A river's cross profile shows you the cross-section of the river so you can see the shape of the river bed. As rivers flow downhill, they form valleys and channels. The shape of these features changes along the course of the river.
Upper course
In the upper course, erosion dominates over deposition. This means that:
The valley is steep-sided and shaped like a V.
The channel that the water flows through is narrow and not very deep.
Middle course
In the middle course, neither erosion nor deposition is particularly dominant. This means that:
The valley has a gentle slope.
The channel is wider than at the upper course. The channel is also deeper than at the upper course.
Lower course
In the lower course, deposition dominates over erosion. This means that:
The valley is wide and flat.
The channel is wide and deep.
Vertical erosion
Vertical erosion makes the river valley deeper.
Vertical erosion dominates in the upper course.
This is because high levels of turbulence make rough and angular particles drag along the river bed.
Lateral erosion
Lateral erosion makes the river valley wider.
Lateral erosion dominates in the lower course and middle course.
Erosion - Hydraulic action
Hydraulic action is where the force of the water hits against the river or sea bed and banks and causes the rock to break off.
Erosion - Solution
Solution is where soluble rocks are dissolved by the water in the river or sea (limestone and chalk in particular). They are carried along with the river or sea.
Erosion - Abrasion
Abrasion is where pieces of rock are picked up by the river or sea water and hit against the bed or the banks. This wears them away.
Erosion - Attrition
Attrition is where pieces of bedload (material carried by a river or seawater) are hit against one another. This causes them to break apart and become smaller and more rounded.
Transportation - Solution
A solution is where rocks that are soluble are dissolved by water (limestone and chalk in particular). They are then transported within the water.
Transportation - Suspension
A suspension is where silt (sand, clay or other material) and sand particles are held in water (but not dissolved). The silt and sand particles are then transported along with the water.
Transportation - Saltation
Saltation is where small sand and gravel particles bounce along the river bed or sea bed.
They will travel in the direction of the flow of water.
Transportation - Traction
Traction is where large rocks or large particles are dragged along the river bed or sea bed by the current.
Deposition is when the sediment that a river is transporting is dropped. This happens in the middle and lower course of the river and in lower energy conditions. The reasons why rivers lose velocity and drop material are:
Reduced volume of water in the channel
Increased amount of sediment in the channel
Water becomes shallower
The river's mouth is reached
When a river flows over hard rock that overlays softer rock, a waterfall can form. Gorges are formed when waterfalls retreat over time.
Waterfalls and Gorges - Stage 1
The less resistant (softer) rock begins to erode because of abrasion and hydraulic action.
A step or notch forms in the softer rock, creating a river rapid.
Waterfalls and Gorges - Stage 2
Over time, the softer rock gets more eroded. This leaves the hard rock sticking out above the soft rock.
The height of the drop increases as the water pounds the less resistant rock at the base. Eventually a waterfall forms.
Waterfalls and Gorges - Stage 3
The less resistant rock starts to be eroded underneath the hard rock. This process is called undercutting.
The overhanging harder rock collapses because there is nothing underneath it to support it.
The hard rock crumbles into the base and the abrasion between the hard rock and soft rock base creates a plunge pool.
Hydraulic action keeps undercutting behind the waterfall.
If the process repeats over a long time, a gorge may form.
Climate
Heavy rainfall causes high runoff because the water arrives so quickly that there isn't enough time for it to infiltrate the soil.
Prolonged rainfall makes the soil become saturated, and so any more rainfall will run off instead of infiltrating.
Erosion happens faster when there is higher river discharge.
More material (or sediment) is transported when a river has more energy.
Geology
Soft rock erodes faster than hard rock. Therefore, a river will have a lower sediment load if it flows over hard rock.
In areas of harder more resistant rocks, valleys are usually steeper.
Waterfalls are created where soft and hard rock meet.
Interlocking spurs are created when soft rock erodes more quickly than hard rock (a bit like bays and headlands on coastlines).
Slope processes
In steeper valleys, more debris is likely to be pulled by gravity down the valley to the river bed.
Mass movement can add a lot to a river's load. Mass movements are more likely during cold weather or during intense periods of precipitation.
Soil creep is when gravity causes soil to fall down a valley or slope. This can add large amounts of fine material to a river's sediment load.
A hydrograph shows rainfall and river discharge over time.
Hydrographs - Physical factors
A steeper valley will funnel more rainfall into a river faster, increasing river discharge.
Impermeable rocks will absorb less water and channel more rain into a river, increasing discharge (infiltration is lower).
Impermeable soils will have higher runoff and channel more water into a river.
Circle-shaped drainage basins will channel more water into a river, reducing the lag time.
If the weather has been cold and the ground is frozen, the runoff will be higher, decreasing the lag time on the hydrograph.
Hydrographs - Human factors
Deforestation means that less water will be intercepted by trees, reducing the lag time from rainfall to peak discharge.
Impermeable building materials are used for housing and buildings in urban areas. This will reduce infiltration and reduce the lag time.
Precipitation and storms
Heavy rainfall causes high runoff because the water arrives so quickly that there isn't enough time for it to infiltrate the soil.
Prolonged rainfall makes the soil become saturated, and so any more rainfall will run off instead of infiltrating.
If storms are more frequent have more rainfall, then the risk of flooding will rise.
Geology
Runoff is higher in areas that are mainly made up of impermeable soil (e.g. clay) or impermeable rock (e.g. granite).
Relief
Rainfall will reach the river channel quicker in a steep-sided valley.
Land use
Buildings and roads are usually made of impermeable materials. More impermeable surfaces mean more runoff and more discharge.
Trees intercept water and store water that they take up from the ground. Removing trees will, therefore, increase flooding risk as more water will be finding its way into the river channel.
How does flooding threaten people?
Floods can wash people away and kill them. Four people were killed by Storm Dennis in 2020, with 1 person being swept away by the River Tawe.
Floodwater can easily get contaminated with sewage, leaving water unsafe to drink.
Possessions can be destroyed or lost. In the Boscastle flood, 150 cars were swept away.
Businesses and homes can be damaged. People think that the Boscastle flood cost the town £50 million in damage and lost income.
How does flooding threaten the environment?
Habitats can be polluted by floodwater. In the Boscastle flood, 1,850 tonnes of flood debris were recovered from the surrounding area.
Deposited silt can damage agricultural land.
Erosion of river banks can significantly change a river's course and nature.
Vegetation can be uprooted and killed by forceful floodwater.
Hard engineering strategies aim to reduce the effects of flooding by building man-made structures that control a river's flow.
Flood walls
Flood walls are artificial barriers that are built on the banks of rivers.
Increased bank height means increased river capacity.
But, they are expensive to build and can be an eyesore.
Embankments
Embankments are walls built either side of the river channel to raise the height of the banks.
Embankments reduce flooding risk by increasing the capacity of the river.
But, embankments are expensive to build and if the banks do burst, then flooding can still happen.
Flood barriers (floodgates)
Floodgates are built on estuaries to prevent flooding caused by extreme high tides or storm surges.
Shutting them if a storm surge is forecast can save a lot of land.
But, they are expensive to build and require frequent maintenance.