2. Drainage Basins and Flooding


The Amazon River Basin (Sourrce – http://www.clim-amazon.eu/content/view/full/51655)

All rivers receive a water supply and the area of land this comes from is known as a drainage basin. The boundaries of the basin are known as the drainage divide or watershed and will usually be marked by areas of higher land. Drainage basins have many different characteristics that influence how quickly or slowly the main river within them responds to a period of intense rainfall, these are outlined in more detail in the section relating to storm hydrographs.

Drainage basins

A drainage basin is a system. INPUT is largely from precipitation, whereas OUTPUT occurs through river run-off (often into a sea) & evapotranspiration. The PROCESSES that operate with the system include; surface run-off, infiltration, throughflow, percolation, groundwater flow & channel flow.

Drainage basins

On the following blank diagram, add these annotations and a key:

throughflow | groundwater flow into deep rocks | infiltration | precipitation | capillary rise back into soil | evaporation and evapotranspiration | overland flow | percolation | channel flow | throughflow and stemflow | baseflow in bedrock

Drainage Basin System

You need to be familiar with – and be able to explain – these processes: precipitation, evapo-transpiration, interception, infiltration, percolation, channel flow.


Discharge diagram

  • To be able to define [give the precise meaning of a word, phrase, concept or physical quantity] stream discharge.
  • To examine [consider an argument or concept in a way that uncovers the assumptions and interrelationships of the issue] its relationship to stream flow and channel shape.


Discharge ppt

The Bradshaw model, illustrating the changes as a river travels from its source:


Storm or flood hydrographs show how a river handles extra water volume as a result of increased rainfall. It shows how quickly water falling within the river basin will reach the river channel. There are several elements to a storm hydrograph:

Watch the presentation again and use it to help you fit this hydrograph together:

Storm hydrograph exercise

BBC Bitesize has some info on hydrographs.

BBC Rivers – hydrography looks at factors affecting the shape of hydrographs.

Hydrographs will be effected enormously by a whole range of different conditions. What kind of effect would these conditions have on a hydrograph?

small or large drainage basin?
impermeable rock (granite, clay) or permeable rock (limestone, sand)?
human activity – plantations or urbanisation?
vegetation – farmed fields or forest?
shape of the valley – steep sided or sloping gently?
prolonged, heavy rainfall or sudden showers?

This keynote on hydrographs will also be helpful.


The IB Geography syllabus requires you to be able to ‘discuss the natural and human causes and consequences of a specific river flood.’

The command word discuss requires you to ‘offer a considered and balanced review that includes a range of arguments, factors or hypotheses. Opinions or conclusions should be presented clearly and supported by appropriate evidence.’

Take a look at this great interactive animation of flooding and associated concepts:

We’re going to look at the 1998 Bangladesh floods, and use what we find to structure a case study of the flooding which took place in 2012.

These news articles from 2012-13 highlight the causes and effects of the flooding.


Presentation: Bangladesh flooding

Bangladesh in the front line of Climate Change?

Bocastle flood – BBC News looks at before and after (2014)

The Bocastle river flood from the RGS

New flood defence scheme at Bocastle

Bocastle: Then & now (floodblockbarrier.com)

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