? Flashcards
Long cross profile of a river
What the cross profile of a river?
Explain the look of a river from beginning to end
In the upper course, the river flows over a steep gradient from the hill/mountains.
This gives the river a lot of energy, so it will erode the riverbed vertically to form
narrow V-shaped valleys and interlocking spurs.
The middle course becomes less steep with a u-shaped valley
, meander and oxbow lakes.
The lower course of the river widens
further and becomes flatter. Material transported is
deposited at the river end of the river- the mouth.
What involves the deepening of the river bed in the upper course
Vertical erosion
What erodes the banks of the river, mainly in the lower and middle course
Lateral erosion
How does a v-shaped valley form ?
Rivers near their source are a long way above sea level. This means they have high gravitational potential energy. So they erode mainly downwards (vertically).
The valley sides are also weathered.
The slopes cant stay vertical because that’s not stable, so the soil and rocks on the slopes slide down.
This makes the V-shape to the valley sides.
What are interlocking spurs?
Outcrops of land along the river course in a valley.
These are alternate hills on each side of the valley, that stick out like the teeth of a zip in the river’s path.
The river in the upper course doesn’t have enough energy to erode the spurs.
So the river has to flow around the spurs of land.
How do waterfalls form ?
- A waterfall forms when a river crosses a band of hard and soft rock.
- The softer rock gets eroded more quickly by abrasion and hydraulic action.
- This forms an overhang.
- The erosion of the softer rock continues and undercuts the hard rock.
- The hollow at the bottom is called the plunge pool, which is full of bits of rock from above, so there is lots of abrasion.
Eventually the overhang collapses. - The process keep going and the waterfall moves backwards (retreats).
- In front of the waterfall, a steep sided gorge of recession is formed.
How do meanders form ?
- In the middle course the river has lateral erosion which widens the river channel.
- This forms large bends, and then horseshoe-like loops called meanders.
- The fastest flow (thalweg) is on the outside of the meander bend. The force of the water erodes and undercuts the river bank, forming a river cliff.
- The main types of erosion are hydraulic action and abrasion.
- On the inside of the bend, where the river flow is slower, material is deposited. A river beach/slip off slope forms here.
How do oxbow lakes form ?
- Meanders become more sinuous (wiggly) due to
lateral erosion. - The meander becomes really tight, known as a swans neck meander.
- During times of high discharge the river erodes across the meander neck through hydraulic action and abrasion.
- The river will now use the new channel.
- The old meander channel is left as an oxbow lake due to sediment being deposited at either end.
- Eventually the water will evaporate leaving a meander scar.
How do floodplains form?
Floodplain Formations:
Either
Meanders migrate across the floodplain due to lateral erosion. When the reach the edge of the floodplain they erode the valley side (bluff). This explains why floodplains are very wide.
OR
When a river floods, it deposits silt, creating a very flat floodplain. Layer upon layer builds up over many years to form a thick deposit of fertile alluvium.
How do leeves form?
- When a river floods, sediment (from upstream) spreads out across the floodplain. Friction with the land causes deposition.
When a flood occurs, the river loses energy. The largest material (sand and gravel) is deposited first on the sides of the river banks and smaller material (finer silt and mud) further away. - After many floods, the sediment builds up to increase the height of the river banks, so the levées become higher than the surrounding floodplain.
What is an estuary and how does it form?
Estuary - Tidal mouth of a river where it meets the sea. Wide banks of deposited mud are exposed at low tide.
Estuary formation:
1. During a rising tide the river is unable to be discharged into the sea.
2. The rivers velocity falls and sediment is deposited.
3. At low tide these fine deposits form extensive mud flats.
4. These develop into saltmarshes and are important habitats for species.
5. They provide protection for boats too.
River tees:
Location and Background
The River Tees is located in the north of England.
The source of the River Tees is located in the Pennines and it flows east 137km to its mouth where the river joins the North Sea.
Upper Course - Vertical erosion has formed a V-Shaped valley. High force Waterfall drops 21m and is the UK’S largest waterfall by volume. An area of harder rock (Whinstone) is located above a layer of softer rock (sandstone and shale). Gradually a gorge has been formed.
Middle Course - Features include meanders and ox-bow lakes due to lateral erosion. These can be found near Barnard Castle in the middle course.
The meander near Yarm encloses the town.
Lower Course- Greater lateral erosion creates features such as floodplains & levees. The River
Physical causes of flooding:
Precipitation - Heavy rain means more water doesn’t have time to infiltrate so there is more surface runoff.
Relief (Steep slopes) - means the water doesn’t have time to infiltrate.
More surface runoff raises the discharge.
Geology - Impermeable rocks /do not allow water to
Flashy hydrograph:
Small basin size, high drainage density, impermeable rocks, urbanised areas, steep slopes, saturated soils and heavy rain all lead to rapid water transfer into the river channel and hence a flashy hydrograph with a short lag time and high peak.
Low, flat hydrograph:
Large basin sizes, low
drainage density, permeable rocks, forested areas, gentle slopes, dry soils and light rain all lead to slower water transfer and hence a low, flat hydrograph with a longer lag time and low peak.
Hard engineering river management
Soft engineering river management
