2.2b+-+Rivers

__From the syllabus__
Candidates should be able to:


 * Demonstrate an understanding of the work of a river in eroding, transporting and depositing.
 * Reference should be made to the erosional processes of hydraulic action, corrasion, corrosion (solution) and attrition.
 * River transport should include the processes of traction, saltation, suspension and solution. Reasons why and where in a river’s course deposition takes place should be studied.
 * It should be realized that the effectiveness of the river processes concerned will vary according to the volume and velocity of the running water and the nature of the load (boulders, pebbles, sand and silt) which, in turn, will be affected by the bedrock along the course of the river.
 * Describe and explain the landforms associated with these processes. A study should be made of the following forms of river valleys: long profile and shape in cross section, rapids, waterfalls, potholes, meanders, oxbow lakes, deltas, levées and flood plains.
 * Demonstrate an understanding that the natural environment presents hazards and offers opportunities for human activities. Reference should be made to flooding. (NB: You should also be aware of the benefits of locating settlements near rivers, in terms of suitability for agriculture, availability of transport, etc.)

**Inputs**

 * Precipitation – rain, hail, sleet or snow

**Stores**

 * Groundwater – water stored in porous rocks deep below the ground
 * Interception – when water droplets collect on trees and plants
 * Soil Moisture – water stored in the soil and broken rocks near the surface
 * Surface Storage – when water lies on the ground as puddles or lakes

**Transfers**

 * Infiltration – when surface water soaks down into the soil
 * Groundwater flow – the very slow movement of water deep below the ground
 * Percolation – Water moving downwards through the soil into the rocks below
 * Surface Runoff – the movement of water over the surface as a stream or river
 * Throughflow – the movement of water laterally downhill through soils

**Outputs**

 * Evaporation – when water changes to water vapour when the sun heats it
 * Transpiration – water vapour released by trees and plants

**Drainage Basins and their features**

 * Confluence – the point at which a tributary joins the main river channel.
 * Drainage Basin – the area of land drained by a river and its tributaries
 * Mouth – where a river enters the sea.
 * Source – the start of a river
 * Tributary – a small river feeding a large river channel
 * Watershed – the boundary between two drainage basins (a ridge of high land)

River Processes (erosion, transportation & deposition)

 * Erosion**
 * Abrasion (Corrasion) – erosion caused by the rubbing and scouring effect of material carried by the rivers.
 * Attrition – process of river erosion by which particles are reduced in size as they hit against each other
 * Corrosion (solution) – process of river erosion by which chemical action wears away rocks.
 * Hydraulic Action – process of river erosion in which the sheer force of water breaks off rock


 * Transport**
 * Saltation – process of river transport by which small particles bounce along the bed.
 * Solution – process of river transport by which material is carried dissolved in the water
 * Suspension – process of river transport by which material is carried along within the water
 * Traction – process of river transport by which material is rolled along the bed


 * Deposition – when a river loses energy and drops some or all of the material it is carrying.

**River Landforms**

 * Floodplain – an area flooded when a river bursts its bank
 * Gorge – a steep sided valley
 * Meander – a bend in the river
 * Ox-bow lake – a crescent shaped lake formed when a river cuts through the neck of a meander.
 * Plunge Pool – a hollow at the base of a waterfall caused by erosion (hydraulic action)
 * Rapids – area of fast moving water, usually travelling over a gentle slope
 * V-shaped valley – a narrow steep-sided valley formed by vertical river erosion.
 * Waterfall – a vertical or near vertical drop of water resulting from a sudden change in the river gradient.
 * Levee – a raised embankment of alluvium flanking a river channel, formed due to the deposition of alluvium when a river floods its banks.

**Hydrographs**

 * Hydrograph – a graph showing the changes in the discharge of a river over a period of time.
 * Lag Time – the period of time between peak rainfall and peak river discharge.

**General**

 * Long Profile – the height of the river above sea level plotted against distance from its source.

____Case studies____

 * Bangladesh floods (LEDC example) (textbook p. 288)
 * Lynmouth floods (MEDC example) (textbook p. 286)
 * Three Gorges project (example of human use and management)(textbook p. 294)