Hydrosphere

The Global Hydrological Cycle The hydrological cycle describes the global distribution and

movement of water within a system. There is a fixed amount of energy within the system and the movement of water is linked with the transfer of energy at all points in the cycle.

The initial energy is provided by the sun (which is outside the system) in the form of latent heat to the stores of water e.g. icecaps and glaciers which store about 2% of the surface water; rivers and lakes and groundwater which store 1% and oceans 97%. This transfer of energy causes evaporation.

Transpiration takes place from vegetation and when combined with the evaporation from bodies of water on the land's surface forms evapotranspiration. Geothermal energy e.g. by volcanoes contributes very little to evaporation. There is a greater percentage of evaporation from the oceans (84%) than evapotranspiration from the land (16%).

The Global Hydrological Cycle As the moist air moves upwards it cools and condenses

forming clouds. Precipitation from these clouds returns water to the earth's surface. Again there is more precipitation over the oceans (77%) than over the land (23%). As 84% is evaporated over the oceans and only 77% is returned by precipitation, 7% of the evaporated moisture is transferred to the land by winds (advection) and is matched by a return to the oceans by run-off from the land.

Therefore there is no loss or gain of energy within the system.

Advection

(7%)

The Global Hydrological Cycle

Geothermal Energy

Emitted Energy

Solar Energy

Precipitation (23%)Evapotranspiratio

n (16%)Run off

Infiltration

Run off (7%)

Water table Ocean

Storage (97%)

Clouds (0.001%)

Boundary of system

Ice storage

(2%)

Groundwater storage (1%)

SoilSoil Soil

Rivers & Lakes (0.01%)

Precipitation (77%)

Evaporation(84%)

Drainage Basins A drainage basin is an area of land drained by a river and

its tributaries.

The boundary of a drainage basin is called a watershed. This is usually found on higher ground.

The drainage density of a drainage basin is important -

High drainage density on impermeable rocks and clays – greater flooding risk

Low drainage density on permeable rocks and sand.

Unlike the closed system of the hydrological cycle, the drainage basin system is open. It has inputs, stores, flows and outputs.

Drainage Basins

Precipitation

Interception by vegetation

Surface Storage

Infiltration

Soil Moisture

Percolation

Groundwater

Transpiration

Surface runoff (overland

flow)

Evaporation

River carrying water to the ocean/sea

Throughflow

Groundwater flow

Water table

Unsaturated rock/soil

Saturated rock/soil

Input

StorageFlow(transfer)Output

Key

Stream/Storm Hydrographs A hydrograph is a record of channel flow in a stream or river. It shows variations in discharge with time.

Stream/Storm Hydrographs A hydrograph has a number of main characteristics:

Precipitation – not always shown Base Flow – flow supplied by groundwater. This is a slow

movement therefore base flow rises slower and later. Quickflow/Storm Flow – flow supplied by overland flow and

soil throughflow. This gives the peak flows. Rising Limb – includes (a) rainwater that fell into stream

(b) water flowing overland and into stream channels

as the soil water becomes saturated.

(a) & (b) lead to peak discharge.

Stream/Storm Hydrographs

The shape of the rising limb depends on:

The length of the rainstorm – a long period of rainfall causes rise in the water table and increased overland flow.

The intensity of rainfall – if heavy – considerable quick flow if the soil is incapable of

soaking it in and storing it. - if light – less step rise and flatter shape since rain is easier to store.

Stream/Storm Hydrographs

The shape of the rising limb depends on:

The condition of the soil – If the soil is already saturated more water flows overland into streams and this leads to a higher and earlier peak.

Shape of the drainage basin - a long narrow basin with tributaries joining the main channel at regular intervals gives a less steep rise.

- a wide basin with tributaries more closely spaced gives a steep rise.

Stream/Storm Hydrographs

Lag Time – the difference in time between peak rainfall and the peak discharge. Lag time depends on:

The condition of the soil – if the soil is fairly dry, the longer the lag time since rainfall can be stored at first before the excess flows to the channels.

Angle of slopes – the more gentle the slope the longer the lag time compared to steep slopes.

Intensity of rainfall – prolonged, steady rainfall has a longer lag time than intense rainfall.

Size of drainage basin – small basins have a small lag time; larger basins with more tributaries have a longer lag because the tributaries act as temporary storage before run-off increases rapidly.

Stream/Storm Hydrographs

Recessional/Falling Limb – shows the speed at which the drainage of the rivers return to normal (base flow) once the storm has passed. The water level falls off as the effects of the storm die away.

Stream/Storm Hydrographs One type of question that can arise about hydrographs

is a comparison between different hydrographs. When answering these types of questions you must not

only describe the features of the hydrographs, but explain why the hydrographs have certain shapes. It is likely that you will be given a diagram or information to help and you should think about what effects on the hydrograph the following have:

Size of the drainage basin Steepness of the slopes Rock or soil type (Permeable or Impermeable?) Stream density Vegetation

The Work of Rivers There are three main processes in the way that a river

‘works’. These are-

Erosion

Transportation

Deposition

The Work of RiversErosion

The amount of erosion a river can achieve depends on its energy or discharge. A river’s energy increases with its volume, velocity and regime (seasonal flow).

Erosion in a river is caused by 4 processes: Attrition – attacks the rivers load – pebbles etc are eroded

by sticking together, therefore being rounded and reduced in size.

Corrasion – the wearing away of the river bed and the banks by the river’s load. This is the main method of erosion.

Hydraulic Action – the wearing away of the bed and banks by the weight of water hurled against them.

Chemical Solution – dissolving of minerals from the rocks.

The Work of RiversErosion

A river’s valley grows in length by headward erosion i.e. rain wash, soil creep and undercutting where the water table meets the surface extending the valley up the slope.

A river’s valley is deepened by vertical erosion – entirely a river process.

A river’s valley is widened by lateral erosion – affected by weathering on the valley sides and by the river on the river banks.

The Work of RiversTransportation

A river transports its load in 4 ways:

Traction – dragging of pebbles, gravel along its bed. Saltation – bouncing of the load. Suspension – light sediments of silt and clay are held in

suspension by the river’s turbulence ( the greater the turbulence, the greater the size of particles which can be held).

Solution – chemicals are dissolved in the water.

The Work of RiversDeposition

Sediments are transported by a river until it has insufficient energy to move them further and deposition takes place.

A river may lose its energy where:

there is a decrease in gradient. there is widening or meandering of its channel. there is an increase in load.

River Landscapes

Rivers create valleys by eroding the land but slope processes also transfer weathered debris down the valley sides to the bottom of the valley, which is then removed by the river.

An ‘ideal river’ is divided into three sections – the Upper, Middle and Lower courses.

These courses all have characteristic features which form the river landscapes.

River LandscapesIdeal River

River Landscapes

Channel features – Typically, the upper stretches of a river channel are rocky, with boulders of varying shapes sizes. Under normal conditions the discharge is fairly low whereas under flood conditions, hydraulic action and corrasion

processes are at work.

Valley features – The valley bottom is narrow and the valley sides, typically, are steep and in cross-section are ‘V’ shaped. There are interlocking spurs. This shape is a result of the interaction of processes such as vertical erosion of the river itself, physical weather and mass movements.

Characteristic features of the Upper course of a river and its valley

River Landscapes

Long Profile – Generally the gradient is steep and the profile is uneven, particularly where waterfalls and rapids form.

Characteristic features of the Upper course of a river and its valley

Waterfall A waterfall is a steep fall of river

water where its course is markedly and suddenly interrupted. This may be the result of:

Differences in rock hardness The edge of a plateau A fault-line scarp The overdeepening of a valley

through glaciation, producing a hanging valley

The rejuvenation of the area, giving the river renewed erosional power which in turn gives rise to knickpoints where profiles interact.

Waterfall

3

Harder, more resistant rock

4 Waterfall retreats upstream

5 Gorge-like sides develop as waterfall retreats upstream

2 Overhang develops and

collapses when unsupported

River continues downstream

Angular, fallen rocks

Softer, less resistant rock, which is eroded more quickly, which leads to - 1 undercuttin

g

Deep plunge pool. Deepened by hydraulic action and abrasion of fallen, angular rocks, which are swirled at times of high discharge – deepening the pool

River Landscapes

Channel features – Generally speaking, the channel is now wider and deeper and has

smoother banks and floor compared to the upper stretches. The bedload consists of smaller more rounded particles.

Valley features – Lateral erosion by the river’s meanders broadens the valley flow into a

narrow flood plain. Meanders gradually shift their course downstream.

Long Profile – The gradient is now much more gentle, however the river’s velocity is only slightly reduced since there is a smoother deeper channel.

Characteristic features of the Middle course of a river and its valley

Meander A meander is a sinuous bend in a river.

How meanders begin to form is uncertain. They appear to begin during times of flood in sections of the river where there are riffles and pools.

Riffles are shallow areas of fast flowing water. Pools are deeper areas of slow moving water. The riffles and pools are regularly spaced. As they enlarge, the channels become more sinuous.

Water tends to flow in a helicoidal movement (corkscrew motion) and this means that material is moved form the outside of one meander bend and deposited on the inside of the next bend. This increases the sinuosity of the river.

Meander Meanders first develop in relation to riffles and pools -

Riffle

Pool

Deposition

Erosion

Meander Meander becomes more sinuous -

A B

Meander Wavelength = 7-10 X channel width

Deposition on convex bank Velocity is lowest, friction greatest Point-bar deposits; slip-off slopeLargest material deposited on upstream

side

Erosion on concave bank by hydraulic action and corrasion Velocity is greatest, friction minimal. River cliff develops

Meander Cross section A - B

A B

Slip-off slope Shallow,

slow flowing

Helicoidal motion of

water

Deep, fast flowing

River cliff

Meander

Erosion on concave bank – river cliff develops

Deposition on convex bank – slip-off slope develops

Deep fast flowing

Shallow, slow flowing

River Landscapes

Channel features – The channel is now at its broadest and deepest. Bedload is carried entirely in suspension and in solution. Deposition now dominates, particularly during floods when

the river’s energy is expended in spreading its load over the lower course.

Long Profile – As the river reaches base level, the gradient is very gentle in this section. The volume of water

is now at its greatest and velocity differs little from that registered in the upper reaches.

Characteristic features of the Lower course of a river and its valley

River Landscapes

Valley features – Typically the broad, low-lying valley floor has the following features:

1) Floodplain and natural levées2) Braiding

3) Large Meanders4) Ox-bow lakes5) Estuaries and Deltas6) Terraces

Characteristic features of the Lower course of a river and its valley

Floodplain and Natural Levees Over many years, the river has covered the valley floor

with enormous quantities of alluvium (sedimentary deposits). It was deposited by migrating meanders and floodwater. Across the resulting level floodplain, variations in relief are very slight so that any rapid increase in discharge almost inevitably results in flooding.

When a river floods , the floodwater’s speed reduces most quickly at the sides of the channel. Consequently coarser alluvial sediments are deposited at the channel edge, gradually building up into natural ridges or levees. Occasionally levees act as natural embankments and often they have been strengthened by man as a part of flood prevention measures.

Floodplain and Natural Levees

Channel

River

LevéeLevée

Bluff line

Bluff lineWidth of

floodplain

Layers of silt deposited by earlier floods

Bedload causes bed of river to rise

Finer material carried further

Coarser material deposited first

Braiding For short periods of the year, some rivers carry a very

high load in relation to their velocity. When a river’s velocity falls rapidly, competence and capacity are reduced, and the channel may become choked with material, causing the river to braid i.e. the channel splits up into several smaller channels which flow around fresh ‘islands’ of deposited material before rejoining and further dividing.

Ox-bow Lakes An ox-bow lake is a horse-shoe shaped lake separated

from an adjacent river.

Ox-bow lakes

Ox-bow Lakes1)

2)

Erosion on concave banks – gradually narrowing the neck of land

River eventually breaks through neck of land – usually at times of flood

Deposition beginning to seal cut-offCut-off

formed

Ox-bow Lakes

3)

* Note that these three diagrams alone with not gain you the required amount of marks in a question on ox-bow lakes. You must also include the relevant information about meanders.

Ox-bow lake

Deposition completely seals cut-off

River flows in new, straight channel

Estuaries and Deltas When a river enters a loch or the sea, its flow

characteristics are markedly change. Velocity is reduced and load is deposited.

Where the river’s mouth broadens to form an estuary, tidal currents are able to scour out most of the sediments, transferring them to the sea’s transportation system. The sediment that is not removed from the estuary forms extensive sand and mud banks, often colonised by salt-tolerant vegetation and exposed at low tide.

Deltas are essentially the seaward extension of the floodplain and form when tides are weak. They also grow where streams enter freshwater lochs.

Estuaries and Deltas Deltas have been grouped into three basic forms

Arcuate: having a rounded, convex outer margin.

Cuspate: where the material brought down by a river is spread out evenly on either side of its channel.

Bird’s foot / Digitate: where the river has many distributaries bounded by sediment and which extend out to the sea like claws of a bird’s foot.

Estuaries and Deltas

Sea

SeaSea

Sea

Sea Sea

River

River

River

River

Distributaries

Distributaries

Estuary eventually fills up with material

Arcuate Bird’s Foot / Digitate

Cuspate Estuarine

Terraces A river terrace is a remnant of a former floodplain,

which after rejuvenation has been left at a higher level.

Rejuvenation is where a fall in sea level relative to the land or rise in land relative to the sea enables a river to revive its erosional activity.

When a river is rejuvenated, adjustment to the new base level starts at the sea and gradually works its way up the river course and the point of change to the existing profile is known as the knickpoint.

Terraces

Graded profile of a river

source

mouth

Smooth, concave profile, decreasing in angle and gradient towards the mouth

Effect of rejuvenation on the long profile

Original graded profile

Knickpoints

1Original sea level1st fall in sea level2nd fall in sea level and present day sea level

1 1st regraded profile

2

2 Most recent regraded profile

Terraces If a river is rejuvenated, the downcutting (vertical

erosion) of the river produces terraces.

These are paired i.e. the height of the terraces on one side of the river will correspond with the heights of the terraces on the other side

22 11

3 3

Knickpoint

O.S. Rivers Work In the hydrosphere question in the exam it is possible

that you will be given a question relating to the Ordnance Survey map. The most typical type of question is

“Using appropriate grid references describe the physical characteristics of the river ………… and its valley from GR… to GR….”

Notice that the question says “physical characteristics”. This means that you do not include anything altered by man e.g. artificial straightening of the river.

O.S. Rivers Work The following should be taken into account when describing

the river and its valley.

River Valley

Direction of flow Degree of straightness

Width Height and Steepness of valley sides

Tributaries Width and gradient of valley floor

Meandering / Straight Features e.g. glacial, upland course

Features? E.g. waterfalls ox-bow lakes etc.

O.S. Rivers WorkRecognising features on an O.S. map

River is meandering

Ox-bow lake

Confluence of the two rivers

Change in the direction of flow

Width of floodplain (approx. 1km)

Gentle valley sides

O.S. Rivers WorkRecognising features on an O.S. map

Rivers in their Upper courseSteep valley sides

Confluence of rivers

Confluence of rivers

River flowing in a generally straight – little signs of meandering (likely to be flowing quickly)

Waterfalls

V-shaped Valley

Generally, the rivers are flowing from North to south

Decrease in gradient

Small width of the rivers and no floodplain – river takes up the whole width of the valley

O.S. Rivers Work On the following slides, an example of an answer to a

‘describing the river question’ will be shown. The map is divided into a number of sections, however, if you would prefer to follow the whole map, you should be able to borrow the map from the Geography department. The map is the Thirlmere map.

The question is -

“Using appropriate grid references, describe the physical characteristics of the River Derwent and its valley from 235109 (Stockley Bridge) to 263193 (its entrance to Derwent Water).”

O.S. Rivers Work The river flows in a general South to North Direction.

At 235111, just North of Stockley Bridge, three streams meet to form the main River Derwent. This increases the volume of water in the river and the river increases in width. The river is flowing relatively straight in a narrow valley, between steep slopes and falls from 170m to 140m. At 234120 the river enters the floor of a U-shaped valley.

O.S. Rivers Work Here, the river flows along the

West side of the valley in almost a straight line hugging the base of the valley side. A number of tributaries flow down and join the river on the eastern bank in G.S. 2312. At 246136 the river begins to meander and continues to do so until its confluence with the Stonethwaite Beck at 253154. During this stage, the valley varies in width from 0.2km to 1km wide in G.S.’s 2514 and 2515 where both those rivers occupy the valley.

O.S. Rivers Work Unusually, the river valley

then narrows through Borrowdale. Here the river appears to be flowing through a gorge with steep sides. The river has no floodplain at this point. At 253174, the river emerges from the gorge and again meanders across a wide floodplain (1km). The river enters Derwent Water at 263193. Here the river is forming a delta (arcuate), extending the land into the lake. This land is marshy.