How does the long profile of the River Lyn compare to the Bradshaw Model?

If the River Lyn conforms to the Bradshaw model then we can expect to see a long and gentle concave curve in its longitudinal profile from source to mouth through its upper, middle and lower course as shown in the model diagram on this page

To test whether this is true we can use two methods which can be combined:

  • Using the 1:25 000 scale Ordnance Survey map of Exmoor  (extract here) we can construct a long profile of the River Lyn, beginning at the source of one of its major tributaries Farley Water at Grid Reference 764423.  From this point it is 10.8 km to Lyn mouth at Grid Reference 724501 although Farley Water itself only flows for 6.2 km before its confluence with Hoaroak Water (a larger tributary of the East Lyn River) at Hillsford Bridge Grid Reference 741477.  Hoaroak Water itself only flows for another 1 km after this point before joining the East Lyn River at Watersmeet Grid Reference 744487.  From here the East Lyn River has 3 km to flow before merging with the West Lyn River just 0.6 km from the Bristol Channel at Grid Reference 724495.
  • The advantage of using Ordnance Survey contour and spot height data to create a long profile is that information can be collected along the entire length of the river which simply would not be possible for you to manage yourself along an 11 km river during an average fieldwork visit!  However, data collected in the field is likely to be more accurate and detailed and so sample sites can be identified at different points along the river to calculate gradient.  At each sample point two ranging poles should be held upright on the surface of the water 10 m apart.  Using a clinometer measure the slope angle from the sample site upstream to the sample point downstream (see suggestions for fieldwork below).

Consolidate your thinking:

Ordnance Survey map work

Using the Ordnance Survey map extract in , construct a long profile of the River Lyn from the source of Farley Water, through Hoaroak Water and the East Lyn River to its mouth at Lyn Mouth.  Do this by using the edge of an A 3 strip of paper.  Mark the source of Farley water on the edge of the paper and its height which is 440 m.  Now, at intervals of 500 m down Farley Water, Hoar Oak water and East Lyn River make a mark and record the height of the nearest contour to this point that the river has crossed.  Write the height of the contour line next to the mark.  The height of the final point of the long profile (where the Lyn River enters the Bristol Channel) will be sea level or 0 m.

Once you have completed this, you will need a copy of the graph template in Resource 2.3.  The horizontal axis of this template shows distance from source to mouth (11 km) in 500 m intervals.  The vertical axis shows height in meters from 0 to 500 m.  The first height, 440 m, will be at the source of Farley Water and the final height 0 m where the Lyn Rivers enters the sea.  At each interval of 500 m simply place a small cross above it at the correct height.  Join up all of the crosses in a single continuous line.  You now have a long profile of the River Lyn calculated from the Ordnance Survey map.

For support and guidance if required here for either the data collection or presentations go to

Consolidate your thinking

Suggestions for fieldwork

The gradient of a river is a measure of how steeply it loses height.  If the River Lyn adheres to the Bradshaw Model then we would expect the average loss of height in its upper course to be significantly greater than its average loss of height in the middle and lower course.  We will anticipate a steady decline in the loss of gradient along its course to match the classic concave profile predicted by the model. This can be tested by taking measurements at a minimum of 10 sample points along the river’s course.  All that is required are two ranging poles, a 10 m measuring tape and a clinometer (a small piece of equipment that measures angles).

The process of measuring the angle is straight forward:

  • One person needs to stand at a point on the river bank with a ranging pole held vertically;
  • Another person then takes up a position on the river bank 10 m down the river also holding a ranging pole vertically;
  • The first person then holds the clinometer against a precise point on their ranging pole e.g. the top of one of the red bands and looks through it to the same point on the ranging pole held by the second person further downstream;
  • A third person can then record the change in gradient at this sample point measured in degrees

If at least 8 measurements of gradient are made along 8 x 10 m sections the upper course of the River Lyn and a minimum of 8 in its middle course and a further 8 in the lower course then the Mann Whitney U test can be used to assess whether the difference between the medians of two sets of data e.g. Upper and Middle; Middle and Lower and Middle and Upper and Lower are significantly different assuming a null hypothesis of no significant difference existing between the locations.

Here are some possible locations where you can  access the river to carry out your gradient measurements and to consider your risk management strategy.

Assessing risk and identifying control measures to manage risk

When undertaking fieldwork in geography it is important that a researcher is always aware in advance of possible risks to their safety and welfare of the research they are going to undertake.  This is especially important when studying rivers.  In addition examiners will want to see that a researcher has considered risk management and been safe whilst collecting data.  A risk assessment does not need to be excessive or to consider extreme events that might occur but be seen to concentrate on those things over which you have control.  For example, if you are travelling to the River Lyn on a coach organised by the school then this part of the day will be covered by the school’s risk assessment. But once you are working say in pairs or small groups not directly supervised all the time by your teachers then you become responsible for identifying and controlling possible risk. The following pro forma can be used to focus your thinking: Here is a link to a word document that you can adapt.

Potential risk

Possible harm that could be caused

Precautions taken to control or mitigate the risk

Deep peat bogs

Sinking and possible drowning

Ensure only well marked and used paths are followed – do not deviate from marked routes. Always walk with a group never go off on own.

Very cold wind and rainfall


Wear appropriate clothing including waterproof coat and leggings with boots.  Hot drink.

Fast flowing river


Never enter or walk close to the edge of a fast flowing river.

Only choose locations for sampling which are accessible and relatively shallow with moderate flow. Always work in groups.

Secondary Data Sets

If you are unable to undertake your own fieldwork then we have provided some secondary data collected from eleven points along the course of the River Lyn that will still enable you to undertake a statistical analysis.

You can see the data collected at each site in the map below and  the full data set for all eleven sites in an excel spreadsheet here

Please note -  some of these sample points are not suitable for schools groups.

Now go to Investigation 5 - How does the cross section profile of the River Lyn compare with the Bradshaw model?