Describe what rivers consist of
Upper course (closest to source of river)
Middle course
Lower course (closest to mouth of river)
The path of a river as it flows is called a course
Describe the characteristics of the upper course of a river
Steep gradient, steep sides, mountainous relief
Narrow, shallow channel with less sediment loads
More resistant geology
Describe the characteristics of the middle course of a river
Medium gradient
Lower altitude
Gently, sloping valley sides
Wider, deeper channel
Describe the characteristics of the lower course of a river
Gentle gradient
Very wide, almost flat valley, often made from alluvium deposits
Very wide, deep channel
Describe the erosion present in the river courses
In the upper course, vertical erosion is dominant whereas in the lower course, lateral erosion is dominant
State the river landforms formed in the river courses
Upper course: interlocking spurs, V-shaped valleys, waterfalls
Lower course: flood plains, levees, meanders, oxbow lakes, deltas
How are waterfalls formed?
Rivers flow over an area of hard rock followed by an area of softer rock
The softer rock is eroded faster leaving a step in the river
As the water goes over the step it erodes more of the softer rock and a steep drop is created - the waterfall
The hard rock eventually becomes undercut by erosion, becoming unsupported and collapses
The collapsed rocks are swirled around at the foot of a waterfall where they erode the softer rock creating a plunge pool
Over time, more undercutting causes the waterfall to retreat
How are interlocking spurs formed?
In the upper course, most of the erosion is vertical erosion creating steep sides V-shaped valleys
The rivers aren’t powerful enough to erode laterally so they so they wind around the hillsides that stick out into their paths on either side
The hillsides that interlock as the river winds are interlocking spurs
How are meanders formed?
The flow of water is fastest & strongest on the outside of the bend because the river channel is deeper therefore more erosion (abrasion & hydraulic action) takes place on the outer bank forming river cliffs, increasing sinuosity (ability to bend)
The velocity and force is slower on the inside of the bend as the river channel is shallower so eroded material is deposited forming slip-off slopes , increasing sinuosity
How are ox-bow lakes formed?
Erosion causes the outside bends of a neck to get closer until there’s only a small bit of land left between the bends
The river breaks through this land, usually through a flood, and the river flows along the shortest course
Deposition eventually cuts off the meander forming an ox-bow lake
How are levees formed?
When rivers flood, water overflows the channel so it loses velocity and deposits sediment, first the larger & heavier sediment then smaller, onto the river bank
Over time this sediment builds up into natural embankments forming levees
How are deltas formed?
Rivers are forced to slow down when they meed the sea causing them to deposit the material they’ve been carrying
If the sea doesn’t wash away the material it builds up, due to increased rates of deposition, and the channel gets blocked, this forces the channel to split up into lots of smaller rivers called distributaries
Eventually, the material builds up so much that low-lying areas of land called deltas are formed
How are floodplains formed?
Erosion gradually removes any interlocking spurs, creating a wide & flat area on either side of the river
Deposition occurs during a flood, friction causes the river to lose speed and energy
As the material is deposited over a wide area, the floodplain gradually gets higher
As you go downstream, what happens to discharge, erosion, velocity, gradient & sediment load and why?
Discharge increases - more tributaries are joining the river
Lateral erosion increases - as the gradient decreases the channel width increases faster than the depth
Velocity increases - as the river becomes smoother & larger, frictional drag on water is reduced
Gradient reduces - less resistant sedimentary rocks as opposed to more resistant rock in upper course
Sediment load becomes smaller & smoother - due to abrasion and attrition
Define floodhydrographs, discharge (hows it calculated), lag time & base flow
Flood hydrographs shows how a drainage basin responds to rainfall
Discharge is the amount of water passing through a specific point in the river over a given time
Calculated by - measure width, depth to give cross sectional area and multiply by velocity to give cumecs (cubic metres per second)
Lag time is the time difference between peak discharge and peak rainfall
Base flow is the normal river level
Relating to flood hydrographs what are some flood risk factors?
High discharge - large gap between base flow and peak discharge
Short lag time - suggests water is reaching channel quickly which could cause flash floods
Describe the physical conditions of the River Severn
Antecedent conditions of 140mm rainfall in a few hours (highest in 100 years) making the ground saturated, increased run off
Has excessive rainfall due to Atlantic depressions
High discharge due to many tributaries joining the Severn
Its upper course flows over resistant metamorphic rocks resulting steep gradients (contains V-shaped valley)
Its lower course flows over much more easily eroded clay, sands etc (contains meanders)
Describe the human conditions of the River Severn
In Tewksbury there are lots of bridging points settlements increasing vulnerability
Urban areas and development have resulted in more impermeable surface so increased run-off and reduced lag time
This has been carried out by deforestation, also increasing surface run-off
Explain when the River Severn flooded and its repercussions?
In July 2007, Gloucestershire flooded by River Severn in Tewksbury
0.9m of flood water in the town
50,000 homes without power, 420,000 without running water
£140,000 damage for local council
Why is there increased risk in Tewksbury?
Continuous Atlantic depressions crossing the UK, more frequent and stronger storms, due to climate change
Increased population puts pressure on housing resulting in more building on flood plains
More urban development means more impermeable surfaces and thus more surface run-off
How does geology/soil affect flooding?
Impermeable igneous rocks/ impermeable thin, clay soil = surface run off = shorter lag time = more discharge = flood
Permeable sedimentary rocks/ permeable thick, sandy soil = more infiltration = longer lag time = less discharge = no flood
How does slope and drainage basin affect flooding?
Steep slope = quicker surface run-off = short lag time = more discharge = flood
Gentle slop = more infiltration, slower surface run-off = long lag time = less discharge = no flood
Small drainage basin = rainfall reaches river channel quickly
Large drainage basin = rainfall reaches river channel slowly
How do antecedent conditions affect flooding?
Lots of recent rainfall = saturated soil = more surface run-off = short lag time = more discharge = flood
Previous dry conditions = more infiltration into dry soil = long lag time = less discharge = no flood
How do deforestation & urbanisation affect flooding?
More impermeable surfaces = more run off = short lag time = more discharge = flood
Deforestation = less interception & uptake = more run off = short lag time = more discharge = flood
