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Rivers

  • Importance of Rivers
  • Water Cycle
  • Features of a Drainage Basin
  • Rivers impact on Land
  • Waterfalls and Gorges
  • Meanders, Ox-bow lakes, and Floodplains
  • Causes of Floods
  • Flood Management

What are the world's major rivers?

 

Longest vs Largest

The longest river is a river that travels the farthest. From source to mouth it covers the greatest distance. The largest river is a river that carries the most water from source to mouth. It is generally also the widest river. The ten longest rivers in the world are: 

 

River

Location

Distance (Miles)

Nile

Africa

4,180

Amazon

South America

3,912

Mississippi-Missouri-Red Rock

United States

3,710

Yangtze

China

3,602

Ob

Russia

3,459

Yellow

China

2,900

Yenisei

Russia

2,800

Parana

South America

2,795

Irtish

Russia

2,758

Congo

Congo

2,716

 

The River Severn is the longest river in UK, at 220 miles. It starts at an altitude of 2,001 feet in the Cambrian Mountains of mid Wales. It then flows through Shropshire, Worcestershire and Gloucestershire, with the county towns of Shrewsbury, Worcester, and Gloucester on its banks.

Why are rivers important? 

 

A river is water that flows downhill in a channel. 

The map above shows the route of the River Thames, and some of the villages, towns and cities that grew up along it. The river provided a source of transport in order to distribute goods to trade. Historically, settlement was built around rivers for protection from invasions.

 

Rivers are important for many reasons:

  • They shape the land
  • They supply us and our homes with water
  • They can be used to produce hydroelectric power
  • Influence the location of settlements
  • Means of travel
  • Recreation

What is the water cycle?

 

The water cycle is how water is transferred from the hydrosphere (rivers, seas and oceans), the lithosphere (earth's crust) and atmosphere. It consists of evaporation, condensation, precipitation and collection. Evaporation is when the sun heats the water in the puddles, rivers, seas and oceans and it turns into water vapour that rises into the air. Condensation occurs when the water vapour rises and cools down creating water droplets that join together to form clouds. When the clouds become too heavy the water droplets falls from the atmosphere to the lithosphere (earth's surface) as precipitation. Precipitation refers to rain, snow, hail and sleet. The water is then collected in the hydrosphere (streams, rivers, oceans). The cycle starts again when this water is heated. 

Water does not always fall directly into rivers. Sometimes it is intercepted and lands on trees where it is later evaporates. This is known as transpiration. Other times it infiltrates the ground and reaches rivers by throughflow and groundwater flow (where it travels through the soil to reach the river by gravity. A lot of water lands on the surface and reaches the river by travelling overland.

What are the characteristics of a river drainage basin?

 

A river's drainage basin refers to an area of land which provides a river with its water. All of the precipitation that falls into the drainage basin will reach the river by gravity through surface run-off, throughflow and groundwater flow. 

A drainage basin has many features:

 

  • Watershed - the edge of highland surrounding a drainage basin. It marks the boundary between two drainage basins.
  • Source - the beginning or start of a river.
  • Tributaries - a stream or smaller river which eventually joins a larger stream or river.
  • Confluence - the point at which two rivers or streams join.
  • Channel - the main body of water in a river from source to mouth.
  • Mouth - the point where the river comes to the end, usually when entering a sea.

What is the long profile of a river?

 

The journey a river takes from its source to mouth is known as the river's long profile. A river changes dramatically as it travels from its source in the upper course to its mouth in the lower course. The biggest change is its shape as the gradient becomes less steep. 

Upper Course

The gradient (steepness of the land) is very steep in the upper course because the rock is hard and more difficult to erode. The water flows quickly down the steep hill which gives the river energy but, the channel is narrow and so it erodes vertically (downwards), creating v-shaped valleys and interlocking spurs. The river is not powerful enough to erode these spurs and so flows around them.

Middle Course

 

As the river flows through the middle course the gradient becomes less steep and the channel holds more water because tributaries flow into the main channel making it deeper. This gives the river more power to erode sideways (lateral erosion) to make the channel wider. The river starts to bend. A bend in a river is called a meander. Later, these meanders become ox-bow lakes. 

Lower Course

 

As the river flows through the lower course the gradient becomes almost flat. The channel is now very wide as a result of continued lateral erosion and deep. Deposition occurs because the river begins to drop the eroded material it was carrying. Estuaries form in the lower course, this is where the river meets the sea. 

How does water change the land?

 

A river is constantly flowing downhill on its way to meet another river, a reservoir or the sea. As it flows it changes the shape of the land. It wears the land away in some places by taking material from it and it carries this material to deposit it somewhere else. 

 

There are three fluvial processes that shape the land; erosion, transportation and deposition. Each process has a part to play in the formation of landforms. 

 

Erosion means wearing away the land and this happens when the river flows over the land. There are four ways a river can erode the land. 

Hydraulic action - in a fast flowing river, water is forced into small cracks in the banks and bed of a river. Overtime, this breaks bits of rock off the banks and bed and wears it down. 

Attrition - this is where rocks bang against each other and knock bits off wearing them down. This is why pebbles have curved edges, because other rocks have knocked the rough and pointed edges off. 

Abrasion - rocks, stones and sand in the water acts almost like sandpaper and they scrape along the river banks and bed wearing it away.

Solution - the water dissolves the soluble minerals from the river banks and bed. 

Transportation refers to how the river carries the eroded material. This material is caused the river's load. 

Traction - the heavier load is carried along the river bed. The larger stones and rocks roll along the river bed.

Saltation - lighter sand and stones bounce along the river bed because they're too heavy to be carried (suspended) by the river.

Supsension - very light particles can be carried (suspended) by the river. This material can make the water look cloudy or muddy. 

Solution - smaller particles are dissolved and the solution is carried along.

Deposition happens in flatter land when the river loses energy. As the river loses energy, it drops or deposits its load. The material that is deposited is known as sediment.

The biggest and heaviest stones are deposited first and lighter and small stones are deposited after.

How do waterfalls and gorges form?

 

A waterfall is where water falls over a step of hard rock in the long profile of a river. The waterfall forms because there is softer less resistant rock beneath the hard more resistant rock, which is easier to erode. Waterfalls and gorges occur in the upper course of a river along with interlocking spurs and v-shaped valleys. 

Formation of a waterfall:

  1. The water flows over a step of hard rock in the upper course of a river. The hard rock erodes very slowly.
  2. The soft rock under the hard rock is less resistant to erosion. The river picks up large rocks at the bottom of the waterfall and hurls them at the soft rock. This is known as abrasion. 
  3. A plunge pool develops at the base of the waterfall and undercuts the harder rock above creating a notch. The hard rock overhangs. 
  4. The overhang collapses and the rocks at the base of the river erode the softer rock all over again. The process continues and over time the waterfall begins to retreat upstream leaving a steep sided valley known as a gorge behind.

How do meanders and ox-bow lakes form?

 

A meander is a bend in the long profile of a river. It starts as a slight bend but eventually becomes bigger and loopier due to erosion and deposition.

Formation of a meander:

  1. The water flows faster on the outside bend of a meander which causes hydraulic action and abrasion to wear away the river banks and bed. The force of water undercuts the outside river bank creating a river cliff.
  2. The water is slower on the inside bend and deposition occurs forming a slip-off slope. 
  3. Erosion on the outside bend continues which makes the meander migrate across the floodplain and become loopier.

Eventually, two outside bends of a meander migrate towards each other. This makes the neck of the meander very narrow. A river looks for the fastest route and over time cuts through the neck of a meander (during a flood when it has more power to erode) to create a new straight channel. Deposition occurs cutting the loop off from the main river channel; this is known as an ox-bow lake. 

Should we build on floodplains? 

 

A floodplain is the wide and flat land either side of the river in the middle and lower course, it gets its name from the simple fact, this land regularly floods. Floodplains are often used for agriculture because the soil is very fertile from the river. 

 

Floodplains can form in one of two ways. 

  1. A river floods and deposits its load, creating a very flat floodplain. This builds up layer upon layer and over many years to form a thick deposit of alluvium (sand and silt) on either side of the river.
  2. Floodplains can form when meanders migrate across the floodplain through lateral erosion. When they reach the edge of the floodplain they erode the valley side. This makes floodplains wider.

Settlements grew up along rivers because the water could be used for drinking, cooking, cleaning and transport to allow for trade. Floodplains today are still used for settlements. In recent years the homes of those living on floodplains have been damaged and many have to evacuate as a result of prolonged rainfall causing the banks of rivers to burst. This was case in Somerset during the winter of December 2013 and January 2014.

Flooding is not new and we know it has happened for centuries around the Bristol Channel. The sketch below is supposed to depict a flood that happened in Bristol during 1607.

 

The Environment Agency advises Local Authorities against building on floodplains yet, many still develop homes on them. Flooding will always happen on floodplains but, it will only impact people and their livelihoods if they live on floodplains. However, there is the argument that the UK is experiencing a housing crisis and needs to build homes in areas that can be used for development. Due to the risks of flooding, the land either side of a river is often quite cheap; this attracts many property developers. What do you think?

What are the causes of flooding? 

 

Floods occur when water overflows the river's channel and land that is not usually underwater becomes inundated. There are natural and human causes of flooding. 

 

Natural Causes:

  • Prolonged rainfall - when it rains continuously the ground becomes saturated (holds as much water as possible) and water collects on the surface. Water also travels to rivers quicker by surface run-off. Eventually, the discharge in the river becomes too much and the river bursts its banks. 
  • Geology - some rocks are impermeable (water does not pass through them) such as clays and shale. This means that water collects on the surface and reaches rivers by surface run-off much faster, increasing the likelihood of flooding.
  • Relief - relief means the shape of the land. In mountainous environments, the steep slopes allow water to reach the rivers much faster than it would on flatter land. The steep gradient doesn't give time for the water to infiltrate the soil. This increases the likeliness of flooding. 

Human Causes:

  • Urbanisation - building on floodplains creates impermeable surfaces such as tarmacked roads, slate roofs and driveways. Water reaches drains quickly by surface run-off, which then reaches rivers. This quick transfer of water makes flooding more likely.
  • Deforestation - a lot of water that falls on trees is evaporated from them or stored on leaves and branches. The roots also take up water as the tree grows. When humans remove trees, more water reaches the river and therefore increases the likeliness of flooding.
  • Agriculture - arable farming (crops) means the land is often ploughed, which leaves furrows that water can flow through to reach the rivers faster. This might increase discharge to the point of flooding.

How can we stop flooding? 

 

We cannot stop floods because we cannot stop heavy and prolonged rainfall. Floods can ruin homes and business which will lead to temporary homelessness, loss in income and cost a lot of money to repair. People may not be able to get to work or school due to blocked roads or damaged bridges. Flooding can also lead to crops being ruined and farmed being evacuated which might then cause food shortages. There are many ways to prevent flooding causing damage to people, farmland and property. This is done through soft and hard engineering.

 

Soft engineering means working the rivers natural processes to protect communities and habitats from flooding. It is not invasive and does not involve artificial (man-made) structures. 

  • Flood warnings - rivers are monitored using a range of technology. It measures the river level and amount of rainfall. Computers are then used to predict if discharge levels will overflow and what areas at most at risk. The Environment Agency send flood warnings to the emergency services (to allow them to prepare) and the pupil by text and media (often the TV weather report). This allows for people to prepare for flooding by using sandbags to protect their homes or local authorities can install temporary flood barriers. People can also move their valuables upstairs to avoid them being damaged. 
  • Floodplain zoning - this restricts building homes closest to river channels. Instead, the area closest to the channel can be used by animals for grazing, children for playing fields and car parks. Homes should be built as far away from the channel as possible to reduce the cost of damage, it is easier to evacuate farmland, close playing fields and car parks off to people than it is to protect a house. This method will be difficult to implement as many floodplains already have homes built on them. 
  • Planting trees - trees absorb and intercept water and prevent it from reaching rivers. Planting belts of trees along slopes and floodplains reduces the risk of flooding, looks natural and helps to mitigate the causes of climate change too. 

Hard engineering involves using artificial structures to slow down the flow of a river. It is very invasive and expensive. 

  • Dams and reservoirs - dams are large structures (like a wall) that is built across a river's channel. This floods the land behind it, creating a large reservoir. Dams control the water that flows down a river. During high rainfall, technology can hold the water behind the dam to prevent flooding further downstream. Dams can be very useful; they can help with irrigating farmland, supplying water and producing electricity through hydroelectric power. On the other hand, they are very expensive and people have to move to make way for building dams and reservoirs. 
  • Embankments - these are raised river banks that allow the river to hold more water, preventing flooding. This can be done by building concrete walls or it can be done using mud that had been dug from the river bed. This increases the depth of the channel too and is a more sustainable solution. Using mud embankments is cheaper and it looks natural. 
  • Channel straightening - this means cutting through the neck of meanders to create a straight channel. This speeds up the flow of water but it doesn't really stop the risk of flooding, it just makes flooding much more likely to happen further downstream. 

Questions

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