1. River Environments Flashcards
What is physical weathering? (aka freeze-thaw, mechanical)
This happens when there are changes in temperature and rainfall freezes and thaws in rock cracks. This breaks rocks down into smaller and smaller pieces.
What is chemical weathering?
This is when acidic rain seeps into porous rocks, causing them to decay and disintegrate.
What is biological weathering?
When the roots of plants grow into the cracks in the rocks, causing the rock to gradually split apart.
What is slumping ?
When the bottom of a slope is cut away from the river, making the slope unstable, and the weathered material slumps down towards the river. Heavy rain makes it worse as it makes the weathered material heavier and acts as a lubricant.
What is soil creep?
When weathered material moves slowly downhill due to gravity. It collects at the bottom of the valley side and then gets eroded by the river.
What is hydraulic action?
This is the power of the water as it smashes against the river banks. Air becomes trapped in the cracks of the river bank and bed, and causes the rock to break apart.
What is abrasion?
When rocks carried along by the river flow grind along the river bank and bed in a sand-papering effect.
What is corrosion (solution)?
When the water dissolves rock particles, eg limestone, into the river.
What is attrition?
When rocks being carried by the river collide and break apart into smaller, smoother and rounder particles.
What is deposition?
When the river loses energy and drops any of the material it has been carrying. Factors include shallow water, the river mouth and when the volume of water decreases.
What is traction?
When large boulders and rocks are rolled along the river bed.
What is saltation?
When pebbles are bounced along the river bed, most commonly near the source.
What is suspension?
When fine sediment is suspended (carried) within the water, most commonly near the mouth of the river.
Examples of stores in the hydrological cycle:
Ocean
Clouds
Water table
Snow and ice
Examples of transfers in the hydrological cycle:
Evaporation Condensation Precipitation Percolation Groundwater flow Surface run-off Evapotranspiration Throughflow
What is a drainage basin?
The area of a land drained by a river and its tributaries
Inputs in the hydrological cycle:
How water is introduced into the drainage basin system
Precipitation from other drainage basins
Energy from the Sun
Outputs in the hydrological cycle:
How the water is released either back to the sea or back into the atmosphere
River discharge
Precipitation into other drainage basins
Evaporation
What is the source?
The beginning of a river
What is the watershed?
The edge of highland surrounding a drainage basin, marking the boundary between two drainage basins
What is the mouth?
The point where the river comes to an end, usually entering a sea
Factors affecting river regimes:
Weather conditions
Rock and soil type
Land use
Relief
How do weather conditions affect river regimes?
Rainfall can cause the ground to become saturated.
Further rain will then flow as surface run-off.
Sudden storms can cause flash flooding
Continuous rain, or melting snow, increases river discharge, decreasing lag time
How does rock and soil type affect river regimes?
Permeable rocks and soils (eg sandy soils) absorb water easily, very little surface run-off, less discharge
Impermeable rocks and soils (eg clay soils) can’t absorb much water, so lots of surface run-off, increasing discharge
How does land use affect river regimes?
Urban areas have impermeable surfaces (eg roads), increasing surface run-off and discharge
In rural areas, ploughing up and down hillsides creates channels, allowing water to run into rivers, increasing discharge
Deforestation means less interception, so ground becomes saturated more quickly, increasing surface run-off
How does relief affect river regimes?
Steep slopes increase surface run off as infiltration is less likely
On more gentle slopes, infiltration is more likely
What is drainage density?
The total length of streams is a drainage basin divided by the area of the basin
How does permeability affect drainage density?
Basins with impermeable rocks/soil tend to have higher drainage density due to lack of infiltration.
What is a channel network?
The system of surface and underground channels that collects and transports the precipitation falling on the drainage basin.
What is infiltration?
When water soaks or filters into soil/
What is surface run-off?
When water moves across the surface of the earth and becomes a stream, tributary or river.
What is evapotranspiration?
When water vapour evaporates from the trunk and leaves of trees/other vegetation, back into the atmosphere
What is throughflow?
When water moves downhill through soil.
What is percolation?
When water moves from the soil to pores in the rock.
What is groundwater flow?
When water moves slowly through the soil and pores in the rock towards the sea.
How do dams affect river regimes?
Prevent water from flowing all the way down
What are the 4 river transport processes:
Solution
Suspension
Traction
Saltation
What are the 4 forms of river erosion:
Hydraulic action
Abrasion
Solution
Attrition
What are the 3 types of weathering:
Physical
Chemical
Biological
What is deposition?
When a river loses energy and drops and material it has been carrying.
Factors leading to deposition:
Shallow water
At the end of the river’s journey, at the river’s mouth
When the volume of the water decreases
What is a waterfall?
A sudden drop along the river course
Usually in the upper course
How are waterfalls formed?
- The soft rock is eroded quicker than the hard rock and this creates a step.
- As erosion continues, the hard rock is undercut forming an overhang.
- Abrasion and hydraulic action erode to create a plunge pool.
- Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
- This process continues and the waterfall retreats upstream.
- A steep-sided valley is left where the waterfall once was. This is called a gorge.
What are interlocking spurs and how are they formed?
In the upper course there is more vertical erosion. The river cuts down into the valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs of land which link together like the teeth of a zip.
What are meanders?
When the river develops bends.
Found in the middle course
How are meanders formed?
- As a river goes around a bend, most of the water is pushed towards the outside. 2. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion).
- The lateral erosion on the outside bend causes undercutting of the bank to form a river cliff.
- Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope.
- The build-up of deposited sediment is known as a slip-off slope (or sometimes river beach).
How are ox-bow lakes formed?
- Erosion makes the neck of a meander shorter.
- When there is very high discharge (usually due to a flood), the river takes the shortest course through the neck
- New, straighter river course is formed, leaving an abandoned meander (the oxbow lake)
What is a floodplain?
An area of low-lying land next to a river which is very prone to flooding.
Usually found in the lower course
How are floodplains formed?
Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river
Over time, the height of the floodplain increases as material is deposited
Often agricultural land, the area is fertile as it’s rich in alluvium
What are levées?
Banks formed by deposits of alluvium
Found in the lower course.
How are levees formed?
During a flood, water flows over the banks and deposits silts
Forms new levees
In between floods, slow moving river deposits silt in riverbed, forming new river level
With each flood, the levees are built up
Features of a river in the upper course:
Steep-sided v-shaped valleys
Narrow river channel
Steep gradient
Larger sediment size
River landforms in the upper course:
Steep-sided v-shaped valleys Deep channels Interlocking spurs Rapids Waterfalls Gorges
Features of a river in the middle course:
Wider, shallower channels
Shallower gradient
Smaller sediment size
River landforms in the middle course:
Wider, shallower valleys
Meanders
Oxbow lake
Features of a river in the lower course:
Wide, flat bottomed valleys
Shallow/no gradient
Very small sediment
River landforms in the lower course:
Wide, flat bottomed valleys
Floodplains
Deltas
