Hello. I'm RachelAng. :D i'm helping him post because he asked me to. :D He has finished his NLevels today and i dont know where he is. He'll be back soon to update more. :D TwentyOhNine ; Iloveyou. ♥
Kay I'mma going to bath . Spam geog notes later on my blog (:
XD k , bye for now .
Back !
K Geography Spam now .
Rivers and the hydrologic cycle !
1.Rainfall-Water falls as rain or snow if the air is cold enough !
2.Groundwater - Water that seeps into the ground may be absorbed by plants or flow into rivers,lakes or seas
3.Runoff-Water flows from highlands to lowlands into rivers,lakes or seas
4.Evaporation-The sun's heat causes water to evaporate. Water changes into water vapour
5.Transpiration-Plants give out water vapour through their leaves
6.Condensation-Warm air cools as it rises.Water vapour changes into water droplets,Small water droplets combine to form bigger droplets.Millions of water droplets gather together to form clouds !
River System !
A river is a body of water flowing in a channel . It begins at a place known as its source.The source of a river is usually located in the mountains or highlands.This source may be a lake or melting snow on top of a mountain.
Due to gravity,Water always flows downhill from its source.As a river flows,it travels along a path known as its course.The course of a river may be divided into three parts-the upper, middle and lower courses
Nearest to the source is the upper course of the river.Here,the river channel tends to be narrow and shallow,and only a small amount of water is transported.In the upper course, the gradient or slope of the river is usually steep
As the river continues to flow downhill,its characteristics change.In the middle course of the river,the gradient of the slope is usually not as steep as the upper course.Here,the river channel is wider because other smaller rivers or steams may join the main river.These smaller rivers and streams are known as tributaries.
In the lower course of the river, the gradient is very gentle and the river channel is wider than in the middle course.The volume of water that flows in the lower course also increase because more tributaries have joined the main river along the way .
A river ends when it flows intro a lake,sea or and ocean.The part of the river that enters the sea is known as the river mouth .
Factors affecting the speed of a river
Gradient of the channel -Rivers flow downhill from a high altitude due to gravity.The speed at which a river does so depends very much on the steepness of the channel.In other words,it depends on the gradient of the channel.If the gradient is steep,the river will flow quickly.On the other hand ,if the gradient is gentle,the river will tend to flow slowly.
Roughness of channel -As water flows through a river's channel,it encounters obstacles such as rocks,boulders,underwater vegetation or an uneven river bed.These obstacles cause friction between the water and the channel.This friction will slow down the river flow and reduce the speed of the river.Therefore,a river with a smoother channel will have a higher speed than a river with a rougher channel .
Wetted Perimeter -River channels can be of various shapes and sizes.The shape of a river channel affects the wetted perimeter,which is the length and breadth of the channel in contact with the river's water.If the wetted perimeter is large,more water comes into contact with the channel.Thus,a lot of friction is generated as the river flows along the channel.Friction causes the river to flow more slowly.On the other hand,If the wetted perimeter is small,less friction is generated and hence the river will tend to flow faster
Factors affecting the volume of water in a river
Size of drainage basin -In general, the bigger the drainage area,the greater will be the amount of water in a river because when the drainage area is large,there is more surface are for rain to fall on .The rain flows over the land as surface runoff or seeps into the ground before eventually reaching the river.In comparison,smaller drainage areas usually do not collect as much rain water
Presence of vegetation -Vegetation intercepts rain and allows more water to infiltrate or seep into the ground.Hence,abundant vegetation allows more rainwater to infiltrate the ground,thereby reducing the flow of surface runoff.As a result ,less surface runoff flows into the river and the volume of water in the river is smaller.Conversely,sparse vegetation does not allow much rainwater to infiltrate the ground,thus increasing the flow of surface runoff.Thus,more surface runoff flows into the river and the volume of the river is larger .
Permeability of rocks -There is a high tendency for rain falling onto an area with permeable rocks to seep into the ground.This would result in less surface runoff compared to an area with less permeable rocks.Hence,a river flowing in an area with less permeable rocks will most probably have a larger volume of water in its channel compared to a river flowing in an area with more permeable rocks.
Climate -The climate of the area in which a river flows can also affect the volume of water in its channel.In areas with distinct wet and dry seasons,the amount of water in the channel fluctuates according to the amount of rainfall received during each season.In other areas which are generally dry,the volume of water remains low throughout the year.On the other hand,rivers found in hot and wet equatorial areas have a high volume of water in their channels.
River Processes
Erosion -As a river flows, pieces of rock are removed from the river bed and banks,and moved along the channel.This removal and movement of rocks downstream by the river is known as erosion.Erosion causes the river channel to deepen and widen.
Vertical erosion occurs when rocks and other materials on the river bed are removed,causing the channel to become deeper.
Lateral erosion occurs when rocks and other materials along the sides of the channel are worn away.This causes the river channel to become wider
CASH
Corrasion -Corrasion,alson known as abrasion,occurs when rock fragments are dragged along the river bed or against the river banks.This grinding action causes the river channel to widen and deepen as rocks and soil are scraped off the sides and bottom,and carried away with the water.Corrasion is particularly effective during a flood when the river's energy is at its maximum
Attrition -The river carries a load of eroded materials which contain rock fragments and other materials.These materials constantly collide with one another as they move.Overtime,the collision causes the materials to break down into smaller pieces.Through this process,larger rocks are slowly broken down and their rought edges are smoothed out.This process of erosion,whereby the rocks in the sediment load are eroded by colliding with eact other as they are carried along the river is known as attrition
Solution -As rain falls to the earth's surface,it reacts with the carbon dioxide present in the atmosphere,forming a weak acid known as carbonic acid.As this rainwater falls into rivers,the weak acid dissolves minerals present in the rocks on the river bed and banks
For Example,when a river flows over limestone,a chemical reaction occurs between the carbonic acid in the water and the calcium carbonate in the limestone.This chemical reaction,which is called carbonation,dissolves the calcium carbonate to form calcium bicarbonate.Solution occurs because calcium bicarbonate is soluble in water
Transportation -When a river has sufficient enrgy,materials will be carried or transported downstream.These materials are carried downstream by four different transportation processes.These processes depend on the energy of the river and the size of the materials involved.For example,gravel can be transported by different transportation processes depending on the energy of the river.
Traction -Larger sized materials,such as boulders,pebbles and gravel,are found on the river bed.If the river has sufficient energy,these large materials are moved downstream by the process of traction,where they roll or slide along the river bed.
Saltation -Saltation is the movement of materials downstream in a series of bouncing motions.Smaller materails,such as coarse sand particles,may be moved along the river's course in this manner as they get lifted up and dropped onto the river bed over and over again
Suspension -Finer particles such as silt,clay and sand are carried along the river without touching the river bed.These materials,which are held in suspension usually form the greatest proportion of the river's load.They are also responsible for giving the river a murky appearance
Solution -As mentioned earlier,erosion takes place when mierals found in rocks such as calcium carbonate,dissolve in the river water.The running water will continue to transport these dissolved materials downstream.This process is known as solution
Deposition -Deposition occurs when a river does not have enough energy to either erode or transport material.It then puts down or deposits the load it is carrying.This usually occurs when the speed of the river is reduced.For example,during a period of low rainfall,the volume of water in the river drops,and so does the speed of the river.Thus the materials that are eroded in the river channel get deposited
Waterfalls -Waterfalls are sudden,steep,vertical flows of fast-flowing water falling from great heights.A waterfall is usually found in the upper course of a river where the gradient is steep.
Waterfalls are normally formed in two ways.They may be formed through the erosion of rocks of different resistance or though faulting.
A waterfall may also be formed through faulting.Faulting involves the movement of rocks along large cracks in the earth's crust.This movement may result in the lifting of one block of land above the other.When a river flows down from the edge of the uplifted block,a waterfall and a plunge pool are formed.
Gorge -When a river flows through an area with resistant rocks by its sides,it can only erode its channel vertically.Over time, a deep,narrow and steep-sided valley known as a gorge is formed.Gorges are more commonly found in the upper course of a river.
Valleys -Erosion by rivers also create landforms called valleys.A valley is a low area between hills or mountains.Valleys have different shapes.The shape of a valley depends on how the river erodes the land over which it flows.
In the upper course of a river,the gradient of the land is very steep.Thus , the river has sufficient energy to erode the rocks in its path and cut deep into the channel,causing vertical erosion to occer.As a result,V-shaped,narrow and steep-sided valleys are formed.
In the middle course of a river,the gradient of the land is not as steep as in the upper course.Therefore,the river flows with less energy and cuts less deeply into its channel.Lateral erosion occurs more than vertical erosion.As a result the sides of the vallet in the middle course tend to be wider than in the upper course.
Int he lower course of a river,the valleys are exceptionally wide compared to the upper and middle courses becauses the gradient of the land is gentle.As a result,lateral erosion rather than vertical erosion occurs.This gives rise to a broad flat-floored valley
Floodplains and levees -When there is heavy rain,a river may overflow its banks.This causes the land on either side of the river to be flooded.As the floodwater from the river spreads out over a larger area,the water loses speed.When the water recedes,the river deposits the material it is carrying on the land,leaving behind alluvium on the river banks and river beds.
The coarse sediments are deposited closer to the river's edge because they weigh more.The fine particles are deposited further away from the river.The deposited material or alluvium,builds up on both sides of the river as more and more floods occur.Overtime , wide and flat plains called floodplains are formed on either side of the river.The heavier and coarser materials nearer to the river form raised banks called levees.Floodplains and levees are depositional features which can be found at the lower course of a river.
For meanders please refer to your own textbook (:
Oxbow lakes came out last year , So i didn't studied for it
Deltas and distributaries -As a river approaches a sea or lake,its speed decreases.Thus,it deposits its sediments at the river mouth.Some of the deposited sediments may be removed by tides and currents.However,if the rate of deposition is faster than the removal of these sediments,the sediments will accumulate and build up at the river mouth.This results in the formation of a delta,which is an accumulation of alluvium and sediments at the mouth of a river where it enters a sea or lake.
Over time, as more alluvium is deposited,the delta extends outwards into the sea.When this happens,the river channel branches out to form many smaller channels known as distributaries.
Management of river channels
Re-alignment -Re-alignment is the straightening of the river channel.This method removes meanders and reduces the length of the river channel in order to increase the speed of the river.The faster flow of the river allows water to flow away from the area more quickly and wash away sediments which have accumulated on the river bed,hence minimising localised flooding.This also deepens the channel,allowing it to hold more water,thus reducing the occurrence of floods
Re-sectioning -Re-sectioning is the widening and deepening of the river channel.Re-sectioning increases the channel's ability to hold water,thus decreasing the cances of floods.Re-sectioning also increase the amount of surface runoff from the surrounding areas into the river because more surface runoff can now enter the river without causing it to flood.
In addition , the river bed and banks may be smoothened by replacing the soil of the river banks and bed with cement and granite.By doing so,there is less friction between the water and river bed and banks.Thus,the speed of the river will increase and allow water to flow away from the area more quickly
Bank protection -The purpose of bank protection is to prevent floods from occurring and to reduce soil erosion along river banks
A method of bank protection is building artificial levees or dykes.These walls of sand,stone or concrete are built along rivers that often overflow their banks.These walls of sand,stone or concrete are built along rivers that often overflow their banks.These structures increase the capacity of the river to hold water,thereby reducing chances of flooding.For example,in china,dykes have been built for centuries in the lower course of the yellow river. However,these dykes have not been very successful because of the continual build up of sediments on the river bed,making the channel shallower.This has caused water levels to rise higher than the dyke walls over the years.Thus,higher dykes have to be built regularly or the sediments have to be dug up and removed from the river bed .
In some countries concrete structures like gabions and revetments are built along a river channel to divert the flow of water from the river banks to the centre of the channel.This protects the river banks from being eroded by the force of running water,thereby reducing the amount of sediments that flow into the river.When lesss sediments are washed into the river,the capacity of the river to hold water increases .When this happens,the river channel will flood less frequently .
Dykes are stones walls built along the banks of a river to prevent floos and soil erosion from occurring at a river
Gabions are structures of small rocks contained in wire cages.Gabions are placed along river banks to prevent the river banks from being eroded
Revetments are walls made of stone.They are placed along the banks of rivers to prevent erosion .
Planting Vegetaion -Planting vegetation along a river can also protect river banks against erosion as the roots of trees and other plants hold the soil firmly.Vegetation also slows down the rate of surface runoff,preventing large amounts of water from entering the river at any one time.Furthermore,planting vegetation reduces the amount of eroded sediments being transported into the river.Thus,the channel does not become shallower,reducing the chance of floods.
Negative effects of channelisation
Flooding downstream -The increased speed of the river after channelistation may result in flooding downstream.As a result of channelisation upstream,the volume of water flowing downstream is increased.However,the channel in the area downstream may not be able to contain the increased flow of water.The channel overflows and flooding results.Hence,although floods in one part of the river's course are managed,they can still occur in another part of the river
Accumulation of sediments -With channelisation,the river now has a higher speed and more energy to carry the sediments downstream.The sediments,which are normally deposited on the river bed,get washed downstream or into the sea or ocean.The increased sediments in the oceans have resulted in less sunlight reaching the seabed,affecting the growth of marine life like seagrasses and coral reefs
The construction of dykes,revetments and gabions also causes sediments to accumulate behind these structures.Overtime,as sediments continue to build up,the river channel becomes shallower.Thus when prolonged rains occur,the water in the river may overflow these structures and lead to flooding
Loss of marine life and wetlands -Int he process of channelisation,ecosystems may be destroyed.For example,although there are benefits in planting vegetation along river banks,too much vegetation can harm marine life.Excessive shade prevents sunlight from reaching the river bed,killing aquatic plants and affecting aquatic food chains.
Changing the course of the river and lining the natural banks of the river with concrete surfaces in channel re-sectioning,also affect plants and animals as their habitats may be destroyed
Channelisation may also lead to the disappearance of wetlands.In the case of river re-alighnment,for example,by shortening the course of rivers,the area of wetland shrinks because the river flows over a smaller land area now.For instance ,to prevent further destruction of wetlands,the government of the united states of america has now implemented a no nett loss in wetlands policy which aims to discourage channelisation.When channelisation is carried out along a river,it is necessary to create an area of wetland in another area along the river.This policy ensures that plants and animals that live in wetlands continue to have a place to live in.
All in all,we have seen that channelisation has both negative and positive impacts.Therefore,before we make changes to rivers,we need to evaluate the impact of channelisation before altering the course of a river to suit our needs.
