rather than widening the valley. Example: The Rift Valley in Kenya and Uganda, especially near Mount Kilimanjaro, showcases deep V-shaped valleys as rivers cut through the landscape.
4. Dominance of Vertical Erosion
- In the youthful stage, vertical erosion (downward cutting) is the dominant form of erosion. This leads to the creation of deep channels and valleys as the river carves through its bed.
Example: The Nile River in Uganda, particularly near Lake Albert, exhibits vertical erosion where the river plunges through a narrow gorge, creating impressive features.
5. Presence of Interlocking Spurs
- In the youthful stage, the river winds around interlocking spurs—ridge-like features that protrude into the valley, which the river has to navigate through.
Example: The Gulu region in northern Uganda, where the Aswa River flows, features interlocking spurs that the river must maneuver around as it descends into flatter areas.
6. Features Like Waterfalls, Rapids, and Potholes
- The youthful stage is marked by erosional features such as waterfalls, rapids, potholes, and plunge pools, which are created by the fast-flowing waters cutting through rocks and other obstacles.
Example: Murchison Falls in Uganda is a spectacular example of a waterfall formed during the youthful stage of a river, where the Nile River forces its way through a narrow gorge. Rapids and potholes are also evident in many East African rivers, such as the Tana River in Kenya
The youthful stage of a river is critical in shaping the landscape, characterized by steep gradients, fast-moving waters, and deep, narrow valleys. Rivers like the Victoria Nile and Kagera River in East Africa exemplify this phase, creating impressive natural features such as waterfalls, rapids, and V-shaped valleys. Understanding this stage is vital for studying the dynamic processes that shape the landforms of East Africa and beyond.
Features formed in the youthful stage.
Potholes.
The water of a fast flowing river swirls if the bed is uneven. The pebbles carried by a swirling Tiver cut circular depressions in the river bed. These gradually deepen and enlarge hence called potholes those found on the beds of River Ikiwe of Machakos in Kenya, River Manafwa on the slopes of Mount Elgon and River Mubuku on the slopes of Mount Rwenzori
V-shaped valley
Because of intensive vertical erosion, the depth of the river channel increases faster than its width. The rocks in the river bed are usually of less resistance than those at the sides of the valley. Narrow V-shaped valleys result. River such as Isiolo, Sagoma, Thuchi and Timous flowing from Mount Kenya and Mubuku, Nyamugasani and Semliki on Mount Rwenzori all have well defined V-shaped valleys in their upper courses.
Interlocking spurs
In the youthful stage of a river which originates in highland regions, vertical erosion rapidly deepens the valley. The river twists and turns around obstacles of resistant rock but taking advantage of the soft-rocks. The resistant rocks tend to be on the convex slopes while the softer rocks are on the convex slopes. Erosion is pronounced on the concave banks of the beds and deposits the fragments on the convex banks. This ultimately causes spurs to develop which alternate on each side of the river to interlock. Rivers such as Semliki, Mubuku and Nyamugasani of Mount Rwenzori in western Uganda and Isiolo, Sagana, Thuchi and Timau that originate from Mount Kenya flow through interlocking spurs in their youthful stages. Other interlocking spurs can be seen in the Rwampara hills in western Uganda and in the Southern Highland of Tanzania.
Rapids, cataracts and waterfalls
A waterfall is a sharp break in the channel bed over which the river flows from a higher level to sudden lower level. At the base of the waterfall, the river bed may be over deepened by the hydraulic force of the falling water to form a plunge pool. If the slope dips gently downstream,rapids rather than waterfalls are formed.
Formation of waterfalls
There are several ways in which waterfalls are formed. These are discussed below: When a layer of resistant rock lies across a river's course, the less resistant rocks on the downstream side are more easily worn down than the resistant rocks. The riverbed is thus steepened where it crosses the resistant rock and a waterfall is formed. The resistant rocks may be horizontal, dipping gently or vertical e.g. Sezibwa Falls in Mukono, Gura, Tana, Grand andAdamson falls on river Tana and Fourteen Falls on R. Athi.
Waterfalls may also be formed when there is uplift of the land. Faulting for instance leads to the vertical displacement of rocks along the fault line. Rivers flowing across such a fault suddenly flow down a lower level via a waterfall. For example, Murchison Falls in Uganda was formed as a result of water falling off a steep scarp created by faulting. In Kenya, fault scarp falls are common where rivers enter the rift valley e.g. Turkwell falls and Torok falls.
A waterfall can be formed where a river descends the main valley from a tributary valley. The main valley will have been over deepened by the passage of a major glacier eg. the melt wate from Speke glacier join the main Bujuku valley on Mountain Rwenzori via a waterfall.
A lava barrier or landslide across a river may create a waterfall where the river drops over the edge of the barrier e.g. Lily Falls in Madagascar. Where a river falls over a plateau edge, especially when flowing from a high-level plain to a lower one like Augrabies Falls in South Africa. Rejuvenation of rivers creates sharp knick points resulting into waterfalls e.g. Charlotte Falls inSierra Leone.
Plunge pool
This is a broad shallow depression found at the base of the waterfall or an enlarged pothole formed due to progressive drilling and grinding of the valley floor. It is found on rivers with waterfalls especially in the youthful stage. However, they also occur in other stages of a river, which has experienced rejuvenation. For a plunge pool to form there is usually a steep gradient, large volume of water, great erosive power of the river due to the steep gradient and availability of sediments, which contribute to abrasive erosion.
Plunge pools are formed where resistant or hard rocks overly less resistant or soft rocks. Eri of the underlying soft rocks produces a waterfall. A pothole is formed at the base of the soft mod by eddying, scouring action, cavitation, swirling and undercutting. The eddying, cavitation swirling and erosion collectively enlarge potholes progressively to produce plunge po Examples of plunge pools occur at Murchison falls on the Albert Nile, Sezibwa falls inMukono, Sipi falls in Kapchorwa and Kisizi falls in Rukungiri.
Gorge
This is a deep, narrow steep sided V-shaped valley. A wonderful example of a gorge is that one created in Uganda by the Murchison Falls on the Victoria Nile as it enters Lake Albert. A gorge is formed due to a powerful river cutting down its bed at a rate far greater than weathering and man wasting which wear down the valley slopes. The result is the formation of a deep, steep sided valley known as a gorge. Other examples of gorges include the Great Ruaha Gorge on the Rush River in Tanzania and the Mitano gorge through which river Birila flows in western Uganda.
The Mature Stage Of a River.
1. The mature stage of a river is characterised by the following;
2. The valley has a shape of an open V-shape cross-section.
3. The gradient is gentler than in the youthful stage.
4. The speed of the river is moderate due to reduced gradient.
5. Meanders and river bends begin to develop.
6. Spurs are removed by lateral erosion.
7. The valley floor is wide and by the time it enters the old stage it is covered with a layer of sediments.
8. The load consists of mainly rounded and small stones.
The mature stage experiences erosion, transportation and deposition roughly in equal proportions. This leads to the formation of both erosional and depositional landforms. Erosional landforms such as cliffs or bluffs are pronounced at the beginning of the mature stage while depositional features are such as meanders, slip-off slopes, and beginning of a flood plain are pronounced atthe end of the stage.
The old stage of a river.
The old stage of a river is characterised by the following:
The valley cross profile has a shape of a wide flat plain.
The gradient is very gentle.
The speed of the river is very slow due to lowgradient.
Deposition is dominant due to reduced speed of the river.
The valley floor is covered by a layer of sediments.
The load consists of mainly silt and other fine materials.
Meanders, bluffs, slip-off slopes and cut-offs are so pronounced. Sometimes deposition at the river both divides it into several distributaries.
In the old stage, deposition is greater than erosion and transportation. Deposition results from a number of factors. When the gradient of the valley is low, the river flows sluggishly and its energy is reduced. The load becomes too heavy for the river to transport and deposition takes place. Another factor is the cross-profile of the river widening due to lateral erosion which also reduces the river's energy. Where rivers flow to the sea or ocean, the fresh water from the river mixes with the salty seawater. This results in the coagulation of the sediments carried and deposition results. In flood time, deposition results from friction between the sediments carried by the river and its banks. Deposition results in the formation of various landforms such as broad valley, deltas, levees, braiding, and ox-bow lakes are dominant while erosional landforms such as terraces occur due to rejuvenation.
Features formed in the old stage of a river
Wide open valley
The wide-open valley is formed as a result of lateral erosion. This is the erosion which wears down the banks of the river valley resulting into the widening of the river channel. Rivers such as Rufigi and Ruvuma along the East African coast join the Indian Ocean through wide-open valleys.
Meanders
Meanders are curves in the course of a river, which continually swing from side to side in we loops. They form in both the mature and old stages of a river. Their formation is favoured by the presence of a large load carried by the river and low gradient, which enables the deposition of the load. The load deposited on the river bed results in the river flowing in a zigzag manner to w the deposits on its bed. The zigzag manner in which the river flows creates concave and conve bends along the river channel. These are made more prominent by alternate undercutting m deposition at the concave and convex beds of a river channel respectively to form well develope meanders. Meanders can be seen in the lower sections of rivers such as Rwizi in Mbarn, Mpanga in Fort-Portal, Ngaila in Kisumu and Manafwa in eastern Uganda.
Bluffs or river cliffs
The undercutting at the concave bends of a river produces cliff-like features called bluffs.
Slip-off Slope
Deposition at the convex bends of a river leads to the formation of a gently sloping slope known as a slip off slope. Bluffs and slip-off slopes are found on all meandering rivers.
Flood plain
This is a wide flat plain of alluvium on the floor of a river valley cross which the river flows in a meandering or braided channel. It is formed in the lower /old /senile stage of a river. Its formation is favoured by low gradient, excessive load and reduced discharge.
A flood plain develops through the meandering of a river. As the river swings back and forth across the valley, it widens the valley floor. The valley sides are slowly worn back and in time a line of low bluffs or river cliffs are formed on either side of the a level plain. This becomes so broad that the river swings freely without touching the valley sides. During floods, the river bursts its channel and spreads over the entire width of the plain depositing material carried by it. Successive flooding and deposition leads to the building up of a platform of alluvium called a flood plain. Most flood plains are marshy with several swamps and small lakes. On some flood plains levees develop on either side of the channel. Examples of flood plains can be seen along Rivers Manafwa and Rwizi in castern and western Uganda respectively, Ngaila, Nyando, Tana in Kenya and Rufigi in Tanzania.
Ox-bow lake
This is a lake formed when a meandering river having bent in almost complete circle, cuts across the narrow neck of land between the two stretches. Erosion is pronounced on the concave banks. The meander neck is broken through especially at flood time when the river overflows its channel. Deposition takes place at the two ends cutting off the meander loop to form an ox-bow lake e.g. along river Nzoia and Tana in Kenya, river Semliki in western Uganda and river Rufigi in Tanzania.
Meander scarup
This is an ox-bow lake, which has been filled with alluvium from later floods, and gradually dries up.
Levees
These are embarkments built by rivers alongside their channels. They rise above the level of the adjacent flood plain, towards which they slope gradually.
Levees are formed through successive flooding and deposition of sediments by the river Deposition is greatest nearest the river because as the water floods out of the main channel its speed is immediately checked by friction with the banks and the heavier sediments are deposited first. During floods when the river overflows its banks, friction checks the current of the river at the banks the coarser materials are dropped, which gradually build up to be higher than the river's banks. During subsequent floods, the levees continue to build and become higher and at the same time the river also goes on depositing material in its channel and rises to a higher level than the surrounding flood plain or even the entire countryside. Very pronounced levees can be seen on the lower Omo River where it joins Lake Turkana. In addition, rivers Malaba in eastern Uganda and Ngaila, Nyando and Yala in western Kenya have built levees in their lower courses.
In some cases artificial levees have been built to try and contain the floodwaters, but these increase the danger of flooding. River Hwang Ho and Yangtse Kiang in China for instance have built levees, which are higher than the neighbouring housetops, and the river flows at a higher level than the adjoining land. If levees at such a stage collapse, there is obviously serious danger to life and property.
Deferred junction or tributary
This is a tributary of a river that is forced to flow alongside the main stream for some distance before joining it. This is caused by flooding, which builds up levees across the original confluence of the two rivers. Levees, which build across the junction of a tributary to the senile river, cause the tributary to flow for many kilometers without being able to join the main river. This is known as a deferred junction e.g. the deferred junction on river Manafwa at Doho in Butaleja district in eastern Uganda.
Delta
This is a large flat, low-lying plain of river deposits such as silt, boulders, pebbles and gravel where a river flows into the lake, ocean or sea. Deposition at the river mouth forces it to divide into several channels called distributaries. Many deltas are covered by swamp vegetation and lagoons and they bare a triangular shape. The name delta is due to the fact that it resembles the Greek letter (delta)
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