- 3 Courses of a river
- Place where the river begins
- Many tributaries join up to form larger streams which in turn join to create river
- River starts to meander
- Many tributaries join the river
- Meanders are common
- Many distributaries
- River flows toward the river mouth and enters the sea
- Delta (land formed by sediments) may form at river mouth
- Watershed: A stretch of land that surrounds drainage basin (a.k.a. boundary)
- Drainage Basin: A river system that collects rainwater and comprises of streams and rivers
- Tributaries: Streams that flow in the river which is mainly found in the upper course.
- Distributaries: Branch out from the river which is mainly found in the river delta.
- Confluence: Area where the stream meets the river
- River Mouth: Place where the river meets the ocean
- River Delta: Landform created at the mouth of the sea where the river flows into. It is formed by the deposition of the sediment carried by the river as it flows into the sea.
- Factors affecting speed of water in river
|Factor|| Definition||High Speed/Volume || Low Speed/Volume|
|Gradient||Steepness of river|| River is steeper usually at upper course||River is gentler usually at lower course|
|Roughness of River Channel||Roughness affected by obstacles in river||Fewer obstacles --> smoother channel --> faster||More obstacles --> more friction --> reduced speed |
|Wetted Perimeter||Area in contact with the river. Shape of river channel affects the wetted perimeter (length and breadth in contact with the river)||A smaller wetted perimeter --> less friction --> faster||A larger wetted perimeter --> more water --> more friction --> slower|
- Factors affecting volume of water in river
| Factor||Definition||High Speed/Volume ||Low Speed/Volume|
|Permeability of Rocks||Permeability is the ability of rocks being able to hold water ||Permeable rocks holds more water --> less surface runoff||Less permeable rocks --> more surface runoff (more volume of water)|
|Vegetation Cover||Vegetation: Plants at the side of the river||Sparse vegetation --> less rainwater enter ground --> more surface runoff --> large volume||More vegetation --> more rainwater enter ground --> less surface runoff --> smaller volume|
|Climate||How rainfall/temperature affects the river|
Wet/dry weather --> water level fluctuates (drop/unstable)
|Hot/wet (high evaporation) --> high volume of water||Dry --> lower volume of water throughout the year|
|Size of Drainage Basin||Drainage basin is an area which collects water (comprises rivers)||Bigger drainage basin --> larger surface area --> higher volume of water||Smaller drainage basin --> smaller surface area --> smaller volume of water|
- The energy of a river determines the kind of processes that occur as the river moves along its course.
- When the river has high energy, it is able to remove or erode rocks and other materials in its path.
- The river may carry or transport these materials until it loses its energy and deposit them on the river bed.
- 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 ricks downstream by the river is known as erosion.
- Vertical Erosion occurs when rocks and other materials n 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.
The 4 processes of Erosion
- Hydraulic Action
- The force of running water is the main cause of hydraulic action. In some rivers, the force or energy of fast-moving water is strong enough to loosen the rocks and soil along the river bed and banks.
- Over time, the rocks and soil are dislodged and carried down the river. The river channel is eroded as a result.
- Corrasion, also 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.
- 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. Over time, the collisions cause the materials to break down into smaller pieces.
- Through this process, larger rocks are slowly broken down and their rough hedges are smoothed out.
- This process of erosion, whereby the load carried by the river is eroded as the rocks collide with each other when they are carried along the river, is known as attrition.
- 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.
- When a river has sufficient energy, materials will be carried or transported downstream.
- These materials are carried downstream by 4 different transportation processes.
- These processes depend on the energy of the river and the size of the material involved. For example, gravel can be transported by different transportation processes depending on the energy of the river.
- 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 is the movement of materials downstream in a series of bouncing motions.
- Smaller materials, 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.
- 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.
- As mentioned earlier, erosion rakes place when minerals 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.
- Happens 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. E.g. 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.
- Deposition can occur along the lower, middle and upper courses of the river. Generally, larger particles such as rocks and boulders are deposited fist, followed by gravel, then sand and silt.
- Factors that causes deposition
- During a period of low precipitation
- When a river enters a lake or sea, resulting in a decreases in its speed
- When a river flows along the shallower side of a river channel, there is increased friction with the river bank that causes the river to slow down.
- When the river receives a sudden increase in load or when incidents such as a landslide occur
- When the river channel is obstructed by aquatic plants or protruding bed rocks.
Summary of river process
- The Hjulstrom curve shows the different speeds at which particles of different sizes will require to be eroded, deposited or transported in a river. In general, the large the particle, the higher is the speed required for eroding it.
- However, for finer particles that are smaller than 0.1 mm, such as clay, higher river speeds are required for them to be eroded. Higher speeds are necessary as clay particles are highly cohesive and would adhere or stick to each other, hence a higher speed is required to dislodge them
- Larger particles require higher speeds to be transported, but are easily deposited, but once the river speed decreases by a small amount, as they are heavier. Finer particles also require higher speeds to be transported, since their small sizes allow them to be deposited very easily onto the river bed.
Formation of Meanders
- Meanders can occur anywhere along the course of the river; however they are more prominent in the middle/ lower course.
- As the river travels downstream, it tends to find the easiest route, thus it will bend.
- Therefore, bends of the river at middle/lower course allows erosion to occur at the outer bank of a river as there is less friction at the outer bank, the river possess more energy and erosion could occur. Erosion could occur in the form of undercutting, and over time, a steep-sided bank known as a river cliff is formed.
- Eroded materials from the outer bank are deposited at the inner bank where water moves in a spiral fashion. Deposition occurs at the inner bank because there is greater friction; this reduces the speed and cause river to drop its load.
- As more sediment is deposited, the water becomes shallower at the inner bank over time, and the river channel becomes asymmetrical. A gentle slope formed by the deposition along the inner bank (slip-off slope) is formed.
- This process repeats itself and meanders become more and more severe in their curves.
Formation of Oxbow Lakes
- Constant erosion at 2 consecutive concave banks will cause the 2 meanders to more closer, forming a loop. Over time, it becomes more distinct, separated by land.
- As deposition and erosion continues, the 2 meanders eventually meet.
- Sediments deposited at convex banks will dam up the river, forming an oxbow lake which will be separated from the main river. The main river will flow in a straight path.
- Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake. Overtime it could dry up due to evaporation and encourage the growth of vegetation.
- Deltas are found at the lower course of the river.
- As a river approaches sea/lake, speed decreases, thus deposition occurs.
- Although sediments can be removed by tides, if rate of deposition is faster than removal, sediments will build up at mouth.
- Over time, more sediment is deposited and delta extends outwards into the sea. Sediments carried will block flow of Main River.
- As a result, smaller channels known as distributaries are formed to find their way out to reach the sea or lake.
- Deltas only form under certain conditions
- The river must be transporting a large amount of sediment
- The sea must have a small tidal range and weak currents
- The sea must be shallow at the river mouth
Floodplains and Leeves
- Floodplains are found at the lower course of a river.
- When river overflows due to heavy rain, flooding occurs and water spreads over a larger area, losing speed.
- When water recedes, river deposits material carrying on the land, leaving behind sediments on river banks and beds.
- Coarser sediments are deposited closer to river edges and fine particles are deposited away from river. The deposited materials build up on both sides of river due to flooding.
- Over time, the accumulation of sediments at the flat plains called floodplains is formed. Coarser and heavier materials accumulated nearer to the river form raised banks called levees.
Valleys and Gorges
- Valleys and gorges are usually found in the upper course where vertical erosion is prominent.
- In the upper course, the volume of water is small but the gradient is steep.
- The swift flowing water exerts a powerful force on the river bed, forming a V-shaped valley through hydraulic action, abrasion and solution.
- A gorge is an exceptionally deep and narrow valley formed when the river erodes vertically through resistant rocks leaving very steep valley walls.
- River flows through across rocks of different resistance,
- eroding the less resistant rocks more rapidly,
- causing a change of gradient in river course.
- Over time, river plunges from great height to hit river bed below with tremendous force, forming a waterfall.
- Waterfall can also be formed by faulting, where rocks are uplifted, causing displacement of rocks where 1 layer is higher than the other. When the river flows across an area where faulting occurred, gradient drops suddenly.
- Repeated pounding of river bed leaves a depression at base of waterfall.
- Depression is further deepened by rocks and boulders swirling around, forming a plunge pool.
- Over time, as the harder rock is undercut it produces an overhang which will eventually collapse.
- As the rock collapses the waterfall retreats upstream, leaving a gorge.
- Hydrographs visually outlines a river’s discharge overtime. Important for river basin and flood management.
- The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel.
- The rising limb of the hydrograph represents the rapid increase in resulting from rainfall causing surface runoff and then later throughflow.
- Peak discharge occurs when the river reaches its highest level.
- The time difference between the peak of the rain event and the peak discharge is known as the lag time or basin lag.
- The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the river’s level falls.
- It has a gentler gradient than the rising limb as most overland flow has now been discharged and it is mainly throughflow which is making up the river water.
Factors influencing shape of hydrograph
- Drainage Basin
- Size of basin - small basin encourages rainfall to reach the main channel more rapidly than a larger basin, where the water has much further to travel. Lag time would thus be shorter in smaller basin.
- Shape of basin – Circular basins is more likely to produce shorter lag time and a higher peak discharge than an elongated one. Circular basins have all the points on the watershed approximately equidistance from the gauging station, thus water reach from different points reach the river at almost the same, leading to a rapid increase in river level/discharge in the river.
- Soil type – this can control the rate and volume of infiltration, the amount of soil moisture storage and the rate of through flow. Sandy soils for example, with large pore spaces, allow rapid infiltration and thus, a longer lag time for rainwater to reach the main channel. This reduces the possibility of flooding.
- Climatic factor
- Seasonal variation. E.g. annual dry and wet season (tropical Monsoon climate)
- Types of rainfall
- E.g. during intense storms, the rainfall intensity may be greater than the infiltration capacity of the soil, thus surface runoff
- Urbanization and Land Use
- Steeper Rising Limb and shorter time lag: an area that has undergone urbanization, which involves replacing vegetation with impermeable surfaces made from materials such as concrete and asphalt.
- Due to the absence of infiltration of rainwater into the ground, surface runoff/ overland flow increases.
- Overland flow transports water into canals or rivers quickly, which contributes to a shorter lag time, compared to an area where water flows into rivers via through flow/interflow.
- A steeper recession limb due to drainage structures/system constructed, typical of urbanized areas, to help drain away rainwater fast so that flooding does not occur.
- Higher Peak Discharge: because of reduced groundwater storage due to absence of interception by vegetation, decreased evapotranspiration etc, compounded by increased surface runoff, leads to rapid increase in water level/discharge in the canal.
- Permeable floor, increase infiltration of rainwater into the ground, less overland flow, longer time lag.
- Vegetation increase chance of intercepting the rainwater, reducing the amount that is actually injected into the system.
- Vegetation may use evapotranspiration by transpiring and evaporating water, thus decreasing the amount of water injected into the system
Importance of hydrograph
- City developer/ engineers for planning and building of drainage system
- Use data to determine the type/ scale of drainage structure to be constructed / data to calculate amount of peak discharge and lag time
- Can be used to plan to prevent flooding/ warming about flooding
- Flood occurs when land that is usually dry is submerged by large amount of water.
Natural Causes of Floods
- Excessive rainfall
- When moist monsoon winds from the ocean blow over the land, bringing heavy and prolonged rains.
- Floods occur when the excess rainwater is unable to seep into the ground or when rivers overflow their banks because the river channels can no longer contain the excess water.
- E.g. China experiences flooding of its major rivers regularly. The Yangtze River and the Yellow River frequently overflow their banks after heavy rains because the river washed large amounts of sediments into the rivers. These sediments are then deposited on the river beds, causing the rivers to become shallower. With time, flooding occurs.
- Storm Surges
- Storm surges can cause floods in coastal areas.
- A storm surge occurs when strong winds raise the waves in the ocean to exceptionally high levels.
- E.g. Bangladesh has low lying relief and is frequently subjected to regular flooding caused by storm surges.
Human Causes of Floods
- Clearing of forests
- Clearing of forests has caused floods to occur more frequently.
- In China, forests in mountainous regions are clearly for economic gains and houses.
- Without vegetation to hold the soil together on the mountain slopes, the soil is eroded when rain falls.
- The bare slope that remained after the removal of vegetation increases surface runoff.
- This increases the volume of water flowing into river. At the same time, soil and mud will be washed into the river, causing it to be shallower. Flood could occur
- More land is cleared for the development of housing and industries to meet the needs of the urban population, and as a result, built-up area increase.
- In the urban areas, the concrete pavements and tarred roads prevent rainwater from seeping into the ground.
- The clearing of the vegetation to construct settlements means less natural vegetation is available to intercept the rain.
- As a result, there is an increase in the amount of surface runoff flowing into rivers.
Impacts of Flooding
- Loss of lives
- Floodplains around coastal areas and rivers provide fertile alluvium, making them very attractive places for cultivation of crops and are hence densely populated. As such, when flooding happens, the impact is usually higher.
- E.g. In 2003, heavy rainfall causes floods in Anhui, Jiangsu and Henan of China, from the Huai River Basin. The flood killed 298 and more affected.
- Floods usually often occur when people least expect them. As a result, many people are often not prepared and thus drown in a flood.
- Economic Loss
- Properties and infrastructure are damaged and destroyed. Homes are ruined by the floodwaters and roads become inaccessible.
- The flood disrupts power supply and phone lines, cutting off communication.
- People are displaced from their work.
- Such damage is very costly and takes a long time to repair.
- Spread of diseases
- Large number of homeless flood victims are often housed in overcrowded makeshift shelters which lack sanitation and clean drinking water.
- These conditions cause the outbreak of diseases like malaria and cholera, which can spread easily.
- Environmental Damage
- Destroy trees and other plant life, as well as the natural habitats of animals.
- Damage local ecology.
- Flood managements
- Building artificial levees and dykes
- These walls of sand, stone and concrete are built along river banks which are prone to flooding, to increase the capacity of the river to hold water, thus reducing its chances of flooding.
- E.g. China, artificial levees in the lower course of the river.
- Construction of Dams
- It controls the amount of water flowing into the river through the construction of damns. The walls of damns have gates which hold back or release water from the man-made reservoir behind it.
- This controls the amount of water that flows downstream in a river.
- The reservoir also increases the amount of water that a river system can hold upstream, thereby reducing the occurrence of floods downstream.
Three Gorges Dam Case Study
The project stretches more than 2 kilometers across one the of the greatest river in the world, the Yangtze River
- Flood control:
- Water being stored in the reservoir behind the dam, regulates flow of water downstream, thus keeping water level at a steady and safe level downstream all year round.
- This reduces the possibility of flooding as the amount of water in the river does not exceed the holding capacity of river, which helps to benefit the people living along the river, esp. further downstream, can reduce loss of lives and property (social and eco)
- Hydro Electrical Power:
- generation of up to 18,200MW from 26 power turbines, equivalent to the capacity of 15 nuclear power plants. (cleaner energy compared to fossil fuel, contribute less to environmental degradation; sustainable development).
- If this energy were produced with Chinese coal, 50 million more tons would be burned annually, producing 100 million tons of carbon dioxide, 1.2-2 million tons of Sulfur Dioxide.
- Energy generated used in factories and industries to speed up productivity and efficiency of industrial activities.
- Helps to produce more outputs for the people (social –improve standard of living- availability of electricity, may not be available before) and country, aids in economic development (attract investment in industrial development as power supply is more stable and sufficient now, such investment can provide jobs to locals.
- Eco benefit for individual / family and nation as a whole- increase in revenue that can be channeled to provide amenities such as education/ health to improve social standards)
- During the dry season, the dam can release fresh water from reservoir downstream for agriculture and industrial usage, which helps to increase yields and outputs.
- Sufficient water supply for Chinese.
- Resettlement and relocation of farmers and residents along Yangtze River
- The submergence of 13 cities, 140 towns, 1352 villages, 657 factories, 30 000 hectares of cultivated land over 3 stages in 1997, 2003, 2009 has brought about changes to the lives of about 1.3 million people. Residents were relocated to cities, where city dwellers did not welcome them.
- Many relocated to cities were unable to find jobs due to either lack of or irrelevance of skills and knowledge, thus face problems no money for basic necessities.
- This can lead to other social problems like increased crime rate in the cities. Any refugees also end up without compensation due to corruption going on within the project.
- Local government often corrupted, which led to significant resettlement funds ending up in the pockets of the government officials, rather than passing to the refuges.
- Poor local planning also left many relocated people with bad land, homelessness, loss of jobs and social status, and other social ills. There are often tensions and conflicts between the host population and the new migrants.
- Loss of cultural and historical relics
- Reservoir behind the dam has flooded many archaeological and cultural sites – tombs, fossils, ancient inscriptions and other historical sites related to cultures and dynasties of ancient Chinese people.
- This has resulted in the loss of cultural and historical relics. Although some of them were moved to higher grounds before the flooding took place but the undiscovered relics or those which could not be moved due to location, size and design were lost forever.
- Fourteen thousand hectares of agricultural land will be submerged, and will more than 100 archeological sites, some dating back over 12000 years.
- Ecological problems:
- 36% of all freshwater fish species in China, with more than 360 fish species belonging to 29 families and 131 genera.
- Chemical and temperature composition of the water, the character of the natural habitat and food resources available for these fish species is no longer suitable for these fish to live.
- Annual harvest of these commercial fish below the dam from 2003 to 2005 was 50% to 70% below a 2002 pre-dam level and more dramatic declines are being seen on larvae and eggs below the dam
- Disruptions to sediment flow
- deposition river features downstream affected, lack of rich nutrients for farmers, fishermen also affect economically
- Less deposition of sediments at the lower course of the river as much of the load from the upper stream (through inundation of the land upper stream) has been trapped behind the dam
- Impedes the formation of floodplains and deltas at the lower course of the river due to limited deposition of sediments.
- These 2 features are formed when a considerable amount of load is deposited at the lower course of the river or at its mouth.
- The accumulation of sediments is necessary, thus, a reduction in the transportation load may lead to the absence of such features at lower course.
- immediately after the Dam- water without any load may be more powerful in carrying out erosion. But at the lower course, less volume, less energy to erode (esp. Hydraulic action )
- Can include “coastal erosion” at coastal areas where coastline/ delta may reduce in size
- less load to carry, most probably in suspension/ solution mode
- less to deposition, formation of deposition features are affected (Coastal erosion may reduce size of delta)
- Large amount of local and international loan- debt. Estimates of the construction costs made during the mid 1990s for the major parts of the project ranged from a low 25 billion to a high of 60 billion Yuan.
- The TGD is being funded by a complex mix of external and internal sources. Chinese government borrow money from various external sources such as Germany’s banks, Lehman Brothers and such.
- Farming and fishing affected