Rivers Flashcards
Drainage Basin
An area of land drained by a river and its tributaries
Source
The start of a river
Tributary
A stream or river that flows into a larger river
Confluence
Where a tributary joins the river
Long profile shape
Upper course - Steep gradient
Middle course - Gentle gradient
Lower course - Very gentle gradient
Cross profile shape
Upper Course - Steep V-Shaped valley, narrow and shallow turbulent river
Middle Course - Valley with wider, flatter bed. Wider and deeper river
Lower Course - Very wide and flat valley. Wide, deep river with large sediment load
Why does the cross profile change over the course of the river?
- More water joins from more tributaries
- So more energy to erode larger channel
- Through processes such as abrasion, hydraulic action, etc
Why does velocity increase downstream?
- Upper course is shallower, so more energy is lost through friction between water and banks
- In the lower courses, water depth is greater so less water is in contact with the banks so river flows faster DESPITE SHALLOWR GRADIENT
Hydraulic Action
Where the force of water breaks rock particles away from the river channel
Abrasion
The eroding away of the river channel by rocks carried with the flow of the river
Attrition
- Stones carried by river hit each other
- Breaking down and rounding eachother
Solution / Corrosion
Mildly acidic river water dissolves limestone and chalk
Traction
Large boulders and rocks are rolled along river bed
Saltation
Small pebbles and stones bounce along river bed
Suspension
- Small particles are carried along by the water
- Such as silt or clay
Solution
Dissolved load
Typical Landforms in each course
Upper course - Interlocking spurs, V-shaped valley, gorges, waterfalls
Middle course - Meanders, OX-bow lakes
Lower course - Levees, flood plains, river estuaries
Interlocking spurs formation
- V-Shaped valley created by erosion vertically downwards
- Rivers aren’t powerful enough to erode laterally
- So wind around the high hillsides in the way
- Hillsides interlock as river winds round them
- Interlocking spurs is formed
Waterfall formation
- River flows over hard rock lying on top of soft rock
- River erodes soft rock faster, developing a vertical drop where hard rock and soft rock meets
- Plunge pool forms from force of falling water (hydraulic action)
- Rock above plunge pool is undercut and left hanging
- Overhanging hard rock eventually collapses due to force of gravity
- Erosion continues to undercut underneath the hard rock
- So the process repeats and the waterfall over time retreats upstream
Gorge formations
- Waterfall retreats upstream
- Leaving narrow, steep sided valleys called GORGES
- Also form at the end of glacial periods from meltwater
Meander
- Wide bend in the river found mainly in lowland areas
- Most efficient course of river in gentle slopes
Meander formation
- water levels travel faster on the outside of the bend
- so more lateral erosion takes place
- widening meanders
- deposition also occurs mainly in the insides of the bend
What are pools
Deep water found on the outside of bends (faster flow, more vertical Erosion)
What are riffles
Riffles are shallow pools of water formed by the deposition of coarse sediment
Ox-bow lake formation
- Meander is created
- Erosion through hydraulic action bends river so that the meanders travel towards each other
- The neck of the meander eventually breaks creating a straight river
- The old meander becomes blocked from the main river by deposition, creating a separate ox-bow lake
Deposition
- Deposition is the dropping of the river’s load
- when the water velocity decreases so does not have enough energy to transport the sediment
- occurs mainly in the lower course of the river
Floodplain
Wide flat area of marshy land on either side of the river
- occurs mainly in middle and lower courses
Levee
An enlarged bank built up on each side of the river by deposition during river flooding
Estuary
The tidal mouth of a large river, where tide meets the stream
- a transitional zone between fluvial and coastal processes
Physical factors affecting flood risk
Precipitation - high amounts of rain can cause rivers to flood
Geology - impermeable rocks force water to flow, decreasing lag time and increasing flood risk because ground cannot absorb water
Relief - steeper mountain environments cause rapid flow of water
Lag time
The time between the peak rainfall and peak discharge
Human factors affecting flood risk
Urbanisation- impermeable surfaces such as roads aid water flow and decrease lag time
Deforestation - less interception of water, lag time increases and aids water flow
Agriculture - increased surface runoff from unused soil
What are hydrographs
Graph plotting river discharge measured in cumecs, against against time after a storm
Hard engineering strategies
- dams / reservoirs regulate water flow (eg. Clywedog reservoir)
- by-pass channels to cut through meanders to reduce flood risk through urbanised areas
- channel straightening lined with concrete for faster flow of water through urbanised areas
- channel enlarged by dredging
Costs / benefits of channel straightening
- can reduce risk for vulnerable areas
- however shifts problem downstream
- creates an unnatural environment, destroying riverside habitats
Flood relief channels
- Building new artificial channels which are used when a river is close to maximum discharge
- take pressure off main channels when floods are likely
- thus reducing flood risk
- eg. Jubilee Rivee
Soft engineering strategies
- afforestation to increase interception of water
- Wetland areas used for water storage
- river bank conservation, eg. Planting trees
- flood warning / preparation
