Rivers Flashcards
(42 cards)
What are the 4 types of erosion?
Hydraulic action-Water smashes into the river bank and air gets trapped and breaks the rocks apart
Abrasion-Pebbles grind along the river bed and bank
Attrition-River rocks collide in with each and break
Solution-The water dissolving certain chemicals in rocks
What are the 4 types of transportation
Traction - large, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger
Saltation - pebbles are bounced along the river bed, most commonly near the source
Suspension - lighter sediment is suspended (carried) within the water, most commonly near the mouth of the river.
Solution - the transport of dissolved chemicals. This varies along the river depending on the presence of soluble rocks
Define deposition.
When the river loses energy, it drops any of the material it has been carrying.
What are the type of features in a drainage basin?
Watershed - the area of high land forming the edge of a river basin
Source - where a river begins
Mouth - where a river meets the sea
Confluence - the point at which two rivers meet
Tributary - a small river or stream that joins a larger river
Channel - this is where the river flows
Explain upper course and lower course
Upper course - in the upper course, where the river starts, there is often an upland area. The river’s load is large in the upper course, as it hasn’t been broken down by erosion yet. The river erodes vertically.
Lower course - in the lower course, the land is a lot flatter. The river’s load is fine sediment, as erosion has broken down the rocks. The river is much more deeper and the river erodes laterally.
What is a waterfall?
A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned over exposed, less resistant rock (soft rock).
Explain the process of making a waterfall.
The soft rock is eroded quicker than the hard rock and this creates a step.
As erosion continues, the hard rock is undercut forming an overhang.
Abrasion and hydraulic action erode to create a plunge pool.
Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
This process continues and the waterfall retreats upstream.
A steep-sided valley is left where the waterfall once was. This is called a gorge.
What is an interlocking spur?
In the upper course there is more vertical erosion. 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 of land which link together like the teeth of a zip.
What is a meander?
As the river makes its way to the middle course, it gains more water and therefore more energy. Lateral erosion starts to widen the river. When the river flows over flatter land they develop large bends called meanders.
Explain the features of a meander.
As a river goes around a bend, most of the water is pushed towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion).
The lateral erosion on the outside bend causes undercutting of the bank to form a river cliff.
Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope of sand and shingle.
The build-up of deposited sediment is known as a slip-off slope (or sometimes river beach).
How is an oxbow lake formed?
Due to erosion on the outside of a bend and deposition on the inside, the shape of a meander will change over a period of time. Erosion narrows the neck of the land within the meander and as the process continues, the meanders move closer together. When there is a very high discharge (usually during a flood), the river cuts across the neck, taking a new, straighter and shorter route. Deposition will occur to cut off the original meander, leaving a horseshoe-shaped oxbow lake.
What is a floodplain?
A floodplain is an area of land which is covered in water when a river bursts its banks
What are floodplains used for?
Floodplains are often agricultural land, as the area is very fertile because it’s made up of alluvium (deposited silt from a river flood). The floodplain is often a wide, flat area caused by meanders shifting along the valley.
How are levees made?
Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
Sediment that has been eroded further upstream is transported downstream.
When the river floods, the sediment spreads out across the floodplain.
When a flood occurs, the river loses energy. The largest material is deposited first on the sides of the river banks and smaller material further away.
After many floods, the sediment builds up to increase the height of the river banks, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.
What is an estuary?
An estuary is where the river meets the sea. The river here is tidal and when the sea retreats the volume of the water in the estuary is reduced. When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife.
What are the causes of flooding?
Prolonged rainfall
Heavy rainfall
Relief
Geology
Vegetation
Urban land use
Why can lag time be different?
The lag time can be short or long depending on different factors. For example, if there is no vegetation in an area, the water runs off into the river quicker, therefore it would have a short lag time.
What are the factors affecting lag time?
Factors influencing lag time include:
Size of drainage basin
Vegetation
Valley side steepness
Soil type
What is hard and soft engineering?
Hard engineering involves building artificial structures which try to control rivers. They tend to be more expensive.
Soft engineering does not involve building artificial structures, but takes a more sustainable and natural approach to managing the potential for river flooding.
What is a dam?
The dam traps water, which builds up behind it, forming a reservoir. Water can be released in a controlled way.an be used to produce electricity by passing the water through a turbine within the dam.
How does flood warnings and preparation defend against flooding?
Flood warnings and preparation
The environmental agency monitors rivers and issues warnings via newspapers, TV, radio and the internet when they are likely to flood so people can prepare.
Why does the velocity change downstream?
he speed the water travels at is dependent on which course of the river it is travelling through. Friction occurs between the water and the riverbed which slows the water down. The more contact the water has with the floor, the more friction occurs which means the water travels slower.
What are the causes of flooding?
Physical causes of flooding:
heavy rainfall
long periods of rain
snowmelt
steep slopes
impermeable rock (doesn’t allow water through)
very wet, saturated soils
compacted or dry soil
Human factors increasing flood risk:
urbanisation, because towns and cities have more impermeable surfaces
deforestation, because removing trees reduces the amount of water
intercepted and increases
run-off
What is discharge?
Discharge = velocity x cross-sectional area
