RIVER ACTION AND RESULTING LANDFORMS - ADVANCED PHYSICAL GEOGRAPHY

RIVER ACTION AND RESULTING LANDFORMS – ADVANCED PHYSICAL GEOGRAPHY

RIVER

What is the river?

River is a body of running water within a channel.

A small river called stream. A river flows down slope.

CONCEPTS RELATED TO A RIVER

1. Tributaries: Are the branches of a river pouring (drained) its water in the main river. OR Are the smaller rivers that flow into bigger river.

2. Distributaries: Are the branches which collect water from the main river. OR Are the channel of stream detaching from the main river. It is commonly found at the lower stage of the river.

3. Confluence: Is the point at which a tributary joins the main or other river.

4. Drainage basin: Is an area drained (covered) by a river and its tributaries. Sometime known as river basin.

5. Watershed (divided): Is the boundary separating neighboring drained (river) basin.

6. Drainage pattern: Is a layout plan made by a river and its tributaries.

OR Is the arrangement of a river and its tributaries.

7. River source: Is a point where a river is originated. This can be either on mountain, or forest, or spring, or lake, or ice (glacier).

8. River mouth: Is a point where a river pours its loads (water finally).

9. River bed: Are the underlying rocks along which a river flow i.e the floor of a river.

10. River banks: Are the opposite side walls which confine a river within its channel.

11. Load: Is a material being transported by a river.

12. . Alluvium: Are the deposited materials by a river.

13. River rejuvenation: Is the renewed of the erosive power of the river in the valley.

14.. River flow: Is the manner in which the river water flow in a channel from upstream to downstream.

The following are types of river flow;

i. Lamina flow: Is the type of river in which the river flow parallel to the river bed.

ii. Turbulent flow: Is the chaotic flow of river where by water flow in turbulent fashion.

iii. Plunge flow: Is the flow over a vertical or over a hanging slope forming water fall.

iv. Helicoidal flow: Is the river flow which occur when currents take a spiral form of motion in the river meanders.

TYPES OF RIVER

The river can be classified into two (2) main groups i.e temporary and permanent (intermittent and ephemeral) rivers.

In general, there are three (3) types of rivers. These are: –

i. Permanent (Perennial) river:

These are the river with constantly supply of water throughout the year. Eg. river Nile, river Rhine, river Mississippi, rive Amazon, etc.

ii. Intermittent river:

These are the temporary rivers which flow only during the wet seasons in the region receiving seasonal heavy rains. The river continues to flow for sometime after the rains have ceased but when it is exceptionally dry they disappear and their valley become dry. Eg. St. Lawrence river (U.S.A), Orange river (S/Africa), Rufiji river (Tanzania), etc.

iii. Ephemeral river:

These are the temporary rivers which appear during rainy season in area with experience very little rain especially in the desert and semi-desert regions. However, this river tends to disappear immediately after the rains seasons has stopped. Eg. Kuiseb river in Namibia, and Luni and Son rivers in India.

STAGES OF THE RIVER

There are three (3) stages of the river profile. These are: –

i. Youth stage / Upper course / Torrent stage / First stage / Upper section / Young stage:

In this stage the river located in areas of steep slope or the source area of a river.

Characteristics of Youth stage of river:

a. The river flows along a steep slope.

b. The river flows very fast.

c. The river flows in a v-shaped valley.

d. It is characterised by interlocking spurs, therefore flowing in a zigzag course (winding course).

e. There is little or no deposition by a river.

f. Headward erosion may be present.

g. It is dominated by vertical erosion.

h. It is dominated by deposition features like gorge, canyon, v-shaped valley, pot holes, etc.

ii. Middle stage / Middle stage / Second stage / Valley stage (section)

This stage where gradient begins to decrease together with reduce velocity and energy of the river.

In this stage the lateral erosion is dominant and make its valley to be widen.

Characteristics of Middle section of a river:

i. The energy of the river is low because of reduced gradient and speed.

ii. Lateral erosion is more common.

iii. It is associated with a widen (broader) valley compared to a v-shaped valley in the first stage. d. The slope is gentle.

iv. The deposition is beginning to emerge at middle lower section.

v. Both erosion, transportation and deposition occur.

vi. Formation of river cliff and river bluff.

vii. Beginning of meanders formation at the lower parts of this stage.

iii. Old stage / Lower course / Senile stage / Plain stage / Final stage / Mouth stage

It refers to the section of the river valley found at the final section of the river where the river pours (accumulated) it loads. Its final destination of the river i.e river mouth.

Characteristics of the old stage of the river:

a. In this stage the deposition exceeds erosion.

b. The slope is very low i.e flat land.

c. The river flows in a wide area (channel) which has been widened by lateral erosion making it U-shaped.

d. Its characterised by depositional features like meandering, levee, flood plain, delta, etc.

e. The speed of the river is very slow.

f. The volume of the river water is very high.

g. It is dominated by fertile soil i.e alluvial soil, formed by depositional process it is better for agriculture.

FUNCTIONS OF THE RIVER

There are three (3) functions of river. These are: –

i. River erosion.

ii. River transportation.

iii. River deposition.

I: RIVER EROSION

What is the river erosion?

River erosion is the wearing away of landscape (river banks) by action of river water. OR River erosion is the accumulative erosional effects of various processes through which the river degrades and widen its valley.

The river erosion commonly occurs in the upper stage of the river valley due to increased energy and velocity of water.

FACTORS INFLUENCING RIVER EROSION

i. Gradient (slope).

ii. Nature of the geology (rock).

iii. The speed (velocity) of the river.

iv. Amount of erosive materials.

v. Climate of the area, etc.

PROCESS OF RIVER EROSION

There are four (4) processes of river erosion. These are: –

Corrasion (Abrasion):

Is the wearing away of the river valley by gridding action of the materials like sand and pebbles carried by the river. The small materials that transported by a river like sand and pebbles act as abrasion tools for eroded and its most effective process during flood time.

Corrosion (solution):

Is the solvent action of the river on the rock that are soluble to dissolving when come to contact with river water. The calcareous rocks such as limestone rock and chalk rock are easily to dissolve when contact with river water.

Hydraulic action:

Is the wearing away of landscape due to force of running (river) water. Through this process the river water breaks rocks as it hurls the air bubbles in the rock cracks and making them to exert pressure to removing rock materials.

Attrition:

Is the wearing away of river bed by the collision or collide effects of a load transported by a river. In this process the river transported heavy particles like boulders collide against one another with other materials and leading to disintegration into small pieces.

WAYS / TYPES OF RIVER EROSION

There are three (3) ways or types of river erosion. These are: –

i. Headward erosion:

This is the way where by the river increases its length. This is achieved by a river dissecting the its length. This is achieved by a river dissecting the its source on upstream.

The rain wash, soil creep and talus creep also help the river to extent its channel up the slope.

(a) Before headward erosion (b) After headward erosion

ii. Vertical erosion:

The way of river erosion where by the river deepens its channel. In this way the river concentrates its erosive power down wards on the river bed and therefore deepening it to become v-shaped valley.

iii. Lateral erosion:

The way of river erosion in which the river water worn away the sides of the river channel.

This is effective along the river meanders.

FEATURES (LANDFORMS) ASSOCIATED WITH RIVER EROSION

The river erosion is more effective in the youth stage of the river due to steeper gradient and therefore continue into mature stage of the river.

Therefore, the erosional features by a river dived into two (2) major groups. These are:

Erosional features on youth stage. Eg. v-shaped valley, interlocking spurs, water fall, rapids, pot holes, plunge pool, and canyon, gorge.
Erosional features on middle stage. Eg. river cliff and river bluff.

A: EROSIONAL FEATURES ON YOUTH STAGE OF THE RIVER

There are several features is dominant in this stage of the river. These are: –

V-shaped valley:

This is steep sided v-shaped valley formed when vertical erosion is dominant than the lateral erosion. It is a deep steep sided valley with a typical v-shaped in the youthful stage. It formed due to vertical erosion that deepens the river bed.

Interlocking spurs:

These are the ridges like features interlocking one another in the torrent stage of the river profile. This spur is formed by streams which follow a winding course avoiding dissecting the resistant out crop rocks in the channel.

Pot holes:

These are cylindrical holes at the youth stage of the river. They are formed when the river carries the pebbles and swirl them against an even bed opened by cracks and faults. The pot holes are medium in size and almost occupied by water during drought season.

Water fall:

A water falls refers to the sharp breaks in the river channel bed where the river sudden fall down the valley. There are many cases of water fall like faulting process, plateaus, vulcanic sills and dyke, hanging glaciated valley, etc. water falls are sites where H.E.P station can be sited. In general, water fall hinder navigation process. Example; Owen falls in Uganda in Nile river.

Plunge pool:

It is a large, deep and circular depression formed at the base of a water fall at the youth stage of a river. It is formed due to sudden fall of water crossing the sharp escarpment, etc. the plunge pool is large and deep than the pot holes.

Gorge:

It is a deep, narrow and steep sided river valley. It formed when the river retreat upstream or when vertical erosion dissects the landscape made up by resistant rocks overlies softer rocks causing the falls to retreat.

Canyon:

A wide gorge is called canyons. This is a impressive and very large gorge, its formed where by river dissects the landscape made by horizontal layers alternated by hard and soft rocks. Example: Grand Canyon on river Colorado (U.S.A), Fish river Canyon in Namibia, Manambolo gorge in Madagascar.

Rapids:

These are the fast flowing of water in the river course where the river bed is steep and rocky or rough. OR It is a section of the river’s course where the bed is suddenly steepened, causing the water to suddenly flow swiftly. Example: Kora rapids on river Tana below the Usueni falls, etc.

B: EROSIONAL FEATURES ON MIDDLE STAGE OF THE RIVER

The following are the erosional features dominant in this stage of the river. These are: –

i. River cliff:

There are almost steep sided sloping sides of river sides. They are formed as a result of lateral erosion.

ii. River bluff:

These are the spurs ends with steep sided slopes along the river pinks. They are formed when lateral erosion dissects and retreats the interlocking spurs leading to widen up of the river valley.

STRUCTURAL DEVELOPMENT OF WATER FALLS

How water falls develops? OR Describe the geological patterns of waterfalls?

In general, there are many types of water falls depending on their structural development factors of features. These are;

Water fall due to influence of faults:

When a fault leads vertical displacement of rock along the river course. It can cause formation of the water falls. Example Kalambo water fall in Zambia.

Water fall due to intrusion of sill along the river course

Some water falls occur when the river bed is underlined by a hard resistance layer of volcanic sill. As the river dissects its channel eventually meets with sill acting as hard slopping surface where water fall occur. Example Kinkon water fall in Guinea.

Water fall due to influence of a dyke:

Some water falls occur when volcanic dyke cuts across the river valley.

Water fall when glacier melting water falls from hanging valley to the main Ushaped valley:

When glacier melt water across the hanging valley in the glaciated landscape, it can cause water falls down to the main U-shaped valley.

Water fall along the edges of plateaus:

Sometimes water fall can develop along the edges of steep sides of the plateau.

Water fall at the Knick point of river rejuvenation:

Some water falls occur at the Knick points when the river rejuvenates its valley. These water falls may continue upstream as the Knick points recede up streams.

Water fall sometimes occur where streams flow over the edge of a cliff into the sea:

This occur when a vast volume of water poured down to the sea, hence deepening the bed of the stream and cutting the hard stratum. The water has a fall from a valley well above sea level and has cut a small notch in the cliff.

Water fall occur when resistant rock lies horizontally across the river channel

This occur when a layer of resistant lies horizontally across the river channel there must be a river whose velocity increases as it flows over resistant rock layer. The less resistant rock on the downstream side must erode faster than the resistant rocks resulting in steepening of the river bed and the valley must deepen to form a water fall.

Water fall occur where hard rock dips gently upstream;

HOW GORGES AND CANYONS ARE FORMED?

Question. The Long Profile of river contains three stages on which different landscapes are formed. Erosional features are formed in young stage of the river due to different categories of the erosional processes. Explain six (6) points to show how gorges and canyons are formed.

Gorge is a deep, narrow and steep sided river valley. It formed when the river retreat upstream or when vertical erosion dissects the landscape made up by resistant rocks overlies softer rocks causing the falls to retreat. While Canyon is an impressive and very large gorge, it’s formed where by river dissects the landscape made by horizontal layers alternated by hard and soft rocks. Example: Grand Canyon on river Colorado (U.S.A), Fish river Canyon in Namibia, Manambolo gorge in Madagascar.

Conditions (factors/causes) for formations of Gorges and Canyons are;

1. When there is resistant rock on the sides of the established river channels. The river erosion erodes beds vertically than river banks hence development of Gorge and Canyons.

2. Where there is land uplift and at the same time the river undercut the river bed. iii) Where there is slight rainfall such that water available undercut the river bed. iv) Where the powerful exotic rivers enters the deserts.

3. Where the river enters the ground in a limestone areas and keeps on eroding the underground then the roof of the carven collapse.

4. Where the river flows along a line of weakness.

II: RIVER TRANSPORTATION

River transportation is the movement of eroded materials from one place to another place by a river for deposition.

WAYS OF RIVER TRANSPORTAT MATERIALS

The river transports their materials by using the following subprocess (ways). These are: –

Traction:

Is the process of rolling or sliding materials along the river. In this process the river transport heavy particles such as boulders.

Saltation:

Is the process of transportation materials by the river by bouncing motion. In this process the river transport medium size particles.

Suspension:

Is the process of transported materials by lifted along the river bed. In this process the river transport very fine particles such as silt.

Solution:

Is the process of transport materials by dissolving action. Where by the soluble materials such as salt particles are dissolve to become solution.

FACTORS AFFECTING RIVER TRANSPORTATION

Velocity (speed) of the river.
The size of the load. iii. The relief of the area.
Size of the river.
The volume of the river.
Climate of the area.
The nature of geology of the area.
Presence or absence of river meanders.

III: RIVER DEPOSITION

What is the river deposition?

River deposition is the laying down or accumulation of materials that formally transported by a river.

The materials deposited by a river called river alluvium. The process of river deposition occur at the final stage of the river profile especially during floods.

FACTORS THAT INFLUENCING RIVER DEPOSITION

Stream gradient.

Presence of obstacles in the river course. iii. Size of materials transported by the river. iv. Presence of stream meanders, etc.

FEATURES (LANDFORMS) PRODUCED BY RIVER DEPOSITION

There are different features associated with river deposition. These are: –

i. Flood plain:

Is a wide, flat plain of alluvium on the floor of the river valley in the lower course of the river profile. It is formed during flood when the river spreads over the entire width of plain and deposited most of the materials as the river meanders and swings back and forth across the valley. Most of the flood plain are marshy with several swamps and lakes.

ii. Levees:

Are the natural embarkment of sands and boulders build by river along sides (periphery) of the channel at the lower stage. It is formed when the river deposits the larger materials just above the channel during the flooding periods, and cause build up a low ridge like features on either sides of the river.

iii. Yazoo stream (differed tributary):

Is the stream that is forced to follow along side the main river for a long distance before being able to join it on the flood plain. The point at which the Yazoo stream join to main river called different junction or point of confluence. Presence of resistant rock, formation of levee, dense vegetation, etc. cause to the formation of Yazoo stream because act as obstacles.

iv. Alluvial fan / Alluvial fan:

Is the cone or funnel shaped deposits of alluvial materials located where a mountain stream emerges on a flood plain. The cones are steeper, some of them being active and other passive. Many of alluvial fans are the result of tributary stream to the main river on the flood plain. Example; Ombei in Ramisi river near Ramisi sugar factory.

v. Bajada / Bahada:

Are the very large and wide alluvial fan. It is formed when alluvial fans form close to each other, their semi-circular fans may merge (combine) to form Bajada.

vi. Meanders:

Meanders is a curved bend of a river in its valley. The meanders flow in winding course i.e not flow in a straight line. It is formed when the river has widened its valley by lateral erosion and its deposition of sediments slackening the movement of water making it flowing in swinging direction. In general, meanders indicate the lowering speed of the river. The meanders occur both at lower part of middle stage of river and at old stage of a river. Example; It found along river Ruaha in Iringa-Tanzania.

vii. Braided stream:

It is an extremely wide and shallow channel of river with series of inter-connecting minor channels. It is formed when the main river divided and subdivided into series of interconnecting minor channel (distributaries) separated by sand banks and bar Isles of alluvium for long distance then join again to continue the channel flow. Its submerged during the food. Example; River Nile at Bujjagali falls in Uganda, etc.

viii. Ox-bow and ox-bow lake:

What is the ox-bow?

Ox-bow is the horse shore or crescent shaped at final section of river once pronounced river meander but now separated from the main river.

What is the ox-bow lake?

Is the water hole with crescent shaped formed at lower part of river when the ox-bow shape filled with water. It is also called mort lake.

How are formed?

Ox-bow or ox-bow lake are formed due to cut off of the loops of neck of the river meanders or due to horse shoe shaped separated from main river at final stage of the river. If the water of the ox-bow lake dries up, it forms meander scar or ox-bow. Example of Ox-bow lake; Lake Utange in River Rufiji, L. Gambi in R. Tana, L.

Manzala in Nile delta, L. Kanyaboli in R. Yala, L. Avangas in Gabon, etc.

Stages of ox-bow lake formation;

1. A river forms meanders which are so acute that only a narrow neck of land separates the two ends of the meanders.

2. The narrow neck of land is narrowed further by erosion.

3. The neck of land eventually cuts through due to excessive river flooding.

4. The ends of the cut-off are completely sealed by deposition and an oxbow lake forms.

ix. Delta:

Is a large, flat, low lying plain of river deposits laid down where a river enters its mouth into the sea, or ocean, or lake, or another river. It is formed due to progressive deposition of a load at an old stage of a river. Example; Nile delta.

DELTA

Delta is a low-lying tract of alluvium deposits mainly formed at the mouth of a river. The growth of delta is interfered with the formation and development of several minor tributaries called distributaries.

TYPES OF MATERIALS DEPOSITED TO FORM DELTA

According to size and place of deposition there are three types of the materials deposited at the river mouth to form delta. these are;

Bottom set beds:

These are the finest materials such as silts and clays settled out same distance into the sea from the river mouth.

Fore set beds:

There composed by course particles which drop almost immediately upon the entering the sea sea or lake. They form layers that slopes current from the delta front.

Top set beds:

They are composed by materials of very large size which cover the fore set bed. The top set bed are thin horizontal beds deposited during flood.

CONDITIONS NECESSARY FOR DELTA FORMATION

The following are conditions necessary for the formation of delta. these are: –

1. The river course should be free from obstacles such as lakes or swamps for the sea out let delta.

2. The river course should be active erosion in the upper course and middle course, so that the river can attain a large load.

3. The velocity (speed) of river in the lower stage must be sufficiently low to release most of its loads.

4. The river loads should be deposited faster than its removal by the action of the tides, waves, and currents.

5. There should be presence the gently sloping land along the coast (continental shelf) to allow deposition of the materials to form delta.

STAGES OF THE DEVELOPMENT OF DELTA

Accumulation of alluvium:

This takes place in the river mouth within the continental shelf.

Formation of distributaries:

The river is forced to divide into small channels leading their way into the sea or ocean or lake or another river.

Formation of the lagoon:

These are formed when the areas of the water are surrounded by distributaries and their associated deposits.

Extension of the delta after lagoon have been covered by silt

In filling of lagoon with silts causes further division of the distributaries into smaller ones and complete covering of vegetation.

TYPES OF DELTA

The delta can be divided into two major group i.e

Inland delta

That formed when river deposited their load at the lakes or great ponds eg. Lacustrine delta.

Sea outlet delta

Its formed when the river enters their load into the sea or ocean eg. Arcuate delta, Estuarine delta, Bird’s foot delta, and Cuspate delta. In general, the following are the types of delta. these are: –

Arcuate delta:

Is the type of delta which is triangular in shape with convex shoreline on its seaward end. It is characterised by very many distributaries, lagoons are very common in this type of delta and their sediments cover very extensive areas very good for farming. Example; Nile delta in Egypt, Niger delta, Amazon delta in Brazil, etc.

Estuarine delta:

This is the type of delta built by river which deposit materials in the submerged river mouth. It takes the shape of the estuary, they have many distributaries but not extending to the sea due to carrying away by wave actions and tidal currents. Example; River Congo in DRC, River Volta in Ghana, etc.

Bird’s foot delta / Digitate delta / Labote delta:

This is delta which looks like a foot of bird with claws. It has long but very few tributaries bordered by levees just out from the shore. It is form where the waves and currents are very weak. Example; River Mississippi, River Omo, River Nyando in West Kenya, etc.

Cuspate delta:

Is a delta with a tooth like shape. Is is formed when a river reaches a straight coastline along which wave action is vigorous (active or strong). The materials spread out uniformly on either side of a river mouth. Lagoons are almost absent. Example; River Ebro in Spain, and River Tiber in Italy.

Lacustrine delta:

This is the delta formed when the river pours its sediments into the lakes before reaching to the sea. The inland delta are almost areas of extensive deposition. Example; River Okovango in Okovango swamp in Botswana, Lake Geneva and Thur in Switzerland, etc.

IMPORTANCE OF DELTA

The following are the importance of delta. these are;

i. Delta absorb runoff from floods both river and lakes or ocean. ii. Delta filter water, therefore help to reduce the impact of water pollution as it slowly makes its way through the delta’s distributary network.

2. It is home of many species for both plants such as mangroves and animals such as fish like crustaceans such as oysters, etc.

3. Delta contain rich fertile soils such as alluvium soil which support agricultural activities.

4. Delta source of building materials such as sands and gravels.

5. Delta support tourism sector because they attract tourists hence capital revenue.

REVISION QUESTIONS

1. Describe the stages of the river profile development.

2. Discuss the landforms and the factors involved in the development of cross profile of a river. (Necta 1973)

3. Describe features produced by river erosion.

4. Examine six conditions/factors responsible for formation of Gorges and Canyon features in the river valleys.

5. The long profile of river contains three stages on which different landscapes are formed. Erosional features are formed in young stages of the river due to the different categories of the erosional processes. Explain six points to show how gorges and canyons are formed.

6. Express the geological structures/ways which lead to the occurrence of water fall.

7. Describe features produced by river deposition.

8. Briefly explain conditions/factors, types and stages for delta formation.

RIVER CAPTURE / STREAM PIRACY

River capture is the process where by one river diverts (capture) the course of neighboring river into its course.

The stronger river develops into a piracy stream which goes on advancing towards the weaker river and completely captures it neighbor and divert its course.

River capture contain both small scale capture of a single stream and/ large scale capture the entire basin.

CONDITIONS NECESSARY FOR RIVER CAPTURE TO OCCUR

1. The capturing river must be stronger or have enough greater energy for vertical and headward erosion to occur.

2. The pirating river must flow down a much steeper gradient for its victim (captured river), for example the escarpment or plateau.

3. The pirated stream must flow over easily eroded soft rocks such as sandstone or rocks that have been broken by faulting, etc.

4. The pirated river must be weaker enough for its water to be captured.

5. For good capturing process, there must presence of alternate layers of hard and soft rocks enabling occurrence of trellis system of flow.

FEATURES FORMED BY RIVER CAPTURE / STREAM PIRACY

There are several features which provide real evidence of river capture. These are;

Elbow capture:

This is the sharp band or changes in the direction of the river course at the point of capture. It is formed when the river has been diverted. However not all bends are due to the capture.

Wind gap (dry wall):

The dry valley of the beheaded stream below the point of capture on elbow and may be filled up with alluvium. It may be left as a dry valley.

Misfit stream:

The beheaded stream with having lost its head water due to river capture and has been reduced in volume for such that it become too small for its valley. The mist stream latter on disappears completely.

Incision:

The dissected point that occur near to point of capture due to an increased erosive power of capturing river. The pirated stream may rejuvenate and increases (dissecting its valley) below the point of capture.

Pirate river:

This is the small stream that joining between the strong river to the weaker river.

Differed junction:

The point where by the pirate stream join the main river (strong river).

RIVER REJUVINATION

River rejuvenation refers to the renewed of the erosive power of the river in the valley.

During rejuvenation vertical erosion is predominant and therefore leading to deepening of the river valley. There rivers therefore, assume the characteristics of the youth stage in term of erosive power since of become energetic than before. The river rejuvenation process can take place at any stage of the river erosive activity has renewed.

TYPES OF RIVER REJUVINATION

There re three (3) types of river rejuvenation. These are: –

Static river rejuvenation:

This type of river rejuvenation occurs due increases in water volume of river, mainly caused by heavy rainfall, ice melting or increasing of feeder streams due to river capture.

Dynamic river rejuvenation:

This type of river rejuvenation occurs due to changes of land level (river gradient) influenced by tectonic activities like faulting, folding and warping. It’s also known as tectonic or isostatic rejuvenation.

Eustatic river rejuvenation:

This type of river rejuvenation occurs due to fall of sea level or fall of lakes mainly caused by the earth’s movement (diastrophism).

CAUSES OF RIVER REJUVINATION

The following are causes of river rejuvenation. These are;

Eustatic movement:

Negative eustatic movement i.e fall in the sea level may force the river to concentrate its erosive power down a valley so as to allow the change of its base level. Negative sea level change can be caused by earth movement, glaciation, etc. example during ice age periods.

Tectonic activities:

These includes land uplifting caused by earth movements such as faulting, folding, earth quakes, etc. when these process on the river course occur, the river become more energetic and erosive dissecting its valley down ward.

Increasing of the river discharge:

When the volume increases due to heavy rains, presence of feeder streams, ice melting, etc. its erosive power also increases leading to river rejuvenation.

Human activities:

The human activities like construction of dams along the river course, etc as river rejuvenation to occur.

EFFECTS/CHARACTERISTICS FEATURES / LANDFORMS/IMPLICATIONS OF RIVER REJUVINATION

i. Knick point:

This is the sharp peak of the slope in the long profile of the river valley. It is point in the river where the river bed drops sharply and it position marked by water falls or rapids.

If the base level fall or break will appear in the slope of the long profile which gradually work it way of stream. a river may have several Knick points and some may persist in one position for a long time if they are hold by a band of a hard rock. Example Charldtle fall in Sierra Leone.

ii. Paired river terraces:

These are steep or benches on either side of the river valley as a result of undercutting of the river due to renewed erosion and abrupt fall in the sea level.

The terraces are benches out covered by a large of alluvial deposit. The rejuvenation terraces are equal height on either sides of the river valley and therefore called paired terraces.

Sometime are quite wider and may extend along a valley for many kilometers.

iii. Water falls:

This is the sudden fall of the river at Knick point down a valley. It occurs when the slope drops sharply at a point of rejuvenation. The river may appear to have several water fall with several Knick point.

iv. Valley within a valley:

This is the subsidiary (new) valley within a former (main) river valley was formed due to sudden fall of base level. The valley might be so steep sided leading to the formation of gorge.

Number 1 and 2 the old valleys but number 3 the new valley

v. Incised meanders:

These are steep sided curved bend of the river valley formed as a result of undercutting of the river bed by river meanders during rejuvenation process.

Most of incised meanders are the result of rejuvenation of an already meandering stream caused chiefly due to fall in base level.

Types of incised meanders:

There are two (2) types of incised meanders. These are;

Ingrown Incised Meanders:

These are incised meanders with asymmetrical side of a width cross profile. They have alternate steep sides and wider cut slope facing gentle slope (side) with large slip off slopes.

Ingrown meanders develop on resistant rock and where base level falls gradually, therefore meanders tend to shift laterally as they incised producing asymmetrical profile. Example; River Mkomazi in Tanzania.

Entrenched Incised Meanders:

These are symmetrical steep sided meanders with vertical sided formed by vertical erosion on homogenous rock eg. soft rock.

Intrenched incised meanders are formed on weak rocks and where base level falls quickly causing rapid vertical incision. Example; Meanders of River Tyne in Northern England, etc.

vi. Gorge and Canyons:

These are deep sided trough or hollow formed when undercutting become concentrated into the river bed. They are formed when vertical erosion cuts down more rapidly than lateral erosion resulting into hollow, which are very deep compare to their width eg. Grand Canyon in U.S.A.