geography-rivers

Drainage basin

an area of land drained by a river and it's tributaries

Source

the start of a river

Tributary

a small stream that joins a larger river

Confluence

where a tributary joins a larger river

Watershed

the edge of a river basin

Mouth

the end of a river, usually where a river joins the sea

Upper course characteristics

- valley: steep-sided, V shaped

- River: narrow, shallow, turbulent

- Steep gradient

Middle course characteristics

- valley: wider, flat floor

- river: wider and deeper

- gentle gradient

Lower course characteristics

- valley: very wide and flat

- river: wide, deep, with large sediment load

- very gentle gradient

vertical erosion

downward erosion

lateral erosion

sideways erosion

what erosion land forms can you find in the upper course?

waterfalls, interlocking spurs

what erosion and deposition land forms can you find in the middle course?

meanders and transportation

what deposition land forms can you find in the lower course?

levees

Hydraulic action

the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away.

Abrasion

rocks carried along by the river wear down the river bed and banks.

Attrition

rocks being carried by the river smash together and break into smaller, smoother and rounder particles.

Solution

soluble particles are dissolved into the river.

types of transportation

solution

traction

saltation

suspension

deposition

sediment is deposited on the bed and banks of the river and at the mouth, where velocity falls due to increased friction

where does deposition take place

- larger rocks tend to be deposited in the upper course of a river. they are only transported for very short distances, mostly by traction, within high flow

- Finer sediment is carried further downstream, mostly held in suspension. This material will be deposited on the river bed or banks, where velocity is slowed by friction

- a large amount of deposition occurs at the river mouth, where the interaction with tides, along with the very gentle gradient, greatly reduces the river's velocity

How are waterfalls formed?

The soft rock erodes more quickly, undercutting the hard rock.

The hard rock is left overhanging and because it isn't supported, it eventually collapses.

The fallen rocks crash into the plunge pool. They swirl around, causing more erosion.

Over time, this process is repeated and it moves upstream.

A steep-sided gorge is formed as it retreats.

Gorge

A narrow steep-sided valley that is usually found immediately downstream of a waterfall. It is formed by the gradual retreat of a waterfall over many years.

Riffle

an alternating sequence of shallow, fast flowing sections

pools

deeper, slower moving sections

How is an ox bow lake formed?

- the neck of a meander is gradually eroded

- water now takes the shortest route (usually after a flood)

- deposition seals off the old meander

- the meander is cut off forming an ox bow lake

Flood plain

a wide, flat area of marshy land on either side of a river, and found in the middle and lower courses. They are made of a fertile soil called alluvium, formed by sediment (silt) deposited by a river in flood.

- meanders migrate across the floodplain due to lateral erosion. when they reach the edge of the floodplain they erode the valley side. this explains why floodplains are very wide

Levee

a raised river bed found alongside a river in its middle/lower course. it is formed by flooding over many years: the heaviest/coarser material is deposited first forming these ridges of land.

Estuaries

the transitional zones between river and coastal environments and are affected by wave action as well as river processes. There is a mix of fresh and salt water here

what is the name the waterfall on the River Tees?

High force

How was high force formed?

due to a resistant band of igneous rock which cuts across the river valley. unable to erode this tougher band of rock, the river has formed a step in the long profile of the river. this has developed over hundreds of years to form high force waterfall

as the river plunges over the waterfall, it undercuts the weaker limestone forming an overhang. this eventually collapses and the waterfall gradually retreats upstream to form a gorge.

Physical factors of flooding

- Precipitation - river can not hold the capacity of rainfall from storms or flash floods

- geology - impermeable rock encourage water to flow over land and into river channels. this speeds up water flow and makes flooding more likely

- steep slopes - steep slopes encourage a rapid transfer of water towards river channels. this increases the risk of flooding

human factors of flooding

- urbanisation - building on a floodplain creates impermeable surfaces. this means water can't be absorbed as effectively

- deforestation - much of the water that falls on trees is evaporated or stored temporarily on leaves or branches. When trees are removed much more water is transported to the river channels

- agriculture - soil is left unused and exposed to the elements for periods of time. This can lead to more surface run off. This is increased if the soil is ploughed up and down steep slopes, as water can flow quickly along the furrows

time lag

the time in hours between the highest rainfall and the peak discharge

- the shorter the time lag the greater risk of flooding

what affects the shape of a hydrograph? - basin size

- small basins often lead to a rapid water transfer

- large basins result in a relatively slow water transfer

what affects the shape of a hydrograph? - drainage density

- a high density speeds up water transfer

- a low density leads to a slower transfer

what affects the shape of a hydrograph? - rock type

- impermeable rocks encourage rapid overland flow

- permeable rocks encourage a slow transfer by groundwater flow

what affects the shape of a hydrograph? - land use

- urbanisation encourages rapid water water transfer

- forests slow down water transfer, because of interception

what affects the shape of a hydrograph? - relief

- steep slopes lead to rapid water transfer

- gentle slopes slow down its transfer

what affects the shape of a hydrograph? - soil moisture

- saturated soil results in rapid overland flow

- dry soaks up water and slows down its transfer

what affects the shape of a hydrograph? - rainfall intensity

- heavy rain may exceed the infiltration capacity of vegetation, and lead to rapid overland flow

- light rain will transfer slowly and most will soak into the soil

Hard Engineering - Dams and Reservoirs

They are widely used to regulate river flow. Most projects have multiple purposes, such as:

- flood prevention

- irrigation

- water supply

- recreation

- hydro-electric power generation

During periods of high rainfall, water can be stored in the reservoir. It can be released when rainfall is low.

However, during the building of dams, large areas of land are flooded and they cost huge amounts of money. Many have to be moved from their home.

Hard Engineering - channel straightening

Involves cutting through meanders to create a straight channel. This speeds up water flow along the river reducing the risk of flooding, but it can increase flood risk downstream.

In some places the straightened sections are lines with concrete. This can help prevent banks from collapsing and speed up flow but it can cause the channel to silt up. the concrete channel creates an unnatural river environment and can damage wildlife habitats.

Hard Engineering - Embankments

Its a raised riverbank. Raising the level of a riverbank increases the capacity of the river meaning there is more time before flooding occurs.

Some structures involve concrete walls or blocks of stone to prevent flooding of valuable property. Sometimes mud dredged from the river is used. This is cheaper and is more sustainable.

Hard Engineering - flood relief channels

Human made river channels constructed to by-pass an urban area.

During times of high flow, sluice gates can be opened to allow excess water to flow away into the flood relief channel.

What is soft engineering?

involves working with the environment to manage flood risk in a sustainable way

Soft Engineering - wetlands and flood storage areas

Wetland environments on river floodplains are very efficient in storing water. Wetlands are deliberately allowed to flood to form flood storage areas. water can be stored to reduce risk of flooding further downstream

Soft Engineering - floodplain zoning

This restricts different land uses to certain locations on the floodplain. Areas close to the river and at risk from flooding can be kept clear of high-value land uses such as housing and industry. Instead these areas can be used for pasture, parkland or playing fields. It can have an overall lose form flood damage but it can be difficult to implement on floodplains that have already developed.

Soft Engineering - River restoration

Where the river has been changed artificially, It can return back to its original course. It uses the natural river processes and features of a river, such as meanders and wetlands to slow down flow. This might allow floods to happen here like a safety valve preventing floods elsewhere.

Soft Engineering - Preparing for floods

Rivers and river basins are monitored remotely using satellites and computer technology. Instrument are used to measure rainfall and check other river levels. Computer models can then be used to predict discharges and identify areas at risk from flooding.

The environment agency makes maps identifying areas at risk from flooding. These people are encouraged to plan for floods, including:

- planning what to do id there is a flood warning

- using flood gates to prevent floodwater from damaging property

- using sandbags to keep floodwater away from buildings

Local authorities and emergency services use these maps to plan for floods also

- temporary flood barriers

- evacuation

- closing roads