4.6 Landscapes and processes Flashcards
What is meant by the long profile of a river?
The long profile of a river shows changes in the height (altitude) of the course of a river from its source to its mouth. A long profile is usually concave and the slope becomes more gentle towards the mouth of the river.
How does the river Eden’s landscape change along its long profile
(upper/middle/lower). Consider: channel shape, valley profile, gradient,
The source of a river is often - but not always - in an upland area. Near the source, a river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls and rapids. Highland areas are usually composed of hard igneous rocks, which are ideal for forming such features.
As a river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.
As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it approaches the sea.
The discharge (amount of water flowing) will increase as the river approaches the sea.
What are the reasons for these changes?
In the upper course the river erodes vertically, however, as the river becomes wider and flatter, erosion starts to occur horizontally.
Compare hydraulic action with abrasion
Hydraulic action. Air may become trapped in joints and cracks on a cliff face. When a wave breaks, the trapped air is compressed which weakens the cliff and causes erosion.
Abrasion. Bits of rock and sand in waves grind down cliff surfaces like sandpaper.
Define weathering.
Weather (verb) - wear away or change the appearance or texture of (something) by long exposure to the atmosphere.
“his skin was weathered almost black by his long outdoor life”
synonyms: weather-beaten, eroded, worn, disintegrating, crumbling; More
bleached, discoloured, tanned, bronzed;
lined, creased, wrinkled, wizened, shrivelled, gnarled
“a weathered stone urn”
(of rock or other material) be worn away or altered by long exposure to the atmosphere.
“the ice sheet preserves specimens that would weather away more quickly in other regions”
Compare freeze thaw and biological weathering
Freeze-thaw
Water expands slightly when it freezes into ice. This is why water pipes sometimes burst in the winter. You might have seen a demonstration of this sort of thing at school - a jar filled to the brim with water eventually shatters after it is put into a freezer.
The formation of ice can also break rocks. If water gets into a crack in a rock and then freezes, it expands and pushes the crack further apart. When the ice melts later, water can get further into the crack. When the rock freezes again, it expands and makes the crack even bigger.
This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.
Biological weathering
Animals and plants can wear away rocks. This is called biological weathering. For example, burrowing animals such as rabbits can burrow into a crack in a rock, making it bigger and splitting the rock
Compare traction with suspension transport
Traction - large boulders and rocks are rolled along the river bed.
Suspension - fine light material is carried along in the water.
Explain the stages and processes responsible for waterfall formation, v
shaped valleys and interlocking spurs, meanders, ox-bow lakes, flood
plains, levees and deltas. Do each of these in turn – this is a big
question!
Waterfalls often form in the upper stages of a river where it flows over different bands of rock. It erodes soft rock more quickly than hard rock and this may lead to the creation of a waterfall. Formation of a waterfall: The soft rock erodes more quickly, undercutting the hard rock.
A valley formed by flowing water, or river valley, is usually V-shaped. The exact shape will depend on the characteristics of the stream flowing through it. Rivers with steep gradients, as in mountain ranges, produce steep walls and a bottom. Shallower slopes may produce broader and gentler valleys.
As the river erodes the landscape in the upper course, it winds and bends to avoid areas of hard rock. This creates interlocking spurs, which look a bit like the interlocking parts of a zip. When a river runs over alternating layers of hard and soft rock, rapids and waterfalls may form.
In the middle course the river has more energy and a high volume of water. … As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders. The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
After a long period of time, the meander becomes very curved, and eventually the neck of the meander becomes narrower and the river cuts through the neck during a flood, cutting off the meander and forming an oxbow lake.
Floodplains are made by a meander eroding sideways as it travels downstream. When a river breaks its banks, it leaves behind layers of alluvium (silt). These gradually build up to create the floor of the plain.
The formation of levees and floodplains are linked and involve repeated flooding and the build-up of material during the period of flood. Under normal low conditions, the river is contained within its banks and so no sediment is available to form levees of the floodplain.
When a river reaches a lake or the sea the water slows down and loses the power to carry sediment . The sediment is dropped at the mouth of the river. Some rivers drop so much sediment that waves and tides can’t carry it all away. It builds up in layers forming a delta.
This is indeed a big question!
Explain two ways climate, geology and slope processes change the river
landscape
- Permeable rock can mean that water can travel through the rock, forming dry valleys. Impermeable rock will flow on the surface.
- Slope processes can, over time, form V shaped valleys around the river in the upper course.
Label the key features (lag time, rising limb and peak discharge) of a
flood hydrograph
Peak rainfall - Where the storm had the highest rate of rainfall in mm.
- Lag time - Time from peak rainfall to peak discharge.
- Peak discharge - The point with the highest amount of river discharge in Cubic metres or Cumecs.
Compare a flashy and subdued hydrograph
Different river catchments produce different shapes of hydrograph. A ‘flashy’ hydrograph has a short lag time, high peak discharge, and steep rising and falling limbs.In a flashy hydrograph, the river is more likely to flood.
A Subdued Hydrograph has a much longer lag time and a smaller peak discharge, the features of the hydrograph are Periods of steady rain, area with low drainage basin, a rural area, and gentle sloping valleys.
Explain how physical factors (geology, soil, slope, basin type,
antecedent conditions) can affect flood hydrographs.
- impermeable rocks (e.g. granite) and soil (e.g. clay) will not allow water to pass through, resulting in large amounts of surface runoff and a greater flood risk as rivers respond quickly - results in a short lag time.
- permeable rocks and soil have a high infiltration capacity and will absorb water quickly, reducing overland flow - results in a longer lag time
- a drainage basin with a steep gradient will result in greater overland flow and a shorter lag time than where the gradient is less steep allowing more time for infiltration to occur.
- heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff (short lag time)
- slow light rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via throughflow and groundwater flow (longer lag time)
- impermeable man made surfaces such as concrete and tarmac are impermeable therefore rivers in urban drainage basins tend to have short lag times due to higher amounts of surface runoff and drainage systems taking water to rivers quickly.
- vegetated areas help to reduce flood risk by increasing the time it takes for water to reach a river (longer lag time) by encouraging infiltration (roots opening up the soil), intercepting water by their leaves and taking up water in their roots.
- areas cleared by deforestation will respond quickly to rainfall due to the reduced interception
- Large Drainage Basin - water will take longer to reach the river (long lag time)
- Small Drainage Basin - water will enter the river quicker (short lag time)
- If the soil has already been saturated by heavy rain its infiltration capacity will be reduced and further rain will go as surface runoff
- If the soil is dry it will be able to absorb more water during infiltration and therefore the lag time will be longer
- if the ground surface is frozen lag time is short as water cannot infiltrates and passes quickly to the river as runoff
- the presence of a dam will allow flow to be controlled, reducing flood risk and allowing rivers to gradually respond to heavy rainfall in a controlled way;
