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2: The topography of the River Lyn and its catchment

Key Question: How do the characteristics of the River Lyn compare with the Bradshaw model?

A model is an idea or theory that geographers create that they then test out in the real world to see how accurate it is.  In simple terms it involves geographers coming up with an idea or theory about how they think the world works and then comparing their ideas with what actually happens in reality.   How what happens in the real world and why it is different than the model or theory is much more important to geographers than the model itself. A geographer called Bradshaw developed such a model of how rivers should behave as they flow downstream from their source to mouth.  It is a very useful tool to use to understand how real rivers such as the Lyn work.  Through this line of enquiry you are going to assess and attempt to explain using a wide range of secondary data (supported by primary data collected through fieldwork at various locations along the river if this is possible) the extent to which the River Lyn supports or is different from Bradshaw’s model.

1: What is the Bradshaw model and why do geographers use it?

Bradshaw said that as a river flows downstream the following things will always happen:As a river loses height from it source to mouth as it moves towards the sea, its long profile or channel slope decreases gradually and evenly as shown by the concave slope in the diagram below. In the upper reaches of the river close to its source, the gradient of the channel is at its steepest because here most erosion is vertical erosion (down cutting).  As the river flows downstream vertical erosion tends to be replaced by lateral erosion (side by side) and then towards the mouth in the lower reaches most erosion is replaced by deposition as the river flattens out to enter the sea;

Long and Cross Profiles of a Typical River

Bradshaw Model

 from http://www.coolgeography.co.uk/

  • As a result of these patterns of erosion and deposition the cross profile of river channels will be narrow and V shaped in their upper course closer to the source but will become increasingly deeper and wider in their middle and lower reaches as they move closer towards the sea and their mouth (as in diagram above);
  • The discharge of a river (the volume of water passing an area measured in cubic metres per second) will always increase downstream as more water enters the river from tributaries and surface runoff i.e. as the capacity of a river increases then so will its speed;
  • The wetted perimeter (the length of a river’s bed and banks in contact with the water) will always increase with distance from source to mouth as more water enters the river system through tributaries and run off as it progresses downstream;
  • The amount of pebbles, stones, boulders, gravel and sand and sediment on the bed and banks of a river increases its channel bed roughness.  According to Bradshaw channel bed roughness will always decrease in a river from source to mouth.  In addition to channel bed roughness, Bradshaw also said in his model that a river’s load size and shape (average size and shape of material transported) will also become smaller and rounder with distance from the source.  This is because the further downstream material is carried then the greater the time available for it to be eroded by attrition and abrasion which makes rocks and stones smaller and rounder.  Also, during periods of low flow on the river e.g. during the summer only smaller particles can be carried downstream with all of the remaining material being deposited that are then vulnerable to being broken up by weathering such as freeze – thaw processes;
  • The average speed or mean velocity of a river (measured in metres per second) will always increase with distance along its channel.  This is because a river tends to become deeper, wider and have a higher discharge the further downstream it moves.  As a result relatively less water is in contact with the wetted perimeter, so friction on the river water from the bed and banks of the channel is reduced downstream and it consequently flows faster.   In addition in the upper course the wetted perimeter of the river is higher in proportion to the area of the river than it is downstream.  This increases friction on the water and reduces its speed.  As well as this channel bed roughness decreases from source to mouth and therefore so will its frictional effect on water flowing through the channel from source to mouth;
  • Both mean occupied channel width (distance across a river channel measured at the surface water i.e. not the wetted perimeter) and mean channel depth (height from water surface to the stream or river bed) will increase with distance from the source to the mouth of a river.  In both cases this will happen as additional water volume (discharge input) is contributed from tributary streams and rivers as well as surface runoff from a larger land catchment area as a river progresses downstream.

Consolidate your thinking

Go to

http://www.slideshare.net/geographyalltheway/geographyallthewaycom-ib-geography-the-bradshaw-model?next_slideshow=1 

and scan through each of the slides which confirm visually each of the main parts of the Bradshaw model above.  Draw your own graded long profile diagram to summarise the main assumptions of the Bradshaw model for the upper, middle and lower course of a river.


Go to Investigation 2 - How do Geographers go about testing the accuracy of a theory such as the Bradshaw Model.