RIVER
Mass of water flowing in a natural channel over the earth surface from the high land to low land under the influence of gravity.
TYPES OF RIVER
1. Perennial River
These are rivers which flow throughout the year .The source of these rivers is the region with a abundant and well distributed rainfall throughout the year. E.g Nile River(Africa),The Congo river(Africa) and the Amazon in South America.
2.Intermittent Rivers
These are the rivers which flow only during the wet season in the regions which receive seasonal rainfall.
3. Ephemeral Rivers
These are the rivers which appear during the rainy season in the areas which experience very little rainfall especially the desert. They tend to disappear immediately after the rain season has stopped.
River vs stream
– River and stream are used interchangeably.(used the same way)
Terminologies associated with rivers.
1. Rivers head. (River source).
Is a point where the river or stream begins. It makes its first appearance on the surface / it is the highest point on a river system.
Possible river sources
a) Lakes – R. Nile in Lake Victoria.
b) Mountains with plenty rainfall e.g. Rockies and Appalachian ( Mississippi river ) R. Ganges, Indus (Himalayas)
c) Springs.i.e. Thames River in England.
d) Melting ice i.e. Rhine river in France.
2. River mouth.
Is a point where the river ends. The lowest point / base level of the river.
Possible river mouth
a. Oceans – River Rufiji – Indian ocean.
– River Nile – Mediterranean Sea.
b. Lakes – River Kagera – Lake Victoria
-River Malagarasy – Lake Tanganyika.
c. Swamps
d. Others rivers – (Blue Nile meets with White Nile)
3. Water shed/ catchment area/River basin.
Is a collecting ground of single river system. Where a river collects its water.
Tributary:Is a branch of river pouring its water into a main river.
Distributary: Is a branch of a river which collect its water from the main river.This is more prominent in the lower stage as is associated to delta formation.
4. Divide:
Is a highland separating two adjacent river systems.
5. River system:
The main river and its tributaries ( distributes)
6. River valley:
Lowland between two hills of drainage basin where at the bottom the river flows. (At the lowest point a river flows)
7. River bed:
Is the actual part of the river which is covered by water (flowing water).
8. River load:
Is the materials carried by running water.
WORK OF RIVERS
Works of a river includes three processes.
a) Erosion
b) Transport
c) Deposition
1. RIVER EROSION.
Is the progressive removal of materials from the floor and side of the river / progressive removal of materials from the river bed.
Vertical erosion. – Deepens the river channel.
Section across a river channel.
Lateral erosion – Widens the stream.
Head ward erosion – Takes place on the upper course which lengthens the stream.
The process of river erosion is accomplished through four interaction process.
i. Hydraulic action
Refers to the force of moving water which is able to remove loose materials such as gravel, sand and silt and which is able to weaken solid rock by surging into cracks in the rock [from the sides and the floor of the river]
ii. Corrosion
Is solvent action of water (solution). Process of removing soluble materials by moving water e.g. Limestone or Calcium carbonate.
iii. Corrasion
Wearing away of the river bed by the load of the river.
iv. Attrition
Impact of the load of the river upon itself, this takes place because the rock fragment which made up the load are in constant collision with each other.
2. RIVER TRANSPORTATION
Movement of materials from one place to another by the river.
Mechanisms:
i. Suspension.
A lite material whose specific gravity is less than one is carried above the floor as suspended load.
ii. Saltation
Those particles which are large are transported in series of hops (bounces from one point to another, touches the floor at given interval).
iii. Traction.
Transportation of the load by dragging on the floor. Continuously touches the floor.
iv. Solution.
If river passes through soluble rocks, the materials dissolve in water and become soluble and cannot be seen.
– Transportation of the load depends on the energy and power of the river.
Power – ability to do work but power depends on energy to be able to perform a certain work.
Energy of the river
Depends on
i Volume
ii Velocity.
Volume + Velocity = Discharge.
i. The volume of the river is how large the river is (size)
ii. The velocity – How fast the river flows.
– Large volume will have more energy than a slow flowing river also the larger the velocity the greater the energy.
– Volume and velocity of a river is what is called river discharge ( is the number of cubic meters per second passing through a particular section river ) (m3/sec)
– This discharge is measured by a current meter which is placed in the river which has a dialogue (current meter) which automatically records the energy of the river at any point.
-The velocity of a river varies from one place to another across the channel, Due to ;
i.The middle part of the river has maximum velocity.(maximum energy ) because the friction is less.
ii. Velocity of the river varies with gradient. A river with steep gradient has high energy compared to a river.
iii. The shape of the channel also has great influence on the energy of the river.
– A shallow and wide channel has less energy because friction is less compared to a wide channel.
A. Channel B losses more energy through friction than channel A but channel C has the greatest available energy due to the channel.
B. Due to the large size of the channel.
C. It is important to note the difference between the river channel and the river valley.
THE HEIGHT OF A RIVER ABOVE THE BASE LEVEL OF THE RIVER.
Base level of the river.
Is the lowest level of a river can erode. When a river ends on the lakes or oceans is the base the river.
-The height of a river above its base level gives it what called potential energy (P.E). (Energy due to position.)
-When water flows, potential energy is converted into kinetic energy which does the work of the river.
A river uses its potential energy to.
i. To overcome friction in the river bed.
ii To erode
iii. To transport the materials /load.
The rate of erosion along the river channel depends on;
i. Volume of the river.
The larger the volume of the river the higher the rate of erosion.
ii. Velocity.
The higher the velocity the greater the rate of erosion (greater the rate of distraction)
iii.The type of rocks over which a river flows:
A river can flow over soft rocks, erosion will be more and rate of distraction will be high but if flows over hard rocks, erosion will be low.
iv. Type of cutting tools (type of load the river carries).
To do corrosion if it takes large load the rate of erosion will be high compared to a river which carries small load the rate of erosion will be low.
RIVER COMPETENCE AND RIVER CAPACITY
RIVER COMPETENCE
Is the ability of a river to carry large load in term of size individual particles.
-At particular places and particular velocity.
– A river competence is high where a river is narrow because the energy is higher compared to large slow moving water because energy is slow.
River capacity
Is the ability of a river to carry a large load in terms of the volume.
– A large slow moving river has high capacity but low competence and vice versa.
– A fast flowing moving river has high competence but low capacity.
– The ultimate goals of a river are to being the land above the sea level to its base level. But this cannot be achieved because there is adjustment.
3. RIVER DEPOSITION.
What is river deposition?
Lay down /dropping of the load transported by a river.
Why deposition.
– It deposits its load when the energy of the river is insufficient to carry the load further.
When does this occur?
i.When the volume of the river decreases.
When the does the volume of the river decrease and force deposition?. When it enters arid and semi-arid regions (dry land /hot desert) because evaporation increases and percolation increases (more reduction of water in the channel as it Wets the ground) or when it enters regions of porous rock and limestone regions or in the dry seasons (droughts) because no addition of water from rainfall.
ii.When its velocity decreases (speed).
When does the speed decrease? When the gradient falls velocity decreases and energy decreases
iii.When a river enters a lake or swamps the speed decrease because it meets with another force.
iv. When a river enter ocean/sea. Because it encounters some waves and tidal currents (encounter forces).
The stronger the encounter force.
v. When the river channel widens. Friction increase where by the energy decrease and leads to deposition.
DEVELOPMENT OF THE RIVER VALLEY
Long profile and cross profile of a river.
Long profile of a river:
Is the whole length of a river from its source to its mouths.
Cross profile of a river:
Is the width across the river from bank to bank.
– River erosion leads to development of varies features along the valley as erodes from sources to mouth. These features are studies acquired to stages of the river.
THREE STAGES OF A RIVER
Upper / torrential / youthful stage
Middle / mature
Lower / old /plain
LONG PROFILE OF A RIVER FROM ITS SOURCE TO ITS MOUTH
CROSS SECTIONS
CHARACTERISTIC FEATURE OF YOUTHFUL STAGE /UPPER
1) Deep, narrow valley (v-shape) because vertical erosion is dominant. Therefore it deepens the valley.
2) Valley has step gradient –The speed of the river is very high
3) Presence of pot-hole
4) Presence of interlocking spurs
5) Presence of water fall and rapids.
POT –HOLES
–These are circular depression on the river bed.
-H20 swirl when it passing on that depression
– Formed where the rock is softer than the surrounding rocks due to uneven river bed. Erosion of fast flowing water swirls in the depression deepening and widening it.
-A. pothole can be much wider and deeper depression
Form at the base of a water fall and form a plunge pools.
Presence of inter locking spurs.
What are spurs?-is a high land projecting into a lower land. A river at the upper course cannot overcome obstacle it will swirl around the obstacles.
-Spurs alternating on either side of the river interlock/overlap
Waterfalls -A sharp breaks on the river channel where the water falls from higher level to a lower level.
Causes of water falls
1. Difference in rock hardness
2. Uplift of land (tectonic forces ) (earth movement)
3. Glaciated valleys.
4. At a cliff.
5. River rejuvenation.
Waterfalls formed by difference in rock hardness
a) Inclination of the hard rock. Rock layer is horizontal
b) Rock layer dips up stream
c) Rock layer is vertical. (vertical dyke
Example of water falls
1) Gersoppa falls India
2) Victoria falls Zambia
3) Niagara falls between lake Erie and Ontario
4) Living stone falls Zaire river
Rapid -: Is a part of stream where there is sudden increase of speed/velocity of water in a stream/river.
Causes of Rapids/when do they occur
i. When the hard rock dips gently down stream
Rapid.
ii. During recession of water fall (Retreats)
Gorges – An elongated steep sided trough/hollow (deep) always occur where waterfall retreats. (Waterfall migrate up to river)
Canyons – Formed by H2O recession or uplift of the land (areas of up lifting) e.g. the grand canyon.
CHARACTERISTIC FEATURE OF MIDDLE/MATURE STAGE
1) Valley has open V – shaped valley ->because lateral erosion is dominant.
2) The speed of the river has decreased because slope decreased volume/ has increased because more tributaries join it and this in turn means that the river load increases.
3) River beds are pronounce because of maximum erosion on the concave side and even undercutting of the outside of the curve, the concave banks stand up as river cliff ;because there little erosion or even deposition on the inside of the bend ,the convex bend slope gentle as clip – off slopes (smoothed ends of spurs ). The river starts to meander and river plains are formed.
4) Bluffs – As spurs are removed, their remains form a line of bluff on each side of the valley floor.
5) Deposition start to take place on a fully mature valley
CHARACTERISTIC FEATURE OF OLD/PLAIN STAGE
1-The gradient is very low and a river develops meander aimlessly along a wide flood plain. Its energy has decreased hence cannot overcome obstacles it then create banks
2-The main work of the river is deposition and deposition it forms some features on the river bed e.g.Braided stream,Ox-bow lakes,natural leaves ,delta etc.
Braided stream -Due to deposition on the river bed will build some mould and causes the river to split into several channels which rejoin and split again.
Formation of oxbow lakes -: Forms when meander is so acute that only a narrow neck of land separate the two ends of the meander.
a) Neck of the land separate 2 concave banks where erosion is active.
b) The neck is ultimately out through; this may be accelerated/often during river flooding.
c) Deposition seals the cute –off which become an ox –bow deposition take place along the two ends of the cut off and eventually seal off to form Ox-bow Lake.
Production of the natural levees
Formed through successive flooding near the river because as the water flood out of the main channel its speed is immediately checked by friction with the banks and the heavier sediments are drooped first.
-Ridge/embankment on the sides of the river formed caused by river deposition especially after flooding.
-Deferred tributary –Is a tributary which flow parallel to the main river which tries to enter it but the main river is high above the flood plain.
-Deferred junction –Is the point at which the tributary enters the main river.
Example; Ya-zoo streams flows for 200km without managing to join the river Mississippi.
Formation of delta-
What is delta: Is a large, flat low lying plain on the river mouth where deposition takes place.
È.Deposition continues to takes place on the river mouth which causes the river to divide into various channels called Distributaries
Types of delta
There are many types of delta. But the main types are;
i. Arcuate delta.
ii. Estuarine delta.
iii.Bird foot delta
i. Arcuate delta
– It is made up of many distributaries.
-The load is composed of coarse and fine materials.
-It is triangular in shape.
Examples; Nile, Ganges, Niger, Indus and Hwan Ho.
ii. Birds foot (digital)Delta -Has few and long distributaries
-Triangular in shape -Fine and very fine sediments.
-This will occur where the river energy is very low.
Examples; Mississippi, Omo River in Ethiopia. And also the wave energy is low:-deposition takes place comfortably.
iii. Estuarine delta
– Has a shape of estuary (sub merged river mouth) deposition takes place on the sub merged part of the river (estuary). It is also triangular shaped.
It has no distributaries.
Examples; Elbe delta (Germany), Vistula delta (Poland).
Formation of flood
Formation o floodplains
– Is a broad gently sloping surface of alluvium deposits immediately after the river channel.
– Produced by the deposition of alluvial and other materials on the floor of the river valley through which river meanders
Stages in the formation of delta.
The stages in the formation of a delta in an ocean or sea are:-
a) STAGE 1 : Deposition divides the river mouth into several distributaries. Spits and bars arise and lagoons are formed. Depositions on the banks of the distributaries produce levees which extend into the river or sea.
b) STAGE 2: Some lagoons have already begun to fill with sediments which cause further division of distributaries into smaller distributaries. The delta has a more solid appearance though it is still very swampy and is usually well covered with water loving shrubs and threes.
c) STAGE 3: further in filling of lagoons plus the growth of a complete covering of vegetation results in the older parts of the delta coming to stand a above water level and to form dry land.
Conditions necessary for the formation of delta
1) The river must have a large load and this will happen if there is active erosion in the upper erosion of its valley.
2) The velocity of the river must be sufficient low to allow deposition
3) The rate of deposition must be higher than the rate of removal by tidal currents.
4) There shouldn’t be any obstacles in the upper levels of the long profile.
Why a large river like Congo not have delta?
The river Congo has a large load but a high velocity near its mouth which enables most of its load to be carried far out to sea, there by preventing the formation of delta.
River Niger also has a large load but its velocity near its mouth is low. Much of its load is deposited in its mouth where an extensive delta has formed
Value of Rivers / Economic importance of Rivers
1) Water supply for both domestic use, industrial uses, industrial uses and for irrigation purposes example;
2) Rivers are used for local transport (Navigation) they provide inland ports on their courses E.g. St Louis on River Mississippi in U.S.A.
3) Provide sites for hydro-electric power generation. Harnessing of hydro-electric power is common all over the world. E.g. Mtera dam in Tanzania, Seven forks dams on river Tana in Kenya.
4) Rivers are sources of building materials. Sand for building is scooped from the river beds and valleys like in Machakos in Kenya.
5) Rivers also form sources of various alluvial minerals like gold and diamonds e.g. alluvial diamonds are mineral along the course of R. Orange in S.A and Namibia.
6) River deposit alluvial soils a long their valleys during floods and at their deltas. These alluvial soils are fertile and hence for agriculture e.g. along the Nile valley and its delta in Egypt.
7) Rivers have features, which provide tourist attractions such features are like waterfalls and gorges e.g. Victoria falls.
8) Rivers provide rich fishing grounds e.g. R Nile, R Tanah, River Nguruka.
9) Rivers form natural boundaries between communities, districts, provinces and countries e.g. Kagera river between Tanzania, Uganda, Rwanda.
DRAINAGE PATTERNS
-Removal of water from the surface
-Drainage pattern is the actual arrangement or layout of its tributaries over the surface.
Factors which influence drainage patterns
1) Slope –:
This will determine the direction and speed of flow the steeper the flow the higher the speed and vice versa.
2) The function of structure- :
Uniformity whether the rocks have the joints or uniform rock e.g. granite will be different from limestone which has joints.(rock with joints will cause the drainage pattern to follow the lines of weakness but uniformity rocks, the slope is the one which will determine the drainage system.).
3) Nature of rock- :
hard and soft rock .Drainage pattern develop of the soft rock because it is easy for water to penetrate unlike on hard rock, drainage pattern becomes diff to develop Alternating layers, having soft sand hard rock, the drainage pattern will develop on the soft rocks only.
Types of Drainage Patterns
1. Dendrites- This pattern has a shape like the trunk and branches of a tree without leaves. The tributaries join one another at a low angle (less than 90) from many directions.
-Develops where there is no structural control because such pattern develops in a uniform rock, the slope is the only factor which in thence’s the drainage pattern. Example Granite.
DENDRITIC PATTERN
2. Trellis –Pattern develops in a region which is made up of alternate belts of hard and soft rock.
-Shape is rectilinear or almost rectangular in shape
-Tributaries join one another and eventually join the main river at a right angle (90)
-Major control is the rock structure and nature of rock with joints or alternating layers of rocks.
-This drainage pattern gives rise to various types of rivers (stream)
(s) Sub Sequent River – Is any tributary which joins the consequent stream at a right angle.
(c) Consequent river – This is the main river which flows down slope.
TRELLIS:
(o) Consequent stream – Any stream which flows in the opposite direction to the consequent stream and join the subsequent stream. Almost right angle.
(MC) Minor Consequent river– A stream which flows parallel to the consequent stream and joins the subsequent stream.
3. Radial – Shape is like a spokes of a wheel. (Bicycle wheel) that radiates from the centre which can be a conical hill (volcano) ->Develops on a volcano. The major control is the slope. E.g. Granite, volcanic rocks, basalts.
4. Centripetal- Streams from various directions converge to the center. Common in inter – mountain basins or basins between highlands .Major control is slope.
5. Annular – A pattern with streams often joining at sharp angles, but arranged in a series of curves about a dissected dome, basin or crater area. Major control is the nature of the rock.
6. Accordant and discordant
1.Accordant –This is a normal drainage system of the river. In this drainage system, the river flows in accordance with the rock structure and slope. It follows the line of weakness hence hence revealing the relationship with rock structure and slope. This is described as being accordant.
2 Discordant – Drainage systems that are opposed to the dominant structure.(rock structure, slope and land forming processes.
7. Superimposed
– Some rivers have developed a drainage pattern which is in no way related to the structure of the region in which it occurs. The drainage pattern discordant to the structure of the land surface in which it occurs.
-Doesn’t have any relationship with geological structure. It has forced itself to be in a place
STAGES IN THE FORMATION OF A SUPERIMPOSED DRAINAGE PATTERN
a) Original folded surface.
b) Region is reduced to a plain due to erosion
c) Subsidence results in region being buried by new rocks but subsequent uplift sees the formation of a drainage pattern. The main river is draining at right angles to the axis of the original structure.
d) Tributaries to the main river develop wide valleys in the weaker rocks as the main river erodes vertically it cuts across the ridges of strong rock and form gorges. The stronger rock forms ridges because the weak rocks are worn away and not because the region has been uplifted.
e) Antecedent drainage – A river pattern disturbed by earth movement (uplift or folding). A river which is capable of maintaining its course after up lift and erosion is called Antecedent. E.g. Ganges River, Snake River and River Colorado.
RIVER CAPTURE (piracy)
It is process where one river diverse the head water of the neighboring river in its own course/valley (upper course)
Condition necessary for river capture to occur
i. The capturing stream should flow at a lower level than its victim stream (capture stream)
ii. The capturing stream must be stronger and must be flowing at a steeper slope than its victim.
iii. The capturing must be flowing over easily eroded rocks that are weaker rocks.
iv. River rejuvenation.
Processes:
a) Two stream which are adjacent
There is head ward erosion which makes S extend head ward to C2
b) After years of erosion C2 is diverse to C1 where all its water enters C1.
Evidence /feature related to river capture.
i. Elbow of capture.
Is a point where there is a sharp turn of the river. Is the sharp change in the direction of the river course at the point of capture.
ii. Wind gap (dry valley).
This is a gap between elbow of capture and beheaded stream which is dry. The beheaded stream will not dry because it receives water from other sources.
iii. Misfit stream (beheaded river).
A stream carries less water than the valley depth (less water and becomes too small for its valley). A river which is flowing on a valley which is wider than the size of the river.
iv. Rejuvenation.
Feature of river rejuvenation can be seen in the capturing stream.
Examples:
Great berg River capture in S.A, River Volta capture in Ghana, River Niger capture in Nigeria.
RIVER REGIME
Is the seasonal variation of the volume of the water in its channel.
FACTOR INFLUENCING RIVER REGIME.
CLIMATE
a. Precipitation is responsible for the variation of rainfall, snow melt.
– High amount of rainfall and snow melting will increase the volume of the river.
– River regime follows the rainfall regime.
– During winter, volume decreases and early summer volume increases
b. High temperature, melting increase and volume increase also during high temperature, evaporation increases and reduces volume.
– Low temperature freezing increase and volume decreases, this leads to the fluctuation of water volume.
NATURE OF THE ROCK.
Porous rocks and previous rock allow water percolation; hence much of the water sinks in the ground and reduces volume of water.
– Impermeable rock does not allow water to sink hence, increase volume.
SLOPE.
The steeper the slope less the percolation and evaporation, hence volume is maintained. In gentle slope, more loss of water because water stays for a long time hence percolation increases and evaporation increases.
VEGETATION
Variation matter with the area/surface the rivers flowing, the dry/bare land will have less water during dry season because of the increase in evaporation compared to the river passing throughout the forest.
– During rainy season, the bare land will have more volume because there is nothing which will retain the water but the river which passes through the forest, the vegetation retains the water.
NUMBER OF TRIBUTARIES JOINING THE RIVER.
The larger the number of tributaries, the larger the volume and vice versa.
HUMAN ACTIVITIES.
Taking place along the river or the river basin e.g. Irrigation, scheme, during dry seasons tends to reduce the water volume.
– Agriculture on the river basin clears the forest whereby they reduces the water volume.(farming activities)
– Clearing the land leads to increase in evaporation which reduce water volume.
TYPES OF RIVER REGIME.
There are three types of river regime as follows;
i. Simple River Regime.
Is a type of River regime where there is seasonal variation of water volume such that there is one high water volume period and one period of low water volume.
– These occur where there is one dry season and one wet season most common in the tropical regions. Example, Ruvu, Wami, Ruvuma Rivers.
ii. Double River Regime (Regime of first degree of complexity).
This is a type where there are two distinct of high water periods which maybe because of snow melt or double rainfall maxima.
– This occurs on equatorial regions where they are two peaks of high rainfall e.g. River Congo and River Amazon in S. America.
– The volume of the river varies according to those rainfall peaks.
iii. Complex river regime (Regime of second degree of complexity).
It’s a type of river regime which evidenced the longest river in world with wider basins and numerous tributaries of different regimes.
– They cut across different climates. Example River Mississippi, the river volume isn’t much affected because it across different climates in different regions.
IMPORTANCE OF STUDYING RIVER REGIME
All development schemes planned on the river and its valley should have proper knowledge on river regime.
Example, RUBADA – Rufiji Basin Development Project /Authority.
TVA – Tennessee Valley Authority
Kagera River Development Authority
1. NAVIGATION:
Navigation should be planned and the vehicle also should be planned according to the depth of the water.
– During high water level can use a certain vehicle and during low water level certain vehicle can be used (type and size of vehicle)
2. CONSTRUCTION OF HYDRO-ELECTRIC POWER STATION.
Plan for the machines to be placed considering the level of water which keeps on varying. Because during low water level and the machines are placed at a upper position, the water won’t be able to reach and the machines won’t work.
3. CONSTRUCTION OF BRIDGES.
The engineer has to plan before building sometimes water level increase and sometimes decrease. When water level increases, energy increases hence leads to destruction of bridges. To have effective bridges must study river regime so as to make strong bridges which can overcome high energy of the river.
4. FLOOD CONTROL.
Must know the time that flood may occur and the level of water which will increase.
– Construction of dams will control floods but also constructing dams should consider the variation in the level of water so as to be able to prevent floods to continue taking place Eg. In U.S.A TVA builder dams to control floods.
5. AGRICULTURE ACTIVITIES ON THE FLOOD PLAIN.
Example River Nile the consider/depend when the water level is high, irrigation is not important but during dry season, canals are constructed at the level of low water so that irrigation can take place, hence important to know the river variation.
6. SETTLEMENT.
To establish settlement on the river basin must know the river regime, the settlement must be above the area of floods so that during high water level the settlement should not be flooded. (Areas free from floods)
Concept of graded profile
– Consider the river long profile from source to mouth.
– Refers to the profile of the river, which has attained a state of dynamic equilibrium, where is there is balance between the rate of erosion and the rate of deposition.
– In its simplest interpretation, a graded river has gentle slope and long profile with the gradient decreasing towards its mouth.
– It’s concave in shape and smooth due to higher erosion in the middle profile of the river and less erosion at the source (great amount of materials) at the lower course, erosion is less because smaller volume of both of heavy loads and very low speed water and load.
CRITICISMS OF THE CONCEPT OF THE GRADED RIVER PROFILE
There are several obstacles that normally prevail along the river course, which in term distort the equilibrium that river attempts to attain includes;
-The variation in the nature of the rock on the river bed and banks of the river. Hard and soft rock that offer cliff resistance to erosion.
– Climatic variation, for which passes cliff climatic region cannot attain the stage. Variation rainfall and areas.
– The presences of water bodies like lakes in the river course. The lakes become the center of deposition of sediments.
– River rejuvenation. Either sea level changes, river capture interfere with the attempt to attain equilibrium
– Continuous erosion along the river channel can be an obstacle to the attainment of balance.
– Vegetation that occupies certain part of a river channel.
RIVER REJUVENATION:
– Juvenile – young/youth
– Juvenation – process
– Repeat /do again
River rejuvenation;
Is the process of renewal of the erosive activity of the river valley after it has reached its old stage.
After reaching its old stage instead of deposition it starts eroding.
Causes of river rejuvenation:
A. i. Eustatic change (fall in the sea level)
ii. Isostatic change (land uplift and subsidence)
iii. Discharge (increase in the river volume)
I. EUSTATIC CHANGE
Caused by the fall of the sea level (negative movement of base level)
The river profile will have to adjust so as to reach the new sea level.
– The sea level has changed from L1 to L2
– Withdraw of water from the ocean by glacial/during glacial period – fall in sea level.
II. DIASTROPHIC CHANGE. (Isostatic).
Fall of the sea level floor relative to the land or rise of the land relative to the sea floor – this will make the river start erosion again from the upstream.
B STATIC REJUVENATION.
i. Discharge.
Increase in the river discharge by increase in the volume of the river and can be caused by;
a. Increase in precipitation either melting of ice or rainfall
b. By river capture, when a weaker river is captured by the strong river the volume of the strong river will increase. This will make the river to start erosion again and will leave some marks /feature in the river valley.
FEATURES/LANDFORMS RESULTING FROM RIVER REJUVENATION
i. Knick point.
This is a point of a river valley where there is a sharp break of a slope as a old base level joins the new base level after river rejuvenation has occurred.
Note.
– If a gradual fall – gentle slope will cause rapid.
ii. Waterfall /rapid.
Where there is a knick point with a sharp break forms a water fall for the river to join the new base level e.g. Along the river Congo there is a knick point at old base level Bathurst fall.
iii. River terraces.(Paired)
Occur on the flooded area. There are the steps or benches on either side of the river valley formed as a result of undercutting of the river due to renewed erosion.
– If a river on a flood plain is rejuvenated, the down cutting of the river will produce terraces with equal size.
– If the process of river occurs several times a series of terrace will occur.
– Down cutting is vertical erosion, the volume of the river is the same. This will deepen the river.
– Leaves deposition on the terraces.
– The knick point keeps migrating up stream.
iv Incised meanders (incise – cut down).
These are the curved bend of the river valley that has been incised into the land’s surface so that the rivers now wind between steep valley walls.
Two types of meanders
Ingrown meanders – Asymmetrical (the valley is not uniform; one part of the valley is much undercut.)
– Occur due to resistant rocks which do not erode fast compared to the other side (concave side) which erosion takes place at a high rate.
Entrenched meander.
Is a steep sided symmetrical meander with side standing vertically and parallel to each other. This is produced by vertical erosion on rocks with almost uniform resistance.
– There is fast erosion.
v. Valley within a valley
– It is a deep step sided valley within the former valley of river. When the rejuvenation is fairly rapid to cause large fall of the base level producing steep sided valley.
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