# Rivers revision

Drainage Basin
Area of land drained by a main river and its tributaries.
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Watershed
The boundary of high land surrounding the drainage basin.
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Why is the drainage basin an OPEN system?
It is an open system due to the various inputs and outputs and the ways in which it can store and transfer water.
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Groundwater Storage
Rock layers below the surface are saturated, storing water in cracks and pores.
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Interception Storage
Precipitation held above the ground on the surfaces of plants and trees.
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Throughfall
Water dripping from the surfaces of plants and trees to the ground.
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Stemflow
Water running down stems and branches of plants and trees to reach the ground.
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Infiltration
Downward movement of water into the soil
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Percolation
The downward movement of water through rock due to gravity, which occurs below the soil level.
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Run-off
Water that flows out of the drainage basin in the river.
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Precipitation
Water, ice, sleet and hail falling from the sky into the drainage basin.
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Evapotranspiration
The total amount of moisture removed from a vegetated land surface by the combined action of evaporation and transpiration.
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Run-off
All water that enters a river and eventually flows out of the drainage basin.
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Water balance/budget equation
P (precipitation) = Q (streamflow) + E (evapotranspiration) +/- S (changes in storage)
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What does the water balance equation do?
Measures the balance between inputs and outputs.
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What is river discharge?
The water flowing in a river channel, defined as the volume of water that passes a measuring point in a river over a given time period (usually 1 second) Measured in Cumecs (cubic meters per second)
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What does a storm hydrograph show?
Changes in discharge with time.
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Base Flow
Level of flow in the channel without rainstorm effects.
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Rising Limb
Rise in discharge from the peak back to the base level.
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Peak Discharge
Highest level of flow in the channel.
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Receding Limb
Fall in discharge from the peak back to the base level
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Lag Time
Rise in discharge from base level to peak discharge
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Errosion
The break up of rocks by the action of rock particles being moved over the Earth's surface by wind, water and ice.
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Transport
The movement of solid particles from the site of erosion to the site of depostion
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Deposition
The laying down of solid particles as sediment, within a river bed or on the sea floor.
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Factors that determine a river's total energy
1) The weight of the water in the channel 2) The steepness of the channel 3) The height of the river above base level 4) The level of friction in the channel.
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The four main erosional processes
Abrasion, attrition, hydraulic action, corrosion.
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What are the two main sources of sediment that makes up a river's load?
Clay and Silt
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Solution
Minerals are dissolved in the water and carried along in solution.
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Suspension
Fine, light material is carried along in the water.
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Saltation
Small pebbles and stones are bounced along the river bed.
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Traction
Large boulders and rocks are rolled along the river bed.
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Capacity
The maximum load a river can transport
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Competence
The maximum particle size that it is able to transport.
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What is a Hjulstrom graph?
It shows the relationship between river velocity, particle size/competence and the processes of erosion, transportation and deposition.
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Why are small silt/clay particles more difficult to transport than sand?
Cohesion occurs which is when small clay and silt particles stick together so larger velocities are required to transport them.
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Why are large particles deposited soon after being swept up in a flow?
There is a small difference between the critical erosion velocity and the settling velocity for large particles.
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Why do load particles get smaller downstream? (part 1)
River's velocity is usually fastest in the upper course and slower in the middle and lower courses. This drop in velocity leads to a drop in competence as the river moves down the valley. This leads to progressive deposition of the largest particles.
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Why do load particles get smaller downstream? (part 2)
Attrition during load transport acts to break large particles in the flow into small pieces.
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The Long Profile
A river's long profile illustrates the changes in the altitude of the course of the river from its source, along the entire length of its channel, to the river mouth.
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The Cross Profile
A river's cross profile is the view of the valley from one side to another. For example, the valley cross profile of a river in an upland area typically has a V-shape, with steep sides and a narrow bottom.
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The Upper Course characteristics
Narrow steep sided valley where the river occupies all of the valley floor. Result of dominant vertical erosion by the river.
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The Middle Course characterisitcs
Wider valley with distinct valley bluffs, and a flat floodplain. Result of lateral erosion which widens the valley floor.
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The Lower Course characteristics
Very wide, flat floodplain where the valley sides are difficult to locate. Lack of erosion and reduced competence of the river which results in large scale deposition.
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A long profile that displays an even and progressive decrease in gradient moving from source downstream.
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What does the graded profile represent?
Smooth concave shape represents dynamic equilibrium where a balance has been achieved between the process of erosion and deposition.
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What determines the potential energy of a river at a given point?
The potential or stored energy is determined by the altitude of the stream relative to sea level.
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What determines the kinetic energy of a river at a given point?
Due to the movement of water as it flows. The amount of kinetic energy is determined by the volume of flowing water, the gradient down which it is flowing and its velocity.
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Wetted Perimeter
Total length of river bed and bank that is in contact with the water along the channel cross profile.
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How does the length of the wetted perimeter impact upon the velocity of a river flow?
Large wetted perimeters lead to more friction between the water and the river bed and banks. This leads to a slowing of the river velocity as the river expends energy through inefficient flow. Water must flow in a non-linear path around obstructions.
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How is the Hydraulic Radius calculated?
Hydraulic Radius = cross sectional area of the channel / wetted perimeter.
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How is hydraulic radius related to the efficiency of the river?
A high hydraulic radius represents an efficient river which loses proportionally less energy to friction than an inefficient river of low hydraulic radius
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How do potholes form?
Pebbles carried in high turbulence flow become trapped in small holes. Vertical eddies in the flow cause the pebbles to grind holes into the rock by abrasion.
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What conditions to potholes form from?
Turbulent, high velocity water that is loaded with pebbles. Often found in the upper to middle courses.
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What change in gradient in the LP leads to the formation of rapids and waterfalls?
Found where there is an increase in rivers gradient. This increase is normally more sudden in the cases of waterfalls.
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What is the usual cause of such a gradient change?
The river channel encountering bands of erosion-resistant rock.
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How does a waterfall retreat upstream, forming a gorge?
River falls into a deep plunge pool and the surrounding soft rock is eroded more rapidly than the overlying erosion-resistant rock, deepening and widening the pool. Causes the pool to undercut the overlying rock until it collapses.
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What are meanders?
Bends in a sinuous river channel.
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How are meanders formed?
Alternating bars of sediment (riffles) form on river channel in low flow. Water flows around bars finding most efficient path. Centripetal force directs river towards one bank resulting in high velocity flow, erosion + undercutting.
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Braided River
Rivers which are split into several smaller channels that diverge and converge repeatedly as they flow around small islands.
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Characteristics of sediment and discharge associated with braided river formations.
Form when a river has fluctuating discharges and sufficient supply of sediment to result in periodic occurrence of high sediment loads.
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Oxbow Lakes (part 1)
As water flows around the meander in a river, the flow is fastest on the bend, leading to erosion. At the same time, deposition occurs on inside of meander.
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Oxbow Lakes (part 2)
This leads to an increasingly sinuous river in which the gap between the successive arms of the river (neck of the meander) get narrower.
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Oxbow Lakes (part 3)
During a period of high discharge, the river cuts through the neck of the meander. The new channel is now a straighter shorter route downstream.
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Oxbow Lakes (part 4)
Eventually, deposition cuts the old meander off from the channel and the isolated meander becomes an oxbow lake.
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Levee
Levees are small raised banks of sediment found along the sides of a channel.
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How does flooding lead to levee development?
Rivers typically flood during high discharge. As the river over tops its banks, the velocity of the flow drops, resulting in deposition. As the coarsest material will be deposited first, this results in development of levees.
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Floodplain
A relatively flat area of land on either side of a river that is subject to flooding.
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How does a floodplain form?
When a river over tops its banks during a flood event resulting in a loss of velocity and increased wetted perimeter. These combine to cause deposition of flat layers of alluvium.
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Delta
An area of deposited sediment located where a river enters a slow moving body of water (eg. the sea or a lake)
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Why do deltas form?
When a river enters a lake or sea, the velocity of the river decreases, causing deposition.
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Arcuate Delta
Curving convex shoreline with a dendritic (branched/tree-like) drainage pattern.
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Cuspate Delta
Pointed like a cup or tooth, with sediment deposited equally on either side of the channel.
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Birds Foot Delta
Fingers of deposition stretch out into the sea along channels (Mississippi River)
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Rejuvenation
Describes a rivers renewed capacity to erode as its potential energy is increased.
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What causes rejuvenation?
When the potential energy of a river is increased due to an increase in the height of land relative to the level of the sea. This can be due to a fall in sea level (eustatic change) or rise in level of land (isostatic change).
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Kick points (rejuvenation landform #1)
Located where pre rejuvenation long profile reaches new long profile. Sudden breaks in long profile. Level of land rises relative to sea level, river gains potential energy to erode. Causes river to cut deeper into valley. Begins near sea. Changes LP
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Waterfalls (rejuvenation landfrom #2)
Caused when water flows over a suddenly steepened gradient. Such a change in gradient is often associated with knick points and created by rejuvenation also.
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River terraces (rejuvenation landfrom #3)
Narrow areas of flat land raised above the current level of floodplain. Remnants of former floodplains that have been left at higher levels as the river cuts downwards during rejuvenation.
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Incised meanders (rejuvenation landform #4)
They are meanders that have steep cliffs like banks. Form when a meandering river cuts deeply into the floodplain during rejuvenation.
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Flood recurrence interval/flood frequency calculation
Recurrence interval (years) = number of years on record +1 / ranking of flood being considered (in terms of flood size)
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How are recurrence intervals calculated?
River discharge records, collected for a large number of years are ranked from highest to lowest.
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What is a natural hazard?
Any natural event that poses a risk to human life or property.
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How does flooding result from an intense rainfall over a short period of time?
When rainfall is intense, the grounds infiltration capacity may exceed. Rainfall cannot penetrate soil quickly enough, leading to increased flow of water into rivers.
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How does flooding occur when less-intense rainfall occurs over a prolonged period of time?
Prolonged precipitation leads to soil saturation, causing the water table to rise. If the water table rises to the ground surface, increased overland flow occurs, leading to increased flow of water into rivers.
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Characteristics of drainage basins that impact on its likelihood to flood
Topography size, soil type, level of vegetation + geology.
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Other possible physical triggers for floods
Melting snow, storm surges from climatic hazards like cyclones, hurricanes + tsunamis.
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Climate change leading to increased flood frequency #1
Increasing temperatures lead to melting of the polar ice caps
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Climate change leading to increased flood frequency #2
Sea temperatures may lead to increasing occurrence and size of tropical stormes.
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Climate change leading to increased flood frequency #3
Rainfall patterns will change - reduced rainfall in some regions, but increased in others.
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Climate change leading to increased flood frequency #4
There will be an increased frequency of El Nino events.
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How urbanisation exacerbates the effects of flooding (part 1)
Increases the number of people living on flood susceptible floodplains.
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How urbanisation exacerbates the effects of flooding (part 2)
Use of tarmac + concrete make roads + pavements impermeable so precipitation cannot infiltrate slowly into the soils, meaning more rainfall makes its way into rivers.
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How urbanisation exacerbates the effects of flooding (part 3)
Channeling water directs into drains and sewers means that precipitation reaches rivers quickly, reducing lag time between peak rainfall + peak discharge.
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How urbanisation exacerbates the effects of flooding (part 4)
Bridges can constrict river channels, slowing down discharge and reducing the rivers carrying capacity.
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Deforestation and flood risks
When trees are removed, soil erosion occurs and increases. This sediment finds its way into rivers + increases flood risks. Trees also take up water from the soil in forested areas, leading to increased amounts of water reaching river channels.
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What is river management?
This is when engineering projects are undertaken within a river basin in an effort to prevent flooding + ensure that an adequate water supply is maintained.
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Explain how river management might increase flood risk
Engineering projects designed to prevent flooding in one region can have accidental knock on effects, adding to flood risks typically downstream. These include embankments, releasing water from behind dams and channelised rivers in urban areas.
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Graph used to study the water balance
Water budget graph
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Things included in a water budget graph
Evapotranspiration, precipitation, soil moisture recharge, soil moisture utilisation, soil moisture deficit, soil moisture surplus.
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## Other cards in this set

### Card 2

#### Front

The boundary of high land surrounding the drainage basin.

Watershed

### Card 3

#### Front

It is an open system due to the various inputs and outputs and the ways in which it can store and transfer water.

### Card 4

#### Front

Rock layers below the surface are saturated, storing water in cracks and pores.

### Card 5

#### Front

Precipitation held above the ground on the surfaces of plants and trees.