Rivers

Question 1

Define drainage basin (1)

Answer 1

A drainage basin is an area of land that gathers water from a river

Question 2

Define watershed (1)

Answer 2

A watershed is the boundary of a drainage basin, an area of high ground separating drainage down one slope to another (1)

Question 3

Explain the first four components of the drainage basin and state whether they are inputs, outputs, transfers or stores (8)

Answer 3

1. Precipitation (input) - water falls from rain, sleet or snow from the clouds towards the surface
2. Interception (store) - the precipitation is caught and held for a short time by vegetation before reaching the soil store, more vegetation means more interception
3. Stem flow and through fall (transfers) - the movement of water from the interception store to the surface store either from flowing down branches (stem flow) or dripping off leaves (stem flow)
4. Surface store (store) - water is stored temporarily on the surface e.g in puddles

Question 4

Explain the next four components of the drainage basin (five - eight) and state whether they are inputs, outputs, transfers or stores (8)

Answer 4

5. a) Infiltration (transfer) - water enters the soil from the surface store
   b) Overland flow / surface runoff (transfers) - water runs over the surface of the land following a rainstorm, this occurs when the soil has became too saturated, rain intensity exceeds the infiltration capacity or rain falls on impermeable surfaces
6. Soil storage (store) - water that has infiltrated is stored in the surface layers of the soil before experiencing through flow or percolation
7. Throughflow (transfer) - water moves downhill through the soil, close to and parallel with the surface to the river
8. Percolation (transfer) - water moves further down into the soil from the soil store to the groundwater store

Question 5

Explain the next four components of the drainage basin (nine - twelve) and state whether they are inputs, outputs, transfers or stores (8)

Answer 5

9. Groundwater storage (store) - the permanent store of water in the lower layers of the soil and the bedrock
10. Groundwater flow (transfer) - the movement of water from the groundwater store in the lower layers of soil and the bedrock to the river
11. Evaporation (output) - water is changed into water vapour from from various stores such as interception and surface storage, the main factor affecting this is temperature
12. Transpiration (output) - water vapour is taken from vegetation and plants into the atmosphere, this is affected by vegetation type and moisture availability

Question 6

Explain three factors that affect the transfers and stores in a drainage basin (6)

Answer 6

Vegetation - thick vegetation creates more opportunities for interception and for evaporation from the leaves of trees, any water that does not directly reach the ground will take a much longer time to get their

Soil type - some soils are porous and they can infiltrate quickly (e.g sandy soils). Whereas some are the opposite (e.g clay soils) infiltration is slower making surface runoff the main transfer

Seasons - the warmer the temperature is in summer the more evapotranspiration lowering discharge levels in the river. When deciduous leaves fall in the autumn there is less interception, in the winter the ground could be frozen preventing infiltration

Question 7

Define discharge (1)

Answer 7

The volume of water passing through one point at any given time (1)

Question 8

Explain the difference between an annual hydrograph and a storm hydrograph (2)

Answer 8

An annual hydrograph shows the variation in river discharge over a year whereas a storm hydrograph records the impact of one specific event of precipitation on discharge (2)

Question 9

Explain the difference between base flow and storm flow (2)

Answer 9

Base flow is the background flow contributed slowly and steadily by groundwater flow, whereas storm flow is the additional flow contributed by precipitation (2)

Question 10

Define lag time (1)

Answer 10

Lag time is the difference between peak precipitation and peak discharge

Question 11

Explain the difference between the rising limb and the falling limb (2)

Answer 11

The rising limb is the point of the storm hydrograph that rises steeply towards peak discharge whereas falling limb falls slowly as the main flow is now through flow which is slower than surface runoff (2)

Question 12

Explain how the nature of the storm, I.e the length of precipitation affects a hydrograph (2)

Answer 12

More intense rain causes the soil to become saturated leading to infiltration capacity and surface runoff. This produces a short lag time and higher peak discharge of a flashy hydrograph (2)

Question 13

Explain how nature of the drainage basin I.e basin size, basin shape and soil type affect the storm hydrograph (6)

Answer 13

Basin size - a smaller basin the precipitation has less distance to travel before it reaches the mouth, so the hydrograph will be shorter and steeper (2)

Basin shape - shorter, round basins are more likely to be flashy as the water from the basin tends to arrive more quickly at the mouth. In a longer, thinner basin, the water falls near the source has much further to travel which produces a flat hydrograph (2)

Soil type - clay soils are impermeable meaning they allow for water to enter slowly, as a result surface runoff is more likely and so the water reaches the channel quickly, vice versa for sandy soils (2)

Question 14

Explain how land use, i.e urban areas, ploughed fields and afforestation affect the storm hydrograph (6)

Answer 14

Urban areas - the impermeable surfaces increase runoff and the drains and sewers are designed to take the surface water to the river quickly

Ploughed fields - where vegetation is removed for agriculture, it leaves bare soil which reduces interception and so the water gets to the channel quicker

Afforestation - this increases interception and slows the speed which the water reaches the channel, more evaporation occurs and peak discharge is lowered

Question 15

Explain how climate I.e precipitation, temperature and snowmelt influence the annual hydrograph (6)

Answer 15

Precipitation - the total amount of rainfall affects discharge including its timing. Some climates experience monsoon rainfall which is when most rain falls during summer giving large peak in river discharges)

Temperature - the higher the temperature, the greater the rates of evaporation. This reduces the amount of water in a river channel making peak discharge lower

Snowmelt - linked to temperature, this can have a significant impact on rivers that have large amounts of snow or ice near them

Question 16

Define abrasion/corrasion (2)

Answer 16

Abrasion is an erosional process which involves using the rivers load to erode the bed and banks by scraping and scouring, it is particularly effective at times of higher discharge when the river has enough energy to transfer heavier particles (2)

Question 17

Define hydraulic action (2)

Answer 17

Hydraulic action involves the physical force of water against the bed and banks. On the outside bends of meanders, the currents push water into cracks causing pressure leading to erosion. It is more effective in waterfalls and rapids. (2)

Question 18

Define corrosion/solution (2)

Answer 18

Corrosion is the dissolving of soluble materials in the bed and banks by weak acids in river water, this is a chemical reaction rather than a physical process so it is not dependent on the velocity of a river (2)

Question 19

Define attrition (2)

Answer 19

Attrition occurs when the load particles clash with each other and the bed and banks, as a result the rough edges are smoothed and the particles become smaller and more rounded (2)

Question 20

Explain the difference between vertical and lateral erosion (2)

Answer 20

Vertical - creates river valleys which is more common in the upper course of the river

Lateral - as meandering rivers widen floodplains

Question 21

Define suspension (1)

Answer 21

The smaller particles of clay, silt and sand can be carried along by the turbulence of the river, this tends to be the most effective form of transportation

Question 22

Define solution (1)

Answer 22

Solution involves the material eroded from corrosion, it is carried along and dissolved in the water

Question 23

Define saltation (1)

Answer 23

The smaller bedload, such as pebbles and gravel, can be bounced along the riverbed by turbulence during times of higher discharge

Question 24

Define traction (1)

Answer 24

The largest boulders in the river can be rolled along the riverbed during times of very high discharge

Question 25

Where does deposition occur on meanders and floodplains (2)

Answer 25

Meanders - point bar deposits inside the bends of meanders Floodplains - as a river outflows it’s banks and in deltas

Question 26

What are the two lines the Hjulström Curve shows? (2)

Answer 26

1. The critical erosion velocity/pick-up curve (i.e the speed needed to pick up the particles) 2. The critical deposition velocity/drop curve (i.e the speed at which there is no longer enough energy to transport the particles)

Question 27

Why do clay particles need more energy as their particle size decreases (4)

Answer 27

They are cohesive - they stick together and so need more energy to separate one another, however once separated they are so small that they are easily transported Smooth surfaces - this means that as the river flows over them, there is less turbulent flow and so fewer eddies in in the water that could scour off particles from the surface

Question 28

With reference to places, explain how a waterfall is formed (6)

Answer 28

Harder rocks (e.g basalt) overlay soft rocks (e.g chalk), as the river flows from hard to soft rock the softer rock tends to erode quicker. This forms a step where the two rock types meet, this causes the water to fall vertically increasing its speed due to friction (2) Over time erosion (particularly hydraulic action and corrasion) is concentrated at the base of the waterfall, this excavates the softer rock vertically downwards to create a plunge pool, and undercuts the harder rock to form a notch. Overtime as this notch grows in size the overhanging harder rock above it collapses as it is now unsupported leaving behind a steep-sided gorge (waterfalls tend to migrate upstream) (2) Niagra Falls : located in North America, one of the most famous waterfalls in the world, at the end of the Ice Age it was 100m high, which was over 12,000 years ago, this waterfall is marked by the 11km long Niagra Gorge (2)

Question 29

Explain how a rapid is formed (2)

Answer 29

Rapids are found in steeper sections of the river channel, for example as water flows over a waterfall both river velocity and river turbulence are increased here and so erosion rates are higher (2) An example of a rapid is Sheer Wall Rapids in Colorado River

Question 30

Define pool (1)

Answer 30

Pools are found in the channel bed near the outer banks of meanders, they are areas of deep, smooth water flow.

Question 31

Define riffles (1)

Answer 31

Sections of channel between two meanders where the water is shallow and flows through coarse gravel bed sediment (1)

Question 32

Explain the formation of a meander (5)

Answer 32

Due to increased frictional drag caused by the larger particles in the riffles the maximum velocity flow tends to swing to avoid these areas. This creates side-to-side motion within the water and is the start of the process. As the maximum velocity flow swings to avoid riffles, erosion (hydraulic action and corrasion) is concentrated on one of the banks. This deepens the channel and undercuts the river bank producing a steep-sided river cliff. While river velocities lower, point bar deposits occur. Overtime as erosion continues the river becomes more sinuous, it moves laterally across the floodplain while migrating downstream.

Question 33

Give one example of an area that experiences meanders (1)

Answer 33

The Mississippi River - in over 80 years a meander grew laterally by over 1km and migrated downstream 2km (1)

Question 34

Explain how an oxbow lake is formed (4)

Answer 34

An oxbow lake is formed from, continued erosion of the outside bend which causes a gap in the meander loop to become narrower. When there is high velocity and the river is seeking the most efficient way downstream, it might cut across the gap. This produces a straight channel and maximum velocity flows in the middle and deposition occurs at the edges. Over time this deposition build up to cut off the old meander loop, creating the oxbow lake. This lake can silt up and form a meander scar.

Question 35

Give one example of an area that experiences oxbow lakes (1)

Answer 35

Mississippi River

Question 36

Define a floodplain (2)

Answer 36

A floodplain is a large, flat area around lowland rivers characterised by large amounts of deposition, the deposited material is known as alluvium (2)

Question 37

Explain the two sources of deposition in the formation of a floodplain (3)

Answer 37

Migrating meanders - as the meanders weave laterally, they deposit point bars on the inside, leaving alluvium deposits on the floodplain which is the main source of deposits on a floodplain River flooding - if the river overflows its banks, it experiences a significant increase in friction due to contact from the ground and floodplain vegetation, this results in lower velocity

Question 38

Explain how levees form (4)

Answer 38

Levees are often found on floodplains, they form on low ridges along the edges of the river by depositing larger particles. Meanwhile, the finer silts and clays are carried further away from the river and are deposited as the river water slowly infiltrates into the ground. As the levee increases over time, further deposition on the river bed leads to the river flowing at a higher level than the flood plain in.

Question 39

Give an example of where levees are adjusted (1)

Answer 39

Mississippi River, river engineers have increased the height of the natural levees to increase channel capacity and reduce the risk of flooding (1)

Question 40

Define delta (1)

Answer 40

Deltas are depositional features extending from the mouth of the river into a sea or lake (1)

Question 41

Explain the three factors affecting delta formation (6)

Answer 41

When the rate of deposition is higher than the rate of erosion - deposition rates are higher when a river has a large amount of sediment, for example a large river such as the Nile. Erosion rates are lower when the marine environment has a smaller tidal range and weak current Flocculation - When fresh water mixes with seawater , chemical reactions with the salt cause clay particles stick together. This increases their weight and increases depositional rates A gentle sea floor gradient - this increases deposition and aids delta formation

Question 42

Explain the characteristics of a delta (3)

Answer 42

Deltas are regarded as an extension of the river’s floodplain, extending the coastline out into the sea. A common feature is that the river itself breaks up into many channels around the delta, causing braiding. These separate channels are called distributaries as they effectively distribute water and sediment and give rivers numerous mouths (3)

Question 43

Explain how an arcuate delta is formed (4)

Answer 43

These grow out from the coastline with a smooth outer edge, one of the best examples of this delta is The Nile. The deposition at the mouth of a river is usually triangular in shape. This is due to the blocking of the river mouth, which forces the river to break into a number of other channels called distributaries. The distributaries weave back and forth, depositing sediment which overtime forms an arcuate delta.

Question 44

Explain how a birds foot delta is formed (4)

Answer 44

This occurs where delta formation is more river-dominated and less subject to tidal or wave action. The deposition pattern appears more haphazard and the delta can take on multi-lobed shape that resembles a bird’s foot.

Question 45

Give an example of where a birds foot delta occurred (1)

Answer 45

Mississippi river - which drains over 50% of the USA and carries a huge sediment load into the low energy waters of the Gulf of Mexico

Question 46

Explain why some rivers need channelised (3)

Answer 46

Channelisation = this involves the deliberate modification of a river channel from changing its width, depth and form to reduce the risk of flooding and improve hydraulic efficiency. It can also involve the construction of embankments or artificial levees The most common reasons of channelisation are to improve 1. Flood control 2. Land drainage 3. Navigational 4. Erosion problems

Question 47

What is the problem with channelisation (2)

Answer 47

Dynamic equilibrium - where a change in one variable causes change somewhere else in the channel (e.g flooding occurs further downstream)

Question 48

Explain how re-sectioning works (3)

Answer 48

This involves changes to the rivers cross-sectional profile, by widening and/or deepening the river. This results in the river having a greater capacity and a greater efficiency, as it can hold more water before overflowing onto the floodplain and it allows the water to flow at higher velocities therefore water can be removed quicker to reduce the risk of flooding.

Question 49

Explain how realignment works (3)

Answer 49

This involves the straightening of a river channel, either by replacing an entire stretch of a meandering river with a straight channel, or creating cut-offs across meander bends. This improves the rivers efficiency by increasing river velocity.

Question 50

Explain how dredging works (3)

Answer 50

Removing obstructions from the bed such as boulders, sand banks or weed beds to reduce friction and increase velocity. This results in increased ability to erode the beds and banks which means upstream and downstream banks may become unstable needing bank protection (e.g gabions)

Question 51

With reference to places, explain three reasons why some rivers need channelised (9)

Answer 51

1. To increase river capacity and hydraulic efficiency - this is in order to reduce the risk of flooding, if a river can hold more water and channel more water quickly out of the area then the floodplain around it is less likely to flood. For example following devastating floods in 2014 in Somerset Levels, the EA dredged River Tone and Parrett (3) 2. Improve land drainage in agricultural areas - flooding in farmlands can take out land for weeks and even months. By reducing flooding, channelisation can keep this land protected. For example, River Blackwater Land Drainage Scheme (1984-1991) involved deeding and removal of 13 meanders in order to reduce floooding and increase agricultural productivity in the area. (3) 3. To improve navigation in the river - rivers are natural transport routes and vital for economic activity. However given the size of many modern ships and boats channels often have to be dredged and realigned in order to make them more navigable. For example the Lower Mississippi River has been shortened to 235km over the last 90 years by cutting off meander loops. (3)

Question 52

With reference to places, explain two particular situations where hard engineering channelisation is needed (6)

Answer 52

1. In urban areas - urban areas have development along the banks so there may not be space for soft engineering methods beside the river, so it may need to be channelised to protect people and property from flooding. For example, the River Greta is channelised where it runs through Keswick in Cumbria, protecting 141 properties (3) 2. Economic reasons - rivers need to be kept navigable for shipping which is why dredging is used. The Mississippi River, as a US Federal law requires the river to have a depth maintained no less than 3m to aid navigation (3)

Question 53

Explain three reasons why sustainable and environmentally sensitive methods are necessary (9)

Answer 53

1. Limitations of channelisation - channelisation can be effective at reducing flooding in the area in which they are used. However it can lead to increased flooding downstream where there is no channelisation, when increased amounts of water move out of the channelised section they are funnelled into a small river channel which is less efficient of moving water leading to increased risk of flooding (3) 2. Restore natural rivers and increase habitat diversity - another consequence of channelisation is the reduction of habitat diversity within the river channel itself and across the floodplain. To return rivers to a natural state environmentally sensitive river restoration schemes can be used, improving habitat diversity. (3) 3. Increased threat of flooding resulting in climate change - A 2014 report states that by 2050 flood damage will increase 4x in Europe. This would make floods occur from every 16 years to every 10 years. Because of this flood management strategies have been implemented to protect people, property and land. (3)

Question 54

Define washlands (2)

Answer 54

Areas upstream of an urban area that are allowed to flood naturally during times of high discharge, this means that they act as natural stores in the drainage basin

Question 55

Define land use zoning and explain the use of washlands and afforestation (5)

Answer 55

Land-use zoning involves the management of the land to reduce the risk or scale of flooding (1) Washlands are areas upstream of an urban area that are allowed to flood naturally during time of peak discharge. This means they can act as natural stores in the drainage basin, reducing the flood peak of water reaching urban areas (2) Afforestation involves trees intercepting rainfall, slowing down its journey to the river. Consequently the water arrives at the channel slower and peak discharge is reduced. Trees also help output water from the drainage basin via evapotranspiration, which reduces overall discharge. (2).

Question 56

With places for illustration, where were environmentally sensitive and sustainable methods used (6)

Answer 56

In East Belfast, the Connswater Community Greenway is a £32 million scheme to create a 9km linear park. Areas within the drainage basin were prone to flooding, this includes over 1,700 properties. The aim was to improve flood protection to nearby homes and develop the river as a key environmental feature of public park amenity. The new plan involved environmental restoration with a wider and deeper natural channel of the Knock River. Also, wetland habitats were designed linked to a new channel, to store water during high discharge periods. Because of the mixture of soft engineering structures (ponds and wet wildflower meadows) and hard engineering (flood walls and culverts) have been employed in this river restoration scheme. Artificial riffles and wildflower planting are designed to restore plant and animal habitats along the channel.