By the end of the unit…

• Hydrological Cycle

• Hydrographs

• River Processes

• River Landforms

• OS Map Skills

• Past Paper Question

Hydrosphere

“.. If we are not careful, future wars are going to be about water and not about oil.”

15th March 2001

New Delhi

Kofi Annan

Water is vital for all life.

It occurs as a liquid (rivers, seas and oceans), as gas (water vapour) and as solid (glacier and ice-caps).

This is the movement of water and its transportation between gaseous, liquid and solid form.

• Evaporation is the process of water changing from liquid into gas. An input of solar energy causes this change.

• Evaporation from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces.

• Condensation is the process of water changing from a vapour to a liquid. Condensation takes place when water vapour is cooled.

• Precipitation, rain, snow, hail and sleet are the main form.

• Transpiration is the process of water loss from plants. Transpiration takes place when the vapour pressure in the air is less than that in the leaf cells, i.e. transpiration is nil when the relative humidity of the air is 100%.

• Groundwater is derived mainly from precipitation which has percolated through the soil layers into the zone of saturation, where all pores and cracks are filled with water.

The Hydrological Cycle

Explain how a balance is maintained within the hydrological cycle.

Approximate water storage in the cycle:

• 2.1% is stored in ice, snow and glaciers.

• 0.6% is fresh water in rivers and lakes.

• 0.1% is stored as ground water and soil moisture.

• 0.001% is stored in the atmosphere.

Explain how a balance is maintained within the hydrological cycle.

• At any given time the atmosphere only holds, on average, sufficient moisture to give 10 days of rain…

• There must therefore be a constant recycling of water between the ocean, atmosphere and land.

How do humans affect the hydrological cycle?

The Flood HydrographThe flood hydrograph indicates whether a particular river has a high or low flood

risk, by showing the relationship between precipitation and run-off.

Make your own hydrograph with the following information:

The discharge starts at around 8 cumecs. Peak rainfall is 65mm. It reaches this at 10.00am. The lag time is 9 hours. Therefore the peak discharge is at … The Peak discharge is 60 cumecs.

• Add on your axes first.• You will need a bar and a line graph.• Label correctly (eg rising limb, falling limb), or add a key.• Add on throughflow, groundwater flow and overland flow

(surface run-off).• Remember to show the steepness of the line graph.• When you are finished, write a description of what your

hydrograph is showing.

Some hydrographs cover a full year. Precipitation and discharge cover the full year.

Other hydrographs show a few days only. These ones show only one period of precipitation and river discharge which results.

Basic terms:

Run off: the sum of all the rainwater that flows over the surface of the river basin (stream flow and overland flow)

Through flow: the downslope movement of water through soil towards streams and rivers.

Base flow: groundwater movement, which often lags behind precipitation by weeks, months or even years. The usual, reliable, background level of a river, maintained generally by seepage from ground water storage Stormflow: discharge that is not baseflow.

Approach segment: the discharge of a river before the storm.

Some words to do with hydrographs:

Lag time: the time delay between max rainfall and max discharge.

Lag time varies according to the type of drainage basin.

Rising limb: part of the graph which shows increasing

discharge

Falling limb (recession limb): part of the graph which shows decreasing discharge

Bankful discharge: the point at which a river is full.

If the level increases any more, the river floods.

Factors affecting hydrograph

1. Weather factors• Heavy rain -> less

infiltration -> greater overland flow.

• Very cold weather -> frozen ground and little evaporation -> greater overland flow.

Factors affecting hydrograph 2. Natural Factors• Impermeable rock/soil -> very little or no

infiltration. • High water table. • Gradient of the drainage basin. • Small drainage basin area. • High drainage density • Little vegetation • Shape of the drainage basin. (Circular or

elongated shape)

Factors affecting hydrograph 3. Human Factors • Urbanization-> increases the impermeable

surface with tarmac and concrete. • Drainage schemes -> • Deforestation -> less interception -> fewer roots

to take up and hold up the water. • Afforestation is the process of converting open

land into a forest by planting trees or their seeds. ->

• Canalization of the rivers -> straighter river channel->

http://www.pupilvision.com/yeareleven/hydrograph.htm

Precipitation may be intercepted by plants. Large areas of dense vegetation will intercept large amounts of water.

One way of reducing the impact of flood in an areas is to plant trees and slow down the runoff. Deforestation, on the other hand, will increase the risk of flooding.

A drainage basin is the _________area of a river and its_________. The boundary of the catchment area is called the_______. A drainage basin can be viewed as a ______ with inputs such as _________and solar energy and ______such as evapo-transpiration and _____.

A drainage basin is the catchment area of a river and its tributaries. The boundary of the catchment area is called the watershed. A drainage basin can be viewed as a system with inputs such as precipitation and solar energy and outputs such as evapo-transpiration and runoff.

River BasinSource of River

Confluenceof Riverand tributary

Tributaryof River

Flood plain

Mouth ofRiver

Sea

Estuary ofRiver

Watershed of River(boundary between basins)

The Course Of A RiverCharacteristics Upper Course Middle CourseLower Course

Slope

Width

Depth

StraightnessLoad

Main work

Valley width

Type of load

usually steep

narrow

shallow

winding

little

large/small angular

erosiontransportation

quite steep

quite wide

quite deep

meandering

some

medium/smallrounded

transportation

Narrow -V

gentle

quite wide -U

wide

deep

big meanderslots

small+rounded

transportationdeposition

wide

Upper Course of a River

You need to know…

4 different ways in which a river erodes the river channel. Page 54-55 textbook

4 different landforms.

• Waterfall

• V-shaped Valley

• Interlocking spurs

• Rapids

Waterfall

Summary of upper course

A small stream flows from an upland source to the mouth where it enters the sea. The river channel widens as it follows its course to the sea, and the amount of water it carries increases as other streams and rivers join it.

The river’s energy is linked to its velocity (speed). High velocity means high energy. River with lots of energy wear away the channel banks producing the load sand, stone, pebbles, boulders…

When a river has little energy, the load is deposited on the bed and banks of the river.

X-Y Cross-Section

• Flow of water around meander bend in a river follows a corkscrew pattern. Water on the outside of the bend is forced to flow faster than that on the inside of the curve.

• This difference in velocity, together with normal friction grad on the channel walls, produces a corkscrew pattern.

• As a result, erosion occur on the outside bank, where velocity is greatest, and deposition occur on the inside of the bend, where velocity is at a minimum.

• Erosion on the outside of the meander bend and deposition on the inside cause the stream channel to migrate laterally.

http://whs.moodledo.co.uk/file.php/1365/Fluvial%20systems/Ox-bow%20Lakes.swf

Lower River Course Landforms

The river is now nearing the sea and deposition becomes the dominant process.

What’s happening at the Lower Course…

• The river is wide and deep with a more efficient shaped channel

• The river is lined with sand and mud so this results in less friction

• There maybe islands of silt called eyots in the river and the river can split into a number of channels - river braiding

• The river carries a large load of alluvium • There is a wide, flat flood plain either side of the river • There are features such as meanders, ox bow lakes and

deltas • The flood plain is made up of  a thick layer of alluvium • A line of river cliffs are found at the edge of the flood plain

called bluffs.

Levees • Levees are basically caused by floods. In times of low flow

any deposition takes place on the river bed and this raises the height of the river bed.

• In times of flood the water leaves the channel. As it does so it loses energy and the courser and heavier material is deposited near to the river on the banks.

• Finer material is carried further onto the flood plain.

• After many floods the river builds a bank on either side of the river and this can lead to catastrophes.

• As the river has been raised above the flood plain and the banks are often higher still, when the river floods the water can no longer drain back into the river channel and often has to form a new one. 

River Rejuvenation

All your knowledge of rivers should help you to understand this final

process which forms a unique set of landforms.

RejuvenateTo restore to a former state; to make fresh or new again

Changes in base Level

There are two groups of factors which influence changes in base level.

1.Climatic: the effects of glaciations and changes in rainfall. Also Global warming and Climate change.

2.Tectonic: crustal uplift, following plate movement, and local volcanic activity.

Base Level

This is the lowest level to which erosion by running water can take place. In the case of rivers, this theoretical limit is sea level.

Changes• Positive (more water)

Sea levels rise in relation to the land.

Decrease in the gradient of the river

The river loses energy and as such deposition dominates and coastal areas flood.

• Negative (less water)

Sea levels fall in relation to the sea.

Land emerges from the sea, steepening the river gradient thus increasing the rivers erosional power.

Rejuvenation

Key Points

• Rivers begin to regrade from the point nearest the sea.

• It often happens again before a river has reached dynamic equilibrium giving it a partly graded profile.

• Knickpoints

• Incised meanders

• River terraces

• Waterfalls

Changes to the cross profile of the river

River in grade

Flood plain

Waterfall retreats cutting

a lower valley

Fall in sea level

River bluffs

New flood plain forms

New flood plain forms

River bluffs

Waterfall decreases in size to form rapids (knick point)

River Terrace

River Terraces

Knickpoints and waterfalls

Incised Meanders

• The likely story is that the streams once flowed across nearly flat lowlands.

• Then, uplift of the rocks began, giving the streams a steeper slope to the sea and so speeding their flow and causing them to erode.

• But, the uplift was gradual enough that the streams held their old courses. 

• The streams cut downward without a change in pattern, which is called incision. 

At 760545, the River Teme is flowing in an Easterly direction.

It is approximately 100m wide.

It will be slow flowing, as it is in its lower course and the land either side is flat.

The river is also a tributary of the River Severn, with the confluence at 850522.

It is definitely natural, as it meanders often (779548).

There is also an ox-bow lake at 775548.

It is joined at a confluence by the Leigh Brook at 785537.

It continues to meander its way towards the Severn, and there appears to be an eyot at 806533, due to the deposition of sediment.

Eyot

An Ait (or Eyot) is a riverine island in England typically formed by the deposition of sediment building up over a period of time. Aits usually have a bar shape and may become permanent islands; sometimes an ait is eroded however, with the resulting sediment deposited further downstream that may create another ait. A channel with numerous aits is called a braided channel.

There is further evidence of meandering (833524) before it flows into the Severn.

The Valley surrounding the river is flat, indicating a flood plain, although it does have some steep sides a little further out, (20m at 795539).

It is approximately 1km at its widest (7953 and 8252).

At 760545, the River Teme is flowing in an Easterly direction. It is approximately 100m wide. It will be slow flowing, as it is in its lower course and the land either side is flat. The river is also a tributary of the River Severn, with the confluence at 850522. It is definitely natural, as it meanders often (779548). There is also an ox-bow lake at 775548. It is joined at a confluence by the Leigh Brook at 785537. It continues to meander its way towards the Severn, and there appears to be an eyot at 806533, due to the deposition of sediment. There is further evidence of meandering (833524) before it flows into the Severn.

The Valley surrounding the river is flat, indicating a flood plain, although it does have some steep sides a little further out, (20m at 795539). It is approximately 1km at its widest (7953 and 8252).

• Water moves in a curved path round each meander with the maximum velocity and erosion on the outer concave bank.

• The erosion on the concave banks is accompanied by deposition of a point bar on the inner convex bank.

Concave

Convex

Ox-Bow Lake formation

HYDROSPHEREStudy OS map extract number 1326/117: Chester

Describe the physical characteristics of the River Gowy and its valley between 467670 and 431775.

8

The River Gowy between 467670 and 431775 is in its lower course. It is flowing in a northerly direction. At the early part of this section, it is fairly narrow but becomes wider, about 80m (460685). It is only about 12m above sea level at this stage. It is on flat land as there are few contours, and it meanders (458698) its way towards the sea, showing it is natural. It may have been artificially straightened between 431738 and 430760 however. The flood plain becomes larger (4372) and there are embankments built here to prevent flooding. It will have a fairly high discharge due to tributaries flowing into it (4372). By 431759 the river becomes tidal, before it enters the large Stanlow Banks (mudflats) at 431775. Its valley is wide, with a flat valley floor, and is therefore likely to be U-Shaped.

tributary

• A tributary is a stream or river which flows into a main river. A tributary does not flow directly into a sea, ocean, or lake.

confluence

• A confluence of a river is a meeting of two or more bodies of water.