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Factors affecting river discharge
· Discharge can be illustrated using hydrographs ­ show annual patterns of flow
(river regime) in response to climate
The storm hydrograph
· Starting and finishing level show the base flow
· As storm water enters the drainage basin, discharge rises ­ rising limb, to
reach peak discharge.
· The receding limb shows fall in discharge back to base level
· Time delay between max rainfall amount and peak discharge is the lag time
Shape of the hydrograph is influenced by:
·Intensity and duration of the storm ­ if both are high, produce a steep rising limb as
infiltration capacity of soil is exceeded
·Antecedent rainfall ­ heavy rain falling on soil which is saturated from previous period
of wet weather will produce a steep rising limb
·Snow ­ if heavy, may not show for a while, then when it melts, increases discharge.
·Porous soil types and/or permeable rock types produce less steep hydrographs because
water is regulated more slowly through natural systems.
·Impermeable rock types eg granite/clay ­ higher drainage densities, water reaches main
channel faster and rapid discharge increase
·Size of drainage basin ­ small drainage basin responds rapidly to storms ­ shorter lag
time
·Shape of drainage basin ­ rainfall reaches river quicker from a round basin than an
elongated basin
·Slope angle ­ steep sided upland ­ water reaches river more quickly opposite ­ steeper
rising limb and shorter lag times
·Temperature ­ high temp ­ increase evapotranspiration, reduces discharge. Cold temp ­
may freeze ground, restrict infiltration, increase overland flow and increase river
discharge
·Vegetation ­ summer- more leaves ­ higher interception, peak discharge lower. Conifers
have less variable effect
·Land use ­ water runs more quickly on impermeable land eg caravan parks or
agricultural land. Lag time reduced and peak discharge increased
·Urbanisation ­ main human impact ­ reduce lag time and increase peak discharge
·Removal of topsoil + compaction of ground with earth moving machinery
·Building of roads - increases impermeable surface area
·Building of drains & sewers ­ transport water to river rapidly ­ reducing lag time
·Straightening of rivers and lining with concrete ­ faster deliver of water
downstream & increases flood risk downstream…read more

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Channel Processes
·The work of a river involves three main processes: erosion, transportation and deposition
·At any one time the dominant process operating within the river depends on the amount of energy
available
·The is determined by the velocity of the flow and the amount of water flowing within the channel
(discharge)
Erosion
·Rivers erode because they possess energy, their total energy depends on:
·The weight of the water ­ the greater the mass of the water the more energy it will posses
due to the influence of gravity on it movement
·The height of the river above its base level ­ this gives it a source of potential energy, and
the higher the source of the river the more such energy it has
·The steepness of the channel ­ this controls the speed if the river which determines how
much kinetic energy it has
·Most of the energy is lost through friction by turbulence with the flow of the river or
externally with contact with the bed and banks of the river channel
·The rough shape of the channel that the wetted perimeter is large. More energy lost
through friction, and river's velocity, its energy level, is reduced
·In normal condition the river may be unable to perform much erosion. But when the river
contains large quantities of water from heavy rain or snowmelt, it possesses the energy to
perform great amounts of erosion
Four main types of erosion
·Abrasion (corrosion) ­ the scraping, scouring and rubbing action of materials carried along by a
river. It is the most effective in short turbulent periods when the river is at bankfull or in flood.
·Hydraulic action ­ caused by the sheer power of moving water. It is the movement of loose
unconsolidated material due to the frictional drag of the moving water on sediment lying on a
channel bed.
· It is effective at removing loose material in the banks of meanders, can lead to undercutting an
collapse
·Corrosion ­ is most active on rocks that contain carbonates like limestone and chalk. The minerals
in the rock are dissolved by weak acids in the river water and carried away in solution
·Attrition ­ reduction in the size of fragments and particles within a river due to the processes
described above. The fragments strike one another as well as the river bed, becoming smoother,
smaller and well rounded.
Vertical erosion
·Where the land lies high above sea level, the river erosion is predominantly vertical
·Vertical erosion dominates because the river is attempting to cut down to its base level, usually sea
level
·When the river level and velocity are high, the river cuts down into it valley mainly by abrasion and
hydraulic action.
·Such river often produce steep sided valleys…read more

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Lateral erosion
·It occurs more frequently in the middle and lower stretches of the river, where the valley floor lies
closer to sea level
·Here the river possesses a great deal of energy, particularly when close to bankfull
·But the energy is used laterally to widen the valley as the river meanders
·The strongest current is found on the outside of the bend and hydraulic action causes the bank to
be undermined and to collapse
Transport
·River energy not used for erosion or not lost through friction can be used to transport the river's
loads. A river obtains its load from two main sources
·Material that has been washed, or had fallen, into the river from the valley sides
·Material that had been eroded by the river itself from the bed or banks
·A river transports its load in four main ways:
·Traction ­ large stones and boulders are rolled along the river bed by water moving downstream.
This process operates only at times of high discharge (high energy levels)
·Saltation ­ small stones bounce or leap frog along the channel bed. This process is associated with
relatively high energy conditions. Small particles may be thrust up from the bed of the river only to
fall back to the bottom again further downstream. As these particles land they dislodge other
particle upwards, causing more bouncing movements to take place
·Suspension ­ very small particles of sand and silt are carried along by the flow of the river. The
material is not just carried but picked up, mainly through the turbulence that exists within the
water. It contributes the largest proportion of sediment to the load of the river, the suspended load
is the main cause of the brown appearance of many rivers and streams
·Solution ­ dissolved mineral are transported within the mass of the moving water
·Capacity ­ measure of the amount of material it can carry ­ total volume of the load
·Competence ­ the diameter of the largest particle that it can carry for a given velocity
·Faster flowing rivers have greater turbulence and are therefore better able to lift particle from the
river bed
Deposition
·A river deposits when, owing to a decrease in its level of energy, it is no longer competent to
transport its load
·Deposition usually occurs when:
·There is a reduction in the gradient of the river, e.g. when it enters a lake
·The discharge is reduced, e.g. during and after a dry spell of weather
·There is shallow water, e.g. on the inside of a meander
·There is an increase in the calibre of the load. May be due to a tributary bringing in larger particles,
increased in erosion along the river's course, or a landslide into the river
·The river floods and overtops its banks resulting in a reduced velocity on the floodplain outside the
main channel
·The largest fragments are the first to be deposited, followed by successively smaller particles,
although the finest particles may never be deposited
·Pattern is reflected in the sediments found along the course of a river
·The channels of upland rivers are often filled with large boulders
·Gravels, sands and silts can be carried further and are often deposited further downstream
·Sand and silts are deposited on the flat floodplains either side of the river in its lower course…read more

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How are erosion, transport and deposition related to changes in
discharge?
·The Hjulstrom graph shows the relationship between velocity of a river and the size of particles that can
be eroded, transported or deposited
·Velocity increase as discharge rises and generally this enables a river to pick up larger particles from the
bed or banks of the channel
·Similarly, as velocity and discharge reduce, then particles are generally deposited according to their size,
larges first
·Hjulstrom's research showed three interesting relationships:
·Sand particles are moved by lower pick up or critical erosion velocities than smaller silts and clays
or larger gravels. The small clay and silt particles are difficult to pick up because they tend to stick
together. They lie on the river bed and offer less resistance to water flow than larger particles. Much
more powerful flows of water are required to lift them into the water
·Once entrained (picked up), particles can be carried at lower velocities than those required to pick
them up. However, for larger particles there is only a small difference between the critical erosion
velocity and the settling velocity. These particles will be deposited soon after they have been
entrained.
·The smallest particles ­ clays and silts ­ are only deposited at very low velocities. Clay particles may
never be deposited on the river bed and can be carried almost indefinitely. This is why deposits
occur in river estuaries. Where the fresh water meets the salt water of the sea, it causes chemical
settling of the clays and silts to occur and creates extensive areas of mudflats. This is called
flocculation which is when there is a coagulation of clay and silt particles causing them to sink more
rapidly.
River load
·The type, source and character of the load of the river depend upon the nature of the drainage basin, it
location and on human activity
·The Hjulstrom graph categorises the type of river load by size
·Dissolved load consists of soluble materials carried as chemical ions, so there are no measurable particles
·Large particles only form part of the load of the river during and immediately after extreme events that
lead to significant increases in stream discharge
·Temporal changes occur following prolonged heavy rainfall, especially if the ground is also saturated, after
flash floods or after significant snowmelt
·In these circumstances, the competence of the river increases and allows larger particle to be carried
·Boulders and cobbles form part of the load of the upper course because rivers don't have the capacity to
transport these particles great distances
·The further downstream the river travels, the smaller the particles making up the load. Partly due to
attrition.
·Total sediment yields tend to increase with distance downstream, due mainly to increases in both average
discharge and velocity in the lower reaches of the river
·The river possess a greater capacity, so it's able to transport more material
·Spatial variations in load can be seem when comparing rivers in different parts of the world
·The Mississippi river has a massive drainage basin, which is rough one third of USA. Every year, it
transports 136 million tonnes of load in solution, 340 million tonnes in suspension and 40 million tonnes by
saltation. In areas like Australia, there is much small sediment yields…read more

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Spatial variations in load are influenced by the following factors:
·Size of the drainage basin ­ large drainage basins with many tributaries have a
greater potential for transporting sediment than smaller drainage basins
·Rock type ­ where the rock has relatively soft sandstones and clays, the sediment
transported consists mainly of sand and clay particles. Where the rock is limestone,
more material will be transported as dissolved load because limestone is soluble. The
total sediment load in river basins with igneous rock will be low compared to basins
with soft rock
·Relief ­ low relief, there is a small difference in the altitude between the source and
base level and the energy available for erosion and transport is limited. Rivers with
low relief have low loads compared with rivers with high relief
·Precipitation ­ low loads are found in drainage basins with low rates of
precipitation, less water is available as runoff. Seasonal difference in sediment yield
occur in some drainage basins, especially those who have climates with a wet and dry
season and have snowmelt in the spring, adding to normal runoff
·Human activity
oLarge amounts of deforestation, has marked increases in the load carried by
rivers. Caused by increased soil erosion, vegetation that protected the soil
from moving water on its surface has been removed. Also reduced water
uptake by trees and other plants. Soil is washed into river, adding to
suspended load.
oFarmers use nitrates and phosphates as fertilisers, enter the rivers by
throughflow and overland flow and are then transported in solution.
oMajor dams have been made on some river, e.g. the Hoover dam on the
Colorado River. These dams trap sediment, significantly lowering downstream
sediment yields
The effects of channel load on landforms
·A fast-flowing river, at bankfull, has the competence to carry a large load
·The particle erode the river bed and banks by abrasion, creating potholes, waterfalls and
gorges
·If the volume of water in the river falls quickly, the load is deposited because of the fall in
competence
·When this occurs, depositional features such as levees, floodplains and deltas are created
·In some sections of the river, both erosion and deposition occur
·This noticeable on a meander bend, where suspended load carried by the river erodes the
outside edge of the bend by abrasion and load is deposited on the inside of the bend to
form a point bar…read more

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Comments

Alaia E

Thanks Sarah

Really amazing notes, covers everything :)

FMS

Awesome, in depth notes, much obliged!

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