Rivers Flashcards
(32 cards)
Rivers terminology
Tributaries: smaller rivers which join the main river [ increase its discharge]
Confluence: point at which rivers meet.
Drainage basin: land drained by a river system.
Watershed: boundary of the drainage basin, usually made up of highland.
Discharge: amount of water passing a specific point at a given time and is measured in cubic metres per second [cumecs]. Depends on the river’s velocity and volume.
The volume is the amount of water in the river
The velocity is the speed of the river.
River channel: the area in which a river flows.
The characteristics of a river valley and its channel from source to mouth.
The long profile of a river is a graph drawn along the course of a river from the source to the mouth.
The valley cross-profile is the view of the valley from one side to another.
The channel cross-section is a view of the river bed and banks from one side to another at any point on its course.
Types of erosion at different parts of the river
Upper coarse: Vertical erosion
Middle coarse: Lateral erosion
Lower coarse: deposition
V-shaped valley, interlocking spurs
The valley is narrow with a narrow, shallow river channel, steep sides/Channel has a steep gradient
Water is mainly slow flowing as most of the river’s energy is used to overcome the friction of the riverbed & obstructions
The load is mainly large, angular and rough
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.
The river cuts down into the valley to form a narrow, steep-sided V-shaped valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs.
Waterfalls
They occur because the river flows over hard rock which erodes slowly.
Beneath is a 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.
Meanders and oxbow lakes
As the river erodes laterally, to the right side then the left side, it forms large bends, and then
horseshoe-like loops called meanders.
The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
The force of the water erodes and undercuts the river bank on the outside of the bend where
water flow has the most energy due to decreased friction. This creates a steep-sided riverbank on the outer bend called a river cliff.
On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction. This forms a gently sloping inside bend, known as a slip-off slope.
Over time the 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 occur where a river that carries a large amount of sediment meets a lake or the sea
This meeting causes the river to lose energy and drop the sediment it is carrying.
Deltas form where river mouths become choked with sediment, causing the main river channel to
split into hundreds of smaller channels or distributaries.
Example of a delta – the Mississippi Delta, southern USA
Explain the conditions in this area which have created the formation of the delta:
- The Mississippi is a major river that is capable of transporting a lot of sediment
- Its flow slows as it enters the Gulf of Mexico
- Sediment is deposited faster than the tides can remove it
- River flow is blocked by so much deposition that the river splits up into distributaries
- Distributaries deposit sediment over a wide area, extending new land into the sea
- The Mississippi River delta has widely spaced distributaries, making it look like a bird’s foot.
Example of a delta – the Mississippi Delta, southern USA advantage living there
- The fertile soils deposited by the Mississippi River make the delta an ideal area for agriculture. The region is known for its production of crops such as cotton, soybeans, rice, and corn.
- Biodiversity: The delta supports diverse ecosystems, including wetlands, marshes, and swamps, which provide habitats for a wide variety of plant and animal species.
- Economic opportunities: The Mississippi Delta is home to towns and cities that support various industries, including agriculture, fishing, shipping, and tourism.
Example of a delta – the Mississippi Delta, southern USA disadvantages living there
- Flooding: The delta is prone to flooding, especially during hurricanes and heavy rainfall events. This damages to infrastructure, homes, and crops, leading to economic losses.
- Land subsidence: The extraction of groundwater and oil and gas from beneath the delta has contributed to land subsidence (or sinking). This increases the risk of flooding and can cause damage to infrastructure and property.
- Coastal erosion: The Mississippi Delta is experiencing significant coastal erosion, partly due to natural processes and partly due to human activities such as the construction of levees and canals. Coastal erosion threatens
communities, infrastructure, and ecosystems along the coast, leading to loss of land and habitat. - Environmental degradation: Human activities such as pollution, habitat destruction, and drainage of wetlands have contributed to environmental degradation in the delta, including loss of biodiversity and degradation of water quality.
Levées
When a river floods, the coarsest material is deposited first, on the edges of the river, forming a natural embankment called a levée.
Deposition happens due to an increase in friction when the river comes into contact with the bed meaning that energy is lost.
Flood plain
Area of alluvial deposits found beside the river in its lower course.
As meanders move slowly down the course of the river they erode away the valley to create a wide valley floor, and they deposit layers of alluvial material on the slip-off slopes building up into a large flood plain.
Flooding and flood prevention
- Flooding occurs when a river gets more water than its channel can hold.
- A flood occurs when a river bursts its banks and the water spills onto the floodplain.
- Flooding tends to be caused by heavy rain: the faster the rainwater reaches the river channel, the more likely it is to flood.
- The nature of the landscape around a river will influence how quickly rainwater reaches the channel.
The following factors may encourage flooding
- A steep-sided channel - a river channel surrounded by steep slopes causes fast surface run-off.
- A lack of vegetation or woodland - trees and plants intercept precipitation (i.e. they catch or drink water).
If there is little vegetation in the drainage basin then surface run-off will be high. - A drainage basin, consisting of mainly impermeable rock - this will mean that water
cannot percolate through the rock layer, and so will run faster over the surface. - A drainage basin in an urban area - these consist largely of impermeable concrete, which encourages
overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with
sloping roofs further increase the amount of run-off.
Flood hydrograph
- Peak discharge - maximum amount of water held in the channel.
- Peak rainfall – maximum amount of rainfall (millimetres).
- 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.
The lag time can be short or long depending on different factors:
- Geology discharge - Impermeable rocks mean water cannot drain through the rock layer resulting in rapid overland flow and a shorter lag time.
- Soil type – clay soils do not drain easily and become saturated very quickly. This results in
rapid overland flow and shorter lag times. Dry soils slow down water transfer leading to longer
lag times. - Slope - steep slopes lead to rapid water transfer and shorter lag times. Gentle slopes slow
down water transfer making the lag time longer. - Drainage basin shape - a high-density basin has more streams and rivers which speed up
water transfer and shorten lag time. - Antecedent conditions - wet conditions before a storm cause the ground to become saturated.
This speeds up overland flow and shortens lag time. - Vegetation - if there is plenty of vegetation in the
area, the lag time would be longer as the plants would intercept the rainfall.
Dams and reservoirs
Advantages
Can be used to produce electricity by passing the water through a turbine within the dam.
Reservoirs can attract tourists.
Disadvantages
Very expensive.
Dams trap sediment which means the reservoir can hold less water.
Habitats are flooded often leading to rotting vegetation. This releases methane which is a
greenhouse gas
Settlements are lost leading to the displacement of people. In developing countries locals are not always consulted and have little say in where they are relocated.
River straightening and dredging
Straightening the river speeds up the water so high volumes of water can pass through an area quickly. Dredging makes the river deeper so it can hold more water.
Advantages
More water can be held in the channel.
It can be used to reduce flood risk in built-up areas.
Disadvantages
Dredging needs to be done frequently.
Speeding up the river increases flood risk downstream.
Embankments
Raising the banks of a river means that it can hold more water.
Advantages
Cheap with a one-off cost
Allows for flood water to be contained within the river.
Disadvantages
Looks unnatural.
Water speeds up and can increase flood risk downstream.
Flood relief channels
The floodwater flows into the relief channel and is taken either to an area where it can be absorbed, or re-enters the river further down its course.
Advantages
Removes excess water from the river channel to reduce flooding.
Disadvantages
Expensive to build.
If water levels continue to rise, the relief channel may also flood.
Hard engineering rivers
Dams and reservoirs
Embankments
River straightening and dredging
Flood river channels
Soft engineering rivers
Flood warnings and preparation
Floodplain zoning
Afforestation
Flood warnings and preparation
Advantages
People have time to protect their properties, eg 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 for a warning to be effective.
They do not stop land from flooding - they just warn people that a flood is likely.
Floodplain zoning
Advantages
More expensive buildings and land uses are further away from the river, so have a reduced flood risk.
Less damage is caused, leading to fewer insurance claims.
Disadvantages
Not always possible to change existing land uses.
Planners have to decide what type of flood to plan for.
