Physical Geography Rivers Coasts Glaciation
Rivers • Rivers are the single most important agent in shaping the land surface by the processes of erosion and deposition
Terms • Drainage basins – area of land drained by river and its tributaries – basins are separated by watersheds • Stream order – this is the hierarchy of streams in a drainage basin – the smallest source streams without tributaries are called first order – when two of these join the form a second…. 1+1 = 2 2+2=3 but 2+1=2 • Drainage patterns – overall arrangement of rivers in an area. Type of pattern mainly due to rock type and structure on which the river developed & (Relief)
Drainage Types • Dendritic – tributaries converge randomly upon main river on gently sloping land of one rock type – uniform in response to erosion • Radial – radiates out like spokes of a wheel – central high point – isolated hill or dome • Trellised – tributaries join main river at right angles – forms where there are alternate bands of resistant and less resistant rock • Deranged – Rivers flow in chaotic appearance – associated with lowland landscapes – marshy or glacial or karst landscapes
The capacity of a river to erode, transport and deposit will depend on • River discharge • River Velocity (speed of river) • River Flow (water in a river will flow in one of two patterns)
River Discharge • River Velocity (speed of river) • Gradient • Discharge • Shape of river channel • Roughness of river channel • River Flow (water in a river will flow in one of two patterns) • Laminar flow – water movement in a smooth straight manner without mixing or disturbing any sediment on the river bed and banks (usually discounted) • Turbulent flow – water moves in an apparently confused and erratic manner – swirls and eddies – turbulence increased by roughness and speed
River Processes • Hydraulic Action • Abrasion • Attrition • Solution
Erosion • Results in • The deepening of the river valley by VERTICAL EROSION • Lengthening the river valley by HEADWARD EROSION • Widening of the river valley by LATERAL EROSION Headward Erosion Direction of flow
Transportation • Solution • Suspension • Saltation • Traction
Deposition • When a stream loses energy its transporting power drops and it deposits part of its load. • Heaviest first ……. Alluvium • Deposition occurs when • Velocity is reduced • Discharge is reduced • Load is increased
Question re Physical Landforms • Examine the formation of any two Irish Landforms, with reference to the surface processes which formed them
Long profile Typical profile Graded profile
River Features V-Shaped Valley Waterfall Floodplain Levee
Rivers - What do you need to know? • Profile - Stages • Processes - Erosion/Transportation/Deposition • Features - Formation - 2 Erosional/ 2 Depositional • River Rejuvenation • River Capture • River Drainage • The positive and negative effects of man’s interference with rivers
V Shaped Valley • Description: Valley, which has a v shape due to the vertical erosion of youthful river.
Explanation - V-Shaped Valley • This feature is found in the youthful stage of a river. • The gradient here is steep increasing the velocity of the river. This means that the river is full of energy and primarily involved in vertical erosion. This down cutting gives the valley its v shape. • The fast moving water (hydraulic action) erodes the bed and banks providing the river with its load. • The river carries its load, which through a process of abrasion uses it to cut further into the riverbed. • Cavitation also produces further erosion as bubbles of air collapse and form shock waves against the riverbanks. • As the river flows over the bedrock it will dissolve mineral in the rock in a process known as “Solution”. All processes combine to create the feature of a V shaped valley.
V Shaped Valley • Processes: Abrasion, Cavitation, Solution and Hydraulic Action • Example: Devils Glen near Ashford in County Wicklow • Diagram:
Waterfall • Description: A waterfall is a point where there is an interruption in the river profile and the water makes a vertical drop. • Explanation: • Most waterfalls develop where a river meets a band of softer less resistant rock after flowing over harder more resistant rock. • As a result of differential erosion, the water quickly erodes the softer rock and begins to undercut the hard rock by hydraulic action, cavitation and abrasion. (explain what these are here). • If the rock contains minerals that are soluable a process known as solution also will become active. In time the band of hard rock is deeply undercut. • Without the support of the underlying rocks itbecome unstable and eventually collapses. • The rock at the base of the waterfall is quickly eroded because the falling water is unhindered by friction, so its velocity and hence its power to erode are increased. • A combination of hydraulic action and abrasion gouge out a plunge pool at the base of the waterfall. • The processes of undercutting and collapse are repeated many times causing the waterfall to retreat upstream and leave a steepsided gorge.
Over time the river will erode its river bed. When a river erodes through a uniform layer of rock it may reach layers of rock with differing resistance. This may lead to the formation of a waterfall.
Diagram - Waterfall Processes: Hydraulic action, abrasion, solution and cavitation Example: Torc waterfall near Killarney in County Kerry Diagram:
Mature River Valley Meanders Braided Streams Flood Plains
Meanders • Meanders are a series of gently curving bends in the course of the river. • The are a river feature that exhibits aspects of erosion and deposition • They begin to develop as a result of a series of “Riffles”. These are pools and shallows found on the river bed. These pools create currents which cause the river to swing from side to side. • This motion causes the movement of water in the river channel to flow in a particular type of pattern known as helical flow. Helical flow is a cork screw type movement of water. Surface water moves towards the outer bank and then returns along the river bed..
Helical Flow Erosion Deposition
Development of Meander River Cliff Slip off Slope
Floodplain • Description: A flood plain is a level stretch of land along the edge of the river’s channel. It is a wide and flat valley floor that is often flooded with heavy rain. • Explanation and Processes: • A meandering river is one that swings from side to side across level land. • A flood plain is created in this process and each river loop is called a meander. • Lateral erosion occurs at this stage and removes the interlocking spurs and lowers the slopes of the valley sides. Level land is created on both sides of the river’s channel. The level land is called its floodplain. • During times of heavy rain the river overflows its channel and spreads across the flood plain. • Away from the river’s channel, the floodwater is calm and it has lots of fine sediment such as silt and fine sand in suspension. This type of sediment is called alluvium. • The calm water is unable to support its load of alluvium and deposits it on the plain. This happens with each successive overflow of the river. Thousands of years of sediment deposited on the floodplain build up a thick blanket to form extremely fertile level land suited to growing cereal or grazing cattle.
Deltas • When a river enters a body of water such as the sea or lake its velocity is dramatically reduced and deposition takes place. • This sedimentation can lead to the formation of a delta – marine –sea lacustrine – lake • Delta formation requires large amounts of deposition as currents and tides will remove a certain amount of river deposition • As the river enters the sea it will “dump” its heavier load while the suspended load will require time to settle. Flocculation will also occur where sediment particles coagulate or clot on contact with the seawater and settle rapidly on the seabed.
Deltas • The build up of sediment will grow to such an extent that it hinder the river entry into the sea. The river will cut out channels called distributaries • The finest settlements will be carried out furthest then the coarser material will be deposited and lastly the heavier material closest to the river mouth. These three layers are called bottomset beds, foresetbeds and topset beds.
As each bed advances the delta is extended seawards • Estuarine delta is one type of marine delta - lower Shannon – here sediments have been deposited in the shallow water along the sides of the estuary but have not extended out beyond the coastline • Lacustrine delta develops when a river flows into a sheltered lake. Deposits are laid down in two layers fine and coarse – fine – summer deposition and coarse – winter – Often found in ribbon lakes Lough Tay in the Wicklow mountains
Floodplain • Processes: Key process is deposition. Deposition occurs due to a reduction in gradient and velocity of the river. • Example: Blackwater valley near Fermoy County Cork • Diagram:
Levee • Landform: Levee • Description Levees are high banks along the river’s edge and are raised above the flood plain level. They can form naturally along the edges of large silt laden rivers as the rivers slowly wind their way across flat flood plains to the sea. • Explanation: • A natural levee is a broad low ridge of fine alluvium built up along the side of a channel by debris or silt laden floodwater. • As the sediment laden flood water flows out of its completely submerged channel during a flood the depth force and turbulence of the water decreases sharply at the channel margins (edges). • The sharp decrease results in a sudden dropping (deposition) of the coarser materials ( usually fine sand and coarse silt) along the edges of the channel, building up a levee. • Example: Mulkear River in County Limerick
Drainage • Dendritic • Trellised • Radial • Deranged
River Capture • When two rivers exist side by side with an area of high ground (watersheds) separating their basins river capture is possible • Tributaries that flow into these main rivers are constantly involved in headward erosion which means that they are eating back into where they began • Tributaries of the more powerful river will be at a lower level due to more vertical erosion and so this together with the headward erosion can enable it to capture the flow of the neighbouring stream (river capture or stream piracy)
Continued headward erosion until the tributary finally reaches the other primary river Headward Erosion Tributary river eats back into its source Finally this headward erosion leads to river piracy or river capture as the tributary stream re routes the main stream River Piracy or River Capture
Interference with Rivers • Rivers have been used by man from early times as a source of communication, water and food. • Rivers, especially in their lower courses are prone to flooding. This flooding creates a floodplain which is a key feature found in the old stage or lower course of a river. • Flooding has positive and negative results • Rivers have also been harnessed for their energy. HEP is a key renewable resource and fundamental to many the growth of many economies.u • The building of dams across rivers has had positive and negative results.
FLOODING - Positive Results • . A river and its nearby flat flood plain together make up a natural system. In most untouched natural river valleys the water flows over the riverbanks and on to the flood plain every year or so. There Are a number of natural processes that occur because of this flooding • Water and nutrients are stored on the flood plain. • Silt deposits on the flood plain increase the content of the soil • Wetland on the flood plain provide a natural habitat for many birds animals plants and other living organisms • The flood plain acts as a green belt and helps provide diversity of flora and fauna
FLOODING - Negative Results&Flood Control • Natural flooding is not a problem until people choose to build homes and other structures on flood plains. These structures are prone to damage and loss when flooded. People have chosen to build on so many flood plains that flooding is the most universal natural hazard in the world. In 1993 flooding of the Mississippi took over 50 lives and caused over US $10 billion in damages.. • Flooding in Bangladesh in 1970 and 1991 killed more than half a million people. • Today people try to tame nature by building dykes and levees to retain floodwaters. Vast areas of existing farmland, towns and cities lie below water when rivers are in flood.
Man has sought to control and manage river processes- The Colorado RiverA Case Study
Man and the Colorado River • The Colorado river rises in the Rockies in the USA. It flows through 7 states and Mexico and used to enter the sea at the Gulf of Mexico. • One hundred years ago it was a wild uncontrollable river now it has been brought under control and is managed to meet the needs of 20 million people who live in its environs. • How has it been controlled?
This river has been so well utilised that all of its water is used up before it reaches the Gulf of California • The project has five main aims • Flood control • HEP • Irrigation • Urban water supply • Recreational use • The river flow is controlled by ELEVEN DAMS along its course. • Electricity is generated from seven dams • Over two million acres of land are irrigated using water from the river • These interventions have meant that man has interfered with the natural processes of deposition along the lower course of the river • The alluvial soils along the Mexican border are no longer being enriched by sediment as flooding no longer occurs. This is also occurring in the Delta region. This Delta region is now a wasteland as vegetation, fish and bird life have all disappeared. • The Dams have trapped sediment and this has interfered with the fish life of the river. • Downstream from the dams the river’s load is decreased and this has led to vertical erosion Man and the Colorado River
Hydroelectric Power Dams Positive and Negative Outcomes Management and Control of Rivers
Hydro Electric Dams • Example: Ardnacrusha on the River Shannone • Example: Pollaphuca Dam on the River Liffey • Dams are constructed across a river’s channel to generate hydroelectric power. The dams interrupt the natural flow of rivers and reduce the ability of rivers to carry sediment from the upper valleys to their flood plains and their estuaries in lowland areas. • Hydroelectric dams are designed to block and use the flow of rivers. Water builds up behind the dams to form lakes called reservoirs. The depth of the reservoir is regulated by allowing some water to flow through pipes in the dam, called penstocks to generate hydropower. As reservoirs are calm water areas they cause inflowing streams to drop their load of heavy particles but also some of their finest and mineral rich particles that are normally deposited on the flood plain.
Positive results of building dams • Providing over 6 percent of the worlds energy needs • Providing reservoirs for irrigation and water supply • Regulating floodwaters to reduce flooding in lowland areas
Negative results of building dams • Loss of soil fertility e.g. the annual flooding of the Nile in autumn allowed floodwater to cover the flood plain where it remained trapped until it had deposited its silt. This silt enriched the land with minerals carried in tiny grains of soil. The building of the Aswan High Dam, which controlled the waters of the Nile, prevents this from happening today. This has led to additional fertiliser costs for Egyptian farmers. • Submerging farmland and settlements: Damming water for hydroelectric power may involve submerging farmland and settlements on the upstream side of the dam. If people live in the areas to be submerged they have to be evacuated and relocated elsewhere. More than not this causes great upset to the people involved. The submerged farmland is lost.
Negative results of building dams • Limited Lifespan: Although waterpower is considered a renewable resource, the reservoirs created to provide hydro electricity have a limited Lifespan. All rivers carry sediment. This sediment accumulates in the reservoirs behind the dams. Eventually the sediment will fill the reservoir. So each hydroelectric dam has a limited lifespan. Egypt’s huge Aswan High Dam reservoir which was completed in the 1960’s will have half it volume filled with sediment by the year 2005. • Erosion of Deltas:When a river that once entered a sea in a delta is dammed for producing HEP, its load is reduced. This reduction may lead to erosion of its delta because the reduced river load may not be sufficient to balance coastal erosion along the delta shoreline. This has happened in the delta of the Nile due to the construction of the Aswan High Dam. Egypt relies on its Nile Delta for living space and especially for the production of food. The loss of sediment created by deposition in Lake Nasser above the dam has led to erosion of the Nile Delta coastline.
Negative results of building dams • Loss of natural vegetation and wildlife: The rise in water level behind HEP dams also wipes out wild life that once frequented the area. The flooding of habitats forces animals or birds to move or drown when trapped by rising waters behind the dam. Natural vegetation such as tropical forest is cut down and removed or left to rot where it stands often bare of its vegetation after chemical sprays (defoliants) have been used over vast areas such as behind the Tucurui Dam in South east Brazil.
CASE STUDY: Three Gorges Dam in China Positive Aspects • Midway between its icy source in Tibet and the fertile delta at its mouth in Shanghai 6,300 kilometers to the east, China’s Yangtze Kiang (Yangtze River) rushes through a series of vertical sided channels known as the “Three Gorges”. The Chinese government is using these gorges to build the world’s largest hydroelectric dam. • Chinese government leaders argue that the Three Gorges scheme is vital to their country’s future and will be good for the environment as a whole. They say it will prevent the periodic flooding of the Yangtze that claimed 500,000 lives in the 20th Century. • More importantly the production of clean hydroelectric power will reduce China’s reliance on coal, the dirtiest of fossil fuels, which now supplies 75% of the country’s needs. At present the burning of coal has helped make lung disease the nation’s leading cause of death.