Physical Geography

1. Physical Geography Rivers Coasts Glaciation

2. 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

3. River Channel Processes • Erosion • Transportation • Deposition

4. Erosion • Hydraulic Action – sheer force of running water. This action will remove loose material from the bed and the banks of the river. The stronger the current the greater the erosion. Cavitation is a form of hydraulic action where bubbles from the river are forced into cracks in the bedrock on bed or banks. They collapse and the vibration from this leads to further weakening.

5. Abrasion • Abrasion – This is where the river’s load erodes the bed and the banks of the river. The material is hurled agains the channel in a sandpapering effect. This is most effective when the river is in flood. The faster the flow of the river the more material it will carry and the more abrasion that will occur.

6. Attrition • Attrition – This is where the rivers load are rounded and smoothed by the constant collision of particles of rock and debris. Pebbles bounce off one another and produce a rounded smooth appearance. Alluvium is formed in this way

7. Solution • This is the chemical erosion of rock surfaces by water. The rivers are slightly acidic which can react with the minerals in certain rocks. River flowing over limestone react with the calcium carbonate which is changed to calcuim bicarbonate and carried in solution in the river.

8. Deposition • Deposition begins with the heaviest material being deposited first. This is because it take more energy to carry heavy material. Once the river’s energy is reduced it will not be able to carry heavy rocks etc. • When rivers are carrying minerals in solution this will not be deposited. Instead it will be carried out into the sea where it will mix with the sea water.

9. Deposition will occur when.. • The velocity of the river is reduced (when the river slows down) • The discharge of the river is reduced (when there is less water flowing through its channel) • The river’s load is increased (river is carrying a lot of material which slows it down)

10. Rivers - Profile

11. River Features V-Shaped Valley Waterfall Floodplain Levee

12. V Shaped Valley • Description: Valley, which has a v shape due to the vertical erosion of youthful river.

13. Explanation - V-Shaped Valley • This feature is found in the youthful stage of a river. • The gradient 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.

14. V Shaped Valley • Processes: Abrasion, Cavitation, Solution and Hydraulic Action • Example: Devils Glen near Ashford in County Wicklow • Diagram:

15. 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 it becomes 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.

16. Diagram - Waterfall Processes: Hydraulic action, abrasion, solution and cavitation Example: Torc waterfall near Killarney in County Kerry Diagram:

17. 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.

18. 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:

19. WebsiteHow does a river flood?

20. 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

21. Diagram - Levee

22. Drainage • Dendritic • Trellised • Radial • Deranged

23. 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)

24. 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

25. 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.

26. 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

27. 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.

28. Man has sought to control and manage river processes- The Colorado RiverA Case Study

29. 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?

30. 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

31. Hydroelectric Power Dams Positive and Negative Outcomes Management and Control of Rivers

32. 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.

33. 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

34. 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.

35. 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.

36. 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.

37. 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.

38. CASE STUDY: Three Gorges Dam in China Negative Aspects • Besides being a major waterway the Yangtze may be compared to a human artery. The river supports huge numbers of settlers that have grown up along its banks even within the gorge sections. All of these settlements will disappear once the dams are built and the settlements are flooded. People are being rehoused in new settlements on raised land above the gorges. The great upheaval will include the loss of the waterside sites and the traditional character of the river’s settlements that has evolved over generations. • The dams may lead to a dangerous build up of in some part of the river creating new obstacles to navigation.

39. Change in base level • The progress of a river towards achieving a graded profile can be interrupted by a change in base level. Positive changes occur when • The land level rises relative to the sea (isostatic movement). This may result from the uplift of a landmass after the weight of glacial ice has been removed. • The sea level falls relative to the land (eustatic movement). This occurs at the onset of an ice age when more of the earth’s water is stored as ice.

40. Presentation on Rejuvenation

41. Change in base level • In either case the land emerges from the sea resulting in a steepening of the river’s gradient. The river now ha an excess of energy thus increasing it capacity for downward or vertical erosion. This renewed capacity for erosion is called REJUVENATION (making young again). It begins at the mouth of the river and works its way upstream, creating a new river profile.

42. Change in base level • The point at which a new profile meets the original river profile is called a KNICKPOINT and is marked by rapids • If the river has undergone a number of rejuvenations there will be a series of knickpoints each advancing upstream by headward erosion. In this case the river profile is said to be Polycyclic.

43. Change in base level - Features • Paired terraces • During rejuvenation vertical erosion through the combined processes of hydraulic action and abrasion is very effective. The river erodes the floodplain and thus forms a new narrower floodplain at a lower level. Lateral erosion occurs along the sides of the valley. This rapid vertical erosion creates a pair of terraces each at the same height as the margins of the original floodplain. Example River Dodder • Incised meanders • When a meandering river is subject to rejuvenation over a period of time, the vertical erosion causes the meanders to be cut deeply into the floor of the valley and in this way incised meanders are formed. Meanders can be of two types • a. entrenched meanders - produce a river valley with steep sided and symmetrical profile - a product of vertical erosion alone • b. Ingrown meanders are the product of both vertical and lateral erosion - one steep slope and one slip of slope • Example Beside Thomastown on the River Nore

44. Leaving Cert. Questions Rivers

45. 2001 – River Studies ii) “River Flooding cannot be totally avoided but must be controlled” Assess the validity of this statement in the light of examples you have studied. Clonmel has long been at risk from flooding and local residents and businesses have suffered significant hardship. The centre of the town was flooded on six occasions between 1995 and 2003. The flooding occurs when the peak river flow exceeds 250 cubic metres per second. The flooding has intensified in recent years by the location of developments in the floodplain of the river Suir. A flood relief scheme for the town estimated to cost €40 million is now being considered. This will include the construction of flood defence walls and the replacement of two medieval bridges in the town. 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. The construction of 11 dams has helped in to control flooding in this river and reduce the risk of loss of life and property along the river’s course.

46. 2001 – River Studies ii) “River Flooding cannot be totally avoided but must be controlled” Assess the validity of this statement in the light of examples you have studied. • Clonmel • Colorado River

47. 2000 – River Studies ii)Examine briefly two ways in which human societies attempt to manage rivers. Building Dams Building Levees

48. 1998– River Studies ii) Examine one example of how human societies have always attempted to manage or control the natural processes, which operate in river valleys. • The irrigation practices carried on throughout the Mediterranean have been in existence for thousands of years. Even the Roman Empire, based on urban control centres (towns) transferred water from faraway by aqueducts. • Los Angeles city and large areas of irrigated farmland in southern California are supplied by water from the Colorado River and the Sacramento River. 90% of this water transfer is used to irrigate the world’s richest cash crop farmland, which produces one quarter of the US’s fruit and vegetables as well as cotton, rice, soya beans and sugar beet. • Water transferred from the rivers Amu Darya and Sry Darya is used to irrigate vast desert areas that surround the Aral Sea in central Asia. As a consequence, this once waste land now produces cotton, rice and vegetables. However because of the enormous draw of water the Aral Sea itself is said to be shrinking.

49. 1997– River Studies ii) Flooding in river valleys can be worsened by human activity. Examine on example of this. The Rhine is Europe’s most important inland waterway. IT stretches for over 1,3000 km from source to mouth. This was a free flowing system until 100 years ago. Then the main stream was altered to become the most important shipping canal in Europe. The total volume of cargo carried by the Rhine is over 40 million tonnes per annum. In order for this to happen, drastic changes were made to the course of the Rhine, not all with a successful outcome. The overall result is that these man made changes have made the river system less able to accommodate floodwaters. Much of the course of the Rhine has been straightened over the last century. This process has shortened the river by 50 km. This has doubled the speed of the water’s flow from Basel to Rotterdam increasing erosion and caused water to cascade downriver when there is heavy snow or rain upstream. Over the last century oth the frequency of flooding and average floodwater levels have increased significantly. The rhine has flooded annually since the 1990s - the most serious in 1995.

50. 1996– River Studies ii)Examine briefly two examples of human management of rivers. Two way in which human manage rivers are 1. The Construction of Dams (Three Gorges Dam, China) 2. Construction of artificial barriers for flood control (The Rhine, The Mississippi) 1. The Construction of Dams for the generation of HEP - Look at the postive and negative effects of this and mention breifly the Construction of the Three Gorges Dam or discuss the management of the Colorado River 2. Artificial Barriers for Flood Control - For several year the Rhine has been enclosed in a kind of straightjacket. Steep concrete floodwalls were built to channel water from the upper reaches of the river, but this has shifted the problem of flooding downstream. In addition a series of dykes or embankments have been constructed and regularly raised to protect residential and industrial areas. In the US artificial levees have been constucted over ten meters high to protect settlement along the southern part of the Mississippi. Some authorities object to the building of levees that enclose a river so much that the water is high above the countryside. This makes floods even more dangerous when they occur.