Rivers Flashcards
Tributaries
Smaller rivers which join the main river [increase its discharge/waterflow]
Confluence
Point at which rivers meet
Drainage basin
Land drained by a river system.
Watershed
Boundary of the drainage basin, usually made up of hihgland.
Discharge
Amount of water passing a specific point at a given time.
Measured in cubic metres per second.
Depends on the rivers velocity and volume
Volume - River
The amount of water in the river
Velocity - River
Speed of river
River channel
The area in which a river flows.
Long profile of a river
A graph drawn along the course of a river from the source to the mouth.
Split into three sections…
1) Upper course
2) Middle course
3) Lower course
The valley cross-profile of a river
The view of the valley from one side to another.
The channel cross-section of a river
The view of the river bed and banks from one side to another at any point on its course.
Page 5 + 6 of rivers revision booklet
Traction
Rolling stones along the bed
Saltation
Small particles bounce along bed in a leapfrog motion
Suspension
Silt and clay-sized are carried within the water flow
Solution
Minerals dissolve in the water
Attrition
Large particles such as boulders collide and break into smaller pieces (occurs at higher
part of river)
Hydraulic action
The sheer force of the river dislodges particles from its banks and bed
Abrasion
Smaller particles rub against the river banks and bed like sand-paper; occurs at low part
of river (smaller particles)
Solution
Acids in river dissolve rocks (occurs at any part of river)
How does mass movement impact on river landscapes?
Mass movement causes river banks to be washed into the river.
Soil creep creates a rippled effect/ causes soil to slowly slide into the river.
Slumping due to saturation of banks leads to the rotational movement of river banks into the channel which can block the river leading to flooding.
Impacts of mass movement in rivers…
Loss of land, damage to property, damage to river defences, river blockage and flooding, loss of animal habitat
Deposition
When a river lacks the energy to carry its load, it begins with the heaviest particles, happens when there is less water or where the current slows down.
Large boulders = deposited at top of river + small particles at the end –> sorting
Interlocking spurs
In the upper valley a river is in the mountains.
Water takes the easiest path downhill so twists & turns around the high land (spurs) forming interlocking spurs.
How are V-shaped valleys + interlocking spurs are formed?
1) The river erodes vertically into the bed through corrosion + hydraulic action.
2) The exposed channel sides are attached by freeze-thaw weathering.
3) Mass movement (eg. slumping) and overland flow causes the loosened material to move into the river channel.
4) The river transport’s its new load downstrem through traction and saltation.
5) In places, bands of hard rock force the river to ‘wiggle’ these outcrops are known as spurs and usually form an interlocking pattern.
Waterfalls
- They occur because the river flows over hard rock which erodes slowly.
- Beneath is softer rock which is eroded faster to form a “step”.
- The force of the water erodes the bottom of the waterfall to form a plunge pool.
- The hard rock gets undercut as the soft rock erodes so that it eventually collapses.
Interception
When water is taken in by plants
(Part of the water cycle)
Infiltration
When water sinks into the soil
(Part of the water cycle)
Percolation
When water sinks from the soil into the groundwater store
(Part of the water cycle)
Overland flow
When water flows ove rthe land (eg. in a river)
(Part of the water cycle)
Throughflow
When water flows through the soil
(Part of the water cycle)
Groundwater flow
When water flows through the groundwater layer (water bearing rocks)
(Part of the water cycle)
Transpiration / Evopotranspiration
When water is evporated from plants
(Part of the water cycle)
Evaporation
When water is heated + turns from a loquid to a gas
(Part of the water cycle)
Condensation
When water turns from a gas to a liquid
(Part of the water cycle)
Precipitation
Rain, snow, sleet or hail
(Part of the water cycle)
Stages of the water cycle
1) Water starts off in the sea
2) Then evaporates
3) Water vapour condenses + forms clouds
4) Falls in precipitation
5) Precipitation goes back to sea (either by surface runoff or grounwater flow)
Drainage basin
The area drained by a river and its tributaries.
(Any groundwater flow within the area will travel to the river)
Watershed
Makes the edge of a drainage basin
(It is the highest point of land)
Source
The starting point of a river
Mouth
Point at whcih a river enters the sea or a lake
Tributary
A small stream which will join the main channel
Confluence
The point at which a smaller river (tributary) joins the main river.
Changes in a river (from source to mouth)
(From source to mouth)
Discharge increases
Occupied channel width increases
Channel depth increases
Average velocity increases
Load quantity increases
Load particle size decreases
Channel bed roughness decreases
Slope angle (gradient) decreases
Why does the river change from source to mouth?
Discharge increases - (bc more water drains into river.
Channel width increases - (bc sides are eroded by large discharge of water)
Channel depth increases - (Vertical erosion occurs by discharge)
Velocity increase - (Less friction + more discharge)
Load quantity increases - (Seidment around river increases)
Load particle size decreases - (Attrition occurs.)
Channel bed roughness decreases - (Erosion bc of more discharge + velocity)
Slope angle/ gradient decreases - (Because the river isk trying to get down to sea level –> eventually reaches flat land + vertical erosion is fast)
Characteristics of upper course
- Mountainous area
- Small channel width
- Small channel depth
- Small velocity
- Small load quantity
- Large size of load particles
- Large friction against channel bed
- Large gradient of river
(More of a stream than a river)
Main process = Vertical erosion
Characteristics of middle course
- Semi-mountainous area
- Medium channel width
- Medium channel depth
- Medium velocity
- Medium load quantity
- Medium size of load particles
- Medium friction against channel bed
- Medium gradient of river
(More of a river than a stream - Still fairly thin)
Main process = Lateral erosion
Characteristics of lower course
- High volume
- Large discharge
- High speed (because less friction)
- River channel = deep + wide
- Flat relief
- Energy levels are low so deposition takes place
Main process = Deposition
Wetted perimeter
The part of the cross section (bed and bank) that is in contact with the water.
(Bottom + sides of river)
Increases going down the river bc the river widens –> More water in contact with land.
Hydraulic radius
Ratio of the river channel’s cross sectional area to its wetted perimeter.
–> How easy it is for water to move having to overcome friction. (How efficient it is)
Increases going down the river.
More water + Lower wetted perimeter –> Less friction because the river channel is wider + it is smoother.
Less water + Greater wetted perimeter –> More friction because the river channel is thinner + it is more bumpy.
How is a V-shaped valley formed
1) The river erodes vertically into the bed through corrosion + hydraulic action.
2) The exposed channel sides are attached by freeze-thaw weathering /physical etc.
3) Mass movement (eg. slumping) and overland flow causes the loosened material to move into the river channel.
4) The river transport’s its new load downstrem through traction and saltation.
5) In places, bands of hard rock force the river to ‘wiggle’ these outcrops are known as spurs and usually form an interlocking pattern.
How are interlocking spurs formed?
Occur in v-shaped valleys.
- The river cuts down into the valley.
- If there are areas of hard rock which are harder to erode, the river will bend around it.
- This creates interlocking spurs.
How are potholes formed?
- Formed in the upper course of the river.
- Water gets ‘caught’ in depressions/ dents in the river floor.
- Water swirls in these dents eroding them furhter.
- Eventually smaller rocks + sediment gets caught in the depression. The currents cause it to move around –> abrasion.
- Vertical erosion occurs –> pothole deepens.
How are waterfalls formed?
- Found in upper course of a river.
- Occur when there is a band of hard rock (eg. granite) over a band of soft rock (eg. sandstone).
- Softer rock is eroded more quickly than hard rock.
- At the base of the fall, there is a plunge pool formed by the falling water. (rocks + debris –> swept into a plunge pool, cuasing abrasion.)
- After soft rock (bottom layer has eroded away), the hard rock creates an overhang (bc it erodes slower)
- Overhang eventually collapses
- This causes the waterfall to retreat even furhter.
- Process repeats until the waterfall has retreated upstream.
Eventually the waterfall will have created a gorge. (a v-shaped valley)
Upper course of the river - Features
Potholes, Waterfalls (+ gorge) , V-shaped valley
Middle course of the river - Features
Meanders, Oxbow lake
How meanders form
- Middle course = flatter land
- Lateral erosion dominates as river swings in large bends known as meanders.
- Water is pushed to the outer bend.
- This reduces friction with the bed and banks.
- Water in a meander does not all travel at the same speed. (Fastest + most powerful water travels around the outside of the bend)
- On the outer bend –> river has more energy for transporting material which can erode the outside bank via hydraulic action and abrasion.
- Inside of the bend = water is shallower –> has less energy.
–> Leads to deposition (The river no longer has enough energy to carry its load)
Meander
A bend in a river. They are found mainly in lowland areas and are extremely common in the middle course of a river.
Floodplain
The area around a river that is covered in times of flood.
During flooding, the water deposits silt and sediment (alluvium) on the floodplain (due to the friction between the land and the water). This creates fertile land.
Bigger sediment gets deposited closer to the river as it is heavier. This creates an embankment or levees which raise the riverbank.
Erosion on a flood plain
As the river meanders, they create lateral erosion.
When they reach the edge of the floodplain they erode the valley side (bluff line).
Eventually this cuts a wider valley. This explains why flood plains are very wide.
When the river floods it deposits silt, creating a very flat flood plain.
Layer upon layer builds up over many years to form a thick deposit of fertile alluvium.
Levees
Raised riverbanks (ridges of sediment) found on the sides of a river channel that is prone to flooding.
They are found in a river’s lower course.
They are formed by flooding over many years and are made up of gravel, stones and alluvium (silt).
(Bigger sediment is carried a shorter distance than smaller sediment)
Eventually sediment is deposited at the bottom o the river channel, raising it –> causes levees to not become too high —> Flooding will always occur
Causes of river flooding
- Heavy rainfall
- Flash flood (storms over dry areas)
- Glacier melt
- Building dams + Dams breaking
- The addition of concrete (urbanisation)
- Removal of vegetation / deforestation (less interception + abosroption of water + roots give stability to soil stopping it being washed away.)
- Some rock types are more porous/permeable (water can sink through ground easily) eg. Clay = impermeable
- Steep sided channel (fast surface runoff)
Hydrograph
Shows how a river responds to a period of rainfall
Peak discharge
Maximum amount of water held in the channel. (m³/s)
Shown by a line graph on a hydrograph.
Peak rainfall
Maximum amount of rainfall (mm)
Shown as a bar graph on a hydrograph.
Lag time
The time taken between peak rainfall and peak discharge.
Rising limb
Shows the increase in discharge on a hydrograph
Falling limb
Shows the return of discharge to normal/base flow on a hydrograph.
Base flow
The normal discharge of the river.
What affects the lag time on a hydrograph…
- Geology/Soil type (impermeable –> faster drainage)
- Slope (Steep drainage basin –> faster drainage)
- Size of drainage basin (Smaller drainage basin –> Faster drainage)
- Drainage basin shape (Circular drainage basin –> Faster drainage)
- Drainage density (More tributaries –> Faster drainage)
- Antecedent conditions (Dry soil –> Faster drainage)
- Vegetation (Less vegetation –> Faster drainage)
- Urbanisation (More urbanisation –> Faster drainage)
Hard engineering
Building artificial structures which try to control rivers. They tend to be more expensive.
Dams + Reservoirs - Advantages + Disadvantages
Dam traps water forming a reservoir (controlled release of water).
Advantages
- Hydroelectricity
- Tourist attraction
Disadvantages
- Expensive
- Traps sediment (decreased volume of water)
- Rotting vegetation (releasing methane gas)
- Settlements displaced during flooding when building it.
Embankments - Advantages + Disadvantages
Raising banks of river to hold more water.
Advantages
- Cheap + one-off cost
- Allow for flooding to be contained in water
Disadvantages
- Looks unnatural
- Water speeds up + can risk flooding downstream
River straightening + dredging - Advantages + Disadvantages
River straightening –> Water passes through area quicker.
River dredging –> Channel deepends
Advantages
- More water held in channel
- Can reduce flood risk in built up area (where other solutions can’t be used)
Disadvantages
- Dredging needs to be done frequently
- Speeding up river –> increases flooding downstream
Flood relief channels - Advantages + Disadvantages
Floodwater flows into relief channel + taken to area to be reabsorbed / enter river again
Advantages
- Removes excess water to reduce flooding
- Can be controlled with flood gates
Disadvantages
- Expensive to build
- Under intense flooding channels may also flood.
Soft engineering
Does not involve building artificial structures but takes a more sustainable and natural approach to managing the potential for river flooding.
Afforestation - Advantages + Disadvantages
Planting trees in the river valley to intercept rain water and reduce surface run-off.
Advantages
- Economic benefits (i.e. forestry)
- Provides new ecological habitats.
Disadvantages
- Reduces farmland
- Impossible to use if a settlement has already been built
Floodplain zoning - Advantages + Disadvantages
Allows only certain land uses on land near rivers to reduce the risk of damage during flooding.
Advantages
- Limits damages
- Less insurance claims
- Lower need for protection
Disadvantages
- Planners have to decide which flood to plan for
- Not always possibel to change existing land uses.
Flood warnings + preparations - Advantages + Disadvantages
Gov. monitors rivers + issues warnings via radio, TV, newspapers, internet so people can prepare/ evacuate.
Advantages
- People have time to try to protect their properties, e.g. with sandbags.
- Many possessions can be saved, resulting in fewer insurance claims.
Disadvantages
- Some people may not be able to access the warnings.
- Flash floods may happen too quickly to warn.
- Don’t actually prevent flooding, just minimise risk for people.
Benefits of a river
- Flood storage + erosion control on flood plains
- Wetlands have slow water flow, allowing sediment to settle at the bottom of the river –> Cleans water.
- Groundwater recharge
- Provides fertile soils (due to flooding)
- Habitat for wildlife
- Recreational oppurtunities
- Open space
Mississippi river - Case study (Basic information)
- 4th longest river in the world
- Drainage basin covers ⅓ of the USA.
- Flows the entire length of the USA, from Minnesota to the Gulf of Mexico in Louisiana.
- Has over 100 tributaries
- The Mississippi is subject to periodic flooding.
- 1927 river flood = most destructive flood event in US history.
Flood management on the mississippi
Hard engineering
- 6 huge dams and 105 reservoir
- Strengthening the levees with concrete mattresses (prevents erosion)
- Straightening river to get the water passed towns more quickly to the sea
- Diversionary spillways – overflow channels
Soft engineering
- Afforestation to delay runoff
- Less construction on the floodplain e.g. St Louis.
Benefits of living close to the mississippi river
Settlements - Lower course = flat land, easy to build on.
Tourism - Recreational hobbies like canoeing, swimming, boating, fishing etc are popular.
Industry - Water from river = used for cooling in factories and power stations.
Transportation - Cargo boats. 2004 was around 31.5 million short tons consisting of 72% imported cargo and 28% exported cargo.
Commercial fishing (+ shrimping in the delta) - Economic + food supply.
Farming - The floodplain in particular is rich in nutrients and the soils are used for agriculture.
Causes of a river flood in the mississippi
- 1991 eruption of Mount Pinatubo.
- Excess cloud condensation nuclei from the eruption = responsible for the Great Flood of 1993 in the Midwestern United States.
- Flood was caused by large amounts of precipitation over a long period of time
- June and July of 1993 experienced over 150% of normal rainfall.
- Ground was already saturated.
- Development on the floodplain, draining of wetlands and urbaisation –> caused flooding.
- The levees were also poorly built.
Effects of the 1993 Mississippi flooding
- 43 deaths
- 50,000 people evacuated
- 26,000km of land flooded
- $2.46 billion crop losses (Flood submerged crop land)
- $12 billion in damages
- River traffic stopped for several months
- Contents of and the buildings themselves destroyed
- Threat of disease from sewage + threat of water borne diseases
- Insurance claims high
- Stagnant water attracted mosquitoes and rats
- Flooded industrial sites created chemical spills + contamination
Oxbow lake
Over time –> horseshoe becomes tighter, until the ends become very close together.
As the river breaks through, e.g. during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut-off from the main channel.
The cut-off loop is called an oxbow lake.
Deltas
Deltas are found at the mouth of large rivers (eg. the Mississippi)
A delta is formed when the river deposits its material faster than the sea can remove it.
There are 3 main types of delta, named after the shape they create
3 types of deltas
Arcuate or fan shaped
Cuspate
Bird’s foot
Arcuate or fan-shaped (delta)
The land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect.
Land deposited out into sea in a semi-circle
Cuspate (delta)
The land around the mouth of the rive juts out arrowlike into the sea. Stronger waves form a cuspate delta
(This is due to sediment being deposited by the river, faster than erosion of the sea)
Bird’s foot (delta)
The river splits on the way to the sea, each part of the river juts out into the sea, rather like a birds foots.
Features optimising delta formation
- Large amounts of sediment (more to deposit)
- Large river
- Shallow water –> Sediment won’t wash away
- Low wave energy
Formation of a delta
- The river carrying sediment approaches the sea.
- The flow of the river slows down as it meet the calmer ocean currents.
- This causes heavy sediment to be deposited on the ocean floor
- This results in the river channel dividing into two (as sediment is deposited in the middle)
- This occurs until land has been deposited out into the sea.
Formation of the mississippi delta - Case study
Conditions creating the formation…
- Large river transporting a lot of sediment.
- Slowly flowing when entering the gulf of mexico
- Sediment = deposited faster than when the tides remove it.
- Riversplits up into distributaries (due to deposition)
–> Distributaries deposit sediment over a wide aea –> more land entering sea. - Bird’s foot delta (bc of widely spaced distributaries
Advantages and disadvantages of living near the mississippi delta…
Advantages
- Fertile soils deposited –> good for agriculture.
(Cotton, soybeans, rice, and corn) - Diverse ecosystems –> wetlands, marshes, and swamps (habitats for many plants/animal species)
- Agriculture, fishing, shipping, and tourism present in large cities near the delta.
Disadvantages
- Delta floods during hurricanes/heavy rainfall
- Extraction of groundwater + oil/gas –> causes land to sink –> inc risk of flooding.
- Delta is experiencing coastal erosion (natural / caused by buildingof levees + canals)
- Pollution, habitat destruction + drainage of wetlands –> Environmental degradation in delta.
General causes of flooding on the mississippi river
Many settlements have been built along the banks of the Mississippi e.g. St Louis + New Orleans.
–> Reduces permeability in the river basin + increases the flood risk.
Tropical storm systems –> lead to flooding in the lower Mississippi valley eg. Hurricane Katrina in 2005.
Areas affected by the 1993 mississippi flooding
- St. Louis, Missouri (major flooding –> evacuation + infrastructure damage)
- Quincy, Illinois (severe flooding –> evacuations + damage to homes and businesses)
Des Moines, Iowa: Experienced flooding along the Des Moines River and other tributaries, resulting in evacuations and damage to homes and businesses.
