Rivers - George Mather, Chris Hancox, Alex Owen and Michael Soley


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Rivers - George Mather, Chris Hancox, Alex Owen and Michael Soley

  1. 1. Rivers What the Specification says Fluvial processes which result in distinctive landforms Long and cross profiles, processes of erosion (hydraulic power, corrosion, corrasion, attrition), transport (traction, saltation, suspension, solution) and deposition.The characteristics and formation of waterfalls, gorges, meanders, ox-bow lakes, levees, flood plains and deltas. Recognise and describe fluvial features on Ordnance Survey maps and photographs. How a river profile changes downstream and the relative importance of the processes at work such as the typical V shaped valley in the upper course where vertical corrasion dominates. Students will be expected to be able to name an example, describe each feature and to explain its formation making reference to the processes at work. The interaction between people and fluvial environments River basin management issues – the causes and effects of flooding in river basins in the context of both LEDCs and MEDCs.The short, medium and long term strategies used to attempt to manage the floods. Contemporary issues concerning use of ‘hard’ v ‘soft’ strategies and attempts to achieve sustainability.The social, economic, environmental and political issues that have an impact upon strategies and the values and attitudes of interested groups. River basin management issues should be studied in the context of both LEDCs e.g. Bangladesh and MEDCs e.g. the European floods of the 1990s. One case study from the MEDW and LEDW should be chosen to study the physical and human causes of flooding and the impact of the floods. TV news is a good source of up to date information. Flood management should include efforts to prevent flooding and strategies used to ameliorate the impact. Candidates should be aware of how and why the methods used are different between the LEDW and MEDW and now currently there is a move away from hard engineering e.g. dams on the Colorado to softer approaches e.g. flood plain zoning, improved warning system. 1) River Courses Upper course of the river Interlocking Spurs - caused by erosion, Formed in a V - shaped valley (a series of ridges that rise above the river which twist and turn through a valley) Waterfall - Caused by layers of soft and hard rock. The layer of soft rock following the hard rock begins to erode, which forms a drop where the river drops down to the level of the soft rock. This repeats over time to form a large drop at the edge of the hard rock band. Where the water crashes into the band of soft rock, a plunge pool is formed. Here the angular fragments that have been eroded upstream fall into the plunge pool, and rotate with the force of the falling water. Over time this erodes the plunge pool and the waterfall retreats to leave a gorge. Drainage basin - The area of land from which a river and its tributaries drain its water from. The boundary of a drainage basin is usually marked by a ridge of high land, called a watershed. Gorge - See above Rapids - Caused by alternating bands of soft and hard rock. As the soft rock erodes, several bumps and ridges form, making the water writhe and churn.
  2. 2. Middle course of the river Meanders - Bends and curves in the river that undulate back and forth up and down the gently sloping wide valley. Differential erosion causes these meanders to move, and they form slip off slopes and river cliffs. The river flows fastest round the river cliff side, producing alluvium (sediment) that is then deposited on the slip off slope. Lower course of the river River delta - This can only be formed in an area of water with no strong residual currents, in an estuary or mouth of a river. They usually are formed in small enclosed bodies of water, e.g. The Rhone Delta or the Nile Delta. This area of the river is where the alluvium the river carries is deposited and starts to form small sandbanks, and plains in the sea, and sometimes areas of wildlife create habitats. Floodplain - A wide area of land adjacent to the river either side of it when the river starts to widen. It is affected by erosion and deposition. The floodplain can be eroded by
  3. 3. meanders The Nile Delta, pictured above, shows distinct distributaries as alluvium is deposited by the slowing water. migrating downstream, but also deposited upon by slip off slopes on meanders. The majority of deposition occurs in flooding where the river bursts its banks and spills over onto the floodplain, depositing all its carried alluvium. Over time these build up to great masses of alluvium, acting as fertile land for crops. Ox-Bow Lake - These lakes form due to the constant erosion of the river cliffs. When a bend in the meander loops back on itself, the river cliffs are eroded further to cut through the bend and the river begins to flow through the neck of land, which cuts off the meander. Over more time alluvium builds up blocking off the meander and forming an ox-bow lake. If the water in the
  4. 4. ox-bow lake dries out , a meander scar is left. Levées- These occur on the sides of the river, formed of alluvium. They are natural embankments that can reach up to several metres higher than the floodplain. These are formed over numerous cycles of flooding, in which material (alluvium) is deposited by the river as the water loses velocity having become shallow as the river floods. These are only formed on the banks of slow-flowing rivers. 2) River Processes 1. Hydraulic Power - The force of water against the bed and banks of a river. Hydraulic power increases in force when the river floods. Hydraulic force erodes the sides and bed of a river, often removing material from them. 2. Corrasion - Also known as Abrasion. The river carries alluvial material along the courses of the river. The suspended material is thrown against the bed and banks of the river, causing erosion. This is corrasion. 3. Corrosion - Soluble materials, such as limestone (calcium carbonate) begin to dissolve in acidic river water. This causes corrosion of the alluvium. 4. Attrition - The material suspended in the water collides with other suspended alluvial materials, causing erosion of the materials, which is called attrition. 5. Traction - Large boulders rolling along the bottom of the river bed. 6. Saltation - Smaller pebbles bouncing along river bed. They are lifted and dropped along the course of the river. 7. Suspension - Finer sand and silt is carried by the river giving it a brown appearance. 8. Solution - Minerals such as limestone and chalk are dissolved in the water, thus they cannot be seen, but are carried by the flow of the river.
  5. 5. 3) Human involvement in rivers River Regimes This is the difference in river flow at times of the year, normally used to precipitation in the river basin. At times (the winter and spring months) the river reaches its maximum discharge. However when the water goes above this level flooding usually occurs in the lower course with dramatic consequences. Flooding can be increased by human activity in the river valley such as: • Building towns and car parks & urban areas increasing surface flow and decreasing the permeability of the land. • Deforestation in the lower course of the river increases water in the ground flow, leading to more severe floods. • Occasional disasters e.g: burst dams, landslides and storms in the river basin. In spite of this certain precautions can be taken to stop floods, known as management schemes. For example cheap methods of preventing floods are planting trees to decrease ground flow, land use zonation to move residential areas away from the river and an early warning system when heavy precipitation occurs. More expensive solutions are to straighten river meanders to increase the velocity of discharge, dredging river beds to increase volume of discharge and building dams/reservoirs to hold water and release water at a controlled rate. 4) Case Studies Rhone Delta in Southern France The River Rhone flows into the Mediterranean Sea. It has a source in the Alps, but has a major confluence at Lyons with the River Saone. There is a large delta at the mouth of the River Rhone, where the sand and silt has caused the river to split into two distributaries called the Grand Rhone and the Petit Rhone. These also have many islands of silt and sand within them. Because of these islands, the Camargue is home to a wide variety of wildlife. There are many varying types of human involvement. This includes farming, due to the fertile soil caused by the Delta. There is also a lot of tourism, and civil engineering take place there. River Tees in County Durham The River Tees flows from high in the Pennines west into the North Sea. The Tees Valley itself forms a hub for industrial activity, local tourism and agriculture in the area. The most important feature of this case study is the human activity in the upper course of the river. Five major factors are of importance, and they will be key to understanding exam questions • Settlement and communications:- The steep land of a valley makes communications difficult, making construction and employment in this sector weak. In these upland areas the population density is low, in isolated farms and occasional villages • Water supply:- As water quality is high, the Tees valleys acts as a local water supply. A good example of local water storage is Cow Green Reservoir, which supplies the lower course with water, but also helps contain excess rainfall to prevent flooding. • Agriculture:- The major land use in the upper valley is agriculture and specifically sheep farming. Soils are too thin and the valley is too steep for active machinery so at higher grounds, sheep freely roam and graze. But below 400 metres fields are seen with traditional dry-stone walls and on gently sloping land the occasional crop is grown or a small herd of dairy cattle • Tourism:- Many local attractions continue to bring in tourists to the area. The waterfall High Force, market towns, nature trails and footpaths, open moorland and shooting ranges are all good examples of how visitors bring employment for hotels, information centres and restaurants. Yet tourism also has negatives such as litter, the disrupting of livestock and crops, overcrowding, knocking over walls, traffic congestion and erosion of paths.
  6. 6. • Industry:- There used to be a thriving lead mining industry in Upper Tees Valley, however this has slowly died out, although the disused mines have now been converted into tourist areas. Nothing major has replaced this industry, but there is some forestry and specialist craft work employment. A quarry at Holwick is still functioning, mining Whinstone for use in roads. There is also one very large employer in the region, Glaxo-Wellcome, a massive chemical and pharmaceutical manufacturer. Mississippi River in East America The Mississippi River flows through 10 states into the Gulf of Mexico. It is the fourth longest river in the world and has a drainage basin covering most of the United States. During the autumn and winter of 1992 high rates of rainfall and snowfall were recorded. This continued through the spring and summer where storms regularly occurred over the same places again and again. This led to increased run off from the land in places where 48 inches of rain were recorded in 3 months. Over the summer of 1993 consistent heavy flooding was present, and this led to the loss of 28 lives, 36,000 homes destroyed, 6,000,000 acres of farmland flooded, millions of tonnes of silt and sand deposited in the city, and $10,000,000,000,000 of damage in the end. It was all caused by engineers trying their best to stop flooding, by concreting the flood plain, and enforcing and strengthening levees, but eventually, when the flood did occur, this just made it all worse, with no water absorbed by the ground, and a much greater volume of water trapped behind the levees when they burst.