2.2.2 River Processes

Drainage Basins

When studying the river processes we focus on individual drainage basins.  A drainage basin represents the total area drained by a river and its tributaries.

River basin diagram

  • Source: start of the river
  • Tributary: smaller river that flows into a larger one
  • Confluence: the meeting point of two rivers (where a tributary joins the main channel)
  • Mouth: the point at which the river the river flows into the sea.  This could be a delta or an estuary.
  • Watershed: the outer limit of the drainage basin –  any rain falling outside this line will flow into a different river system.

The Hydrological Cycle

To understand the processes happening in river systems you should have an understanding of the hydrological cycle.  Precipitation is an input for drainage basins, after which the vegetation, soil and the relief all affect what happens to the water.

  • Precipitation: water in falling to the earth´s surface (rain, hail, sleet, snow).
  • Interception: precipitation that does not reach the ground, it gets caught on leaves etc and then evaporates.
  • Surface runoff: water flowing over the surface of the land (streams, rivers etc).
  • Infiltration: water soaking into the soil layer.
  • Throughflow: infiltrated water that moves through the soil layer towards the river/sea.
  • Groundwater: water stored in the rocks below the soil layers, aquifers exists here.
  • Groundwater flow: water slowly making its way towards the sea through the rock layers

 

syllabus_link_button

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.

Erosion

Erosion is the wearing away of material (rocks, stones & soil). There are 4 ways in which rivers erode the land that they flow through:

  • erosionHydraulic action: the physical force of the water loosens material in the river banks and bed.
  • Attrition: when rocks and stones knock against each other in the river small bits get knocked or chipped off.
  • Abrasion/corrasion: as material moves along the river it scrapes against the banks and river bed loosening other material.
  • Corrosion: rainwater is often slightly acidic (carbonic acid) resulting in some rocks/minerals being dissolved by the river water.

syllabus_link_buttonRiver transport should include the processes of traction, saltation, suspension and solution.

Transportation

Transportation is the movement downstream of eroded material.  There are 4 ways in which eroded matertransportationial is moved through rivers:

  • Traction: water pushes large stones/rocks causing them to roll along the riverbed.
  • Saltation: smaller, lighter material may be temporarily picked up and moved by the water in a bouncing motion.
  • Suspension: fine particles of soil and sand may be carried long in the currents of the water.
  • Solution: dissolved material is carried along in the water, not visible.

 

 

syllabus_link_button Reasons why and where in a river’s course deposition takes place should be studied.

 

Deposition

When rivers experienbraidingce a decrease in their energy/force they can no longer move the same amount of material.  The result is that they drop it or stop moving it.

Floodplains, braided channels and deltas are examples of the features formed through deposition.

Deposition generally occurs in the lower courses of rivers where the gradient is less and the main channel widens.

 

Itsyllabus_link_button should be realised 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.

Variations Along the River

The velocity of the water in a river channel determines the rates of erosion, transportation and deposition.

The Upper Course

The upper course of a river system usually has the steepest gradient and topsoil is quickly eroded.  The small size of rivers in this section and the rocky nature of the river beds means that the water does not get much chance to build up speed.  In periods of heavy rain, many of the rocks will be moved through traction and saltation resulting in abrasion and attrition.  Most erosion in a river occurs in the upper course often resulting in v-shaped valleys.

The Lower Course

The lower course of a river system is characterised by much much flatter land.  The rivers tributaries are likely to have joined by this point and the river has a large velocity and flows quickly.  Erosion will occur on the outer sections of river bends but deposition is the main process in this section.  Larger rocks and stones are unlikely to have been transported this far and the river bed is mainly silt.  The land it flows through is generally floodplain and consequently flat and fertile.

 

A studsyllabus_link_buttony 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.

River Cross Section

River bends in the lower course have

distinct characteristics.  Erosion on the outside of the bend creates a steep cliff.

Deposition on the inside of the bend creates gentle sloping beach like areas.

Meanders

oxbow1

As rivers flow through floodplain areas the faster flowing water erodes the outside of any bends.  The slower moving water deposits material on the inside.  The result is that the bends get larger and larger known as meanders.

Oxbow Lakes

These form when a meander becomes so wide that it joins up with another meander, when this happens the water takes the shortest route and cuts of the meander.  An oxbow lake remains in the old meander section.

 

Waterfalls

  • waterfallOccur when rivers flow over layers of soft & hard rock.
  • The soft rock is eroded more easily, this leads to the hard rock being undercut.
  • A ledge of hard rock forms until it breaks and collapses into the plunge pool. Each time this happens the waterfall moves further upstream.
  • As the waterfall retreats upstream it often creates a gorge.

Waterfalls occur in the upper and middle courses of rivers.

 

Floodplains

Floodplains exist in the lower course of rivers.  They are areas of flat land created by repeated occurrences of floods over time.  These areas are naturally prone to flooding. When the river floods the water slows down as it covers the surrounding land (due to increased friction as the water is in contact with a larger area of land), this result in significant deposition of material which slowly builds up fertile, flat floodplains.

Floodplains have traditionally been used for agriculture due to the rich alluvial soils that have built up.  Increasing populations and a desire for more scenic surroundings for homes has led to increasing development of floodplains for residential and industrial use.  Floods in these areas now cause significant economic and social damage.

Levees

Levees form naturally in the lower course due to repeated flooding.  When the water initially overtops the river banks and slows down it deposits the largest and heaviest material first.  This process repeats and gradually builds up a natural embankment (levee) alongside the river.

Deltas

Deltas generally exist at the mouth of a river.  They are characterised by the main channel of the river splitting into many smaller channels (distributaries) that fan out over a wider area.  The main cause of deltas is deposition which blocks the channels and causes the water to create new channels, often splitting into two new channels.  The River Nile has an extensive delta region that is used for agricultural purposes.  Since the building of the Aswan Dam, the silt has been trapped behind the dam and the delta region is shrinking by over 100m per year.

Flooding in Bangladesh –  1998

Human Causes

  • Deforestation in the mountain regions has reduced interception rates and increased surface run-off.  The loss of trees reduces the absorption of water through the roots leading to soils becoming saturated more quickly.  Trees are being cut down for their wood (building material and as a fuel source for cooling and heating) but also to clear land for farming.
  • Increasing urbanisation has created more impermeable surfaces which reduce infiltration and increase surface run-off.
  • The growth of informal settlements on marginal land (floodplains) has increased the impact of the floods – more people live in high-risk locations.
  • Climate change leading to increased snow melt from the Himalayas.

Physical Causes

  • Three major rivers flow through Bangladesh, the Ganges, the Brahmaputra and the Meghna.
  • The south of the country is very low lying and flat- one big floodplain.  The land has been created through repeated flooding and deposition.
  • 70% of Bangladesh is less than 1 metre above sea level
  • Snow melt in the Himalayas during Spring and Summer increases river discharge
  • Bangladesh experiences the monsoon season (tropical rains) every year from June to September
  • 10% of Bangladesh is covered in lakes and rivers.

The Effects of the 1998 Flood

  • Over 57% of Bangladesh’s land surface was flooded.
  • Over 1,300 people were killed.
  • 7 million homes were destroyed making 25 million homeless
  • Spread of water-borne diseases like typhoid.
  • Shortage of clean water and food – many rice paddies flooded (2 million tonnes of rice lost)
  • 0.5 million cattle and poultry lost.
  • Roads and bridges damaged.
  • Total costs estimated at about $1 billion

 Test Yourself

Past Paper Style Questions

Grab a pen and start practising your exam technique

1) Explain why erosion occurs more in the upper course of a river system. [5 marks]