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Fieldwork in Wales Day 5

Posted by Lindy on October 31, 2009


Hypothesis 1: The temperature will remain the same at all sites

The data showed very similar values (although slightly cooler near the top of the course). This may have been because site 1 was the most open of the 4 measured

Hypothesis 2: The dissolved oxygen will increase with the presence of white water

This was true. However the lack of sensitivity in this test does not make it particularly useful in testing for example for organic decay causing a reduction in oxygen. A more sensitive test might have shown differences between sites 1, 3 and 4.

Hypothesis 3: The pH will be more acid where there is organic pollutants

The only real evidence of organic pollutants was the sheep hoof marks at the ford and this did have the most acidic results. The anomaly here though was the alkaline (pH8) at the waterfall. Having sort the advice of a chemistry expert, we decided that there are 2 possibilities for this.

(a) Experimental error

(b) High levels of dissolved CO2 can cause a lowering of pH as some of the CO2 becomes dissolved in the water to form a weak acid, carbonic acid. So lack of CO2 may mean that the water has a higher pH than expected. At the waterfall, the water was shallow and the rock was smooth and there was no evidence of either plant or animal life, so it is possible that low CO2 could account for the high pH.

Hypothesis 4: The turbidity will be the same at all sites


Hypothesis 5: The velocity will decrease the further the site is from the source

This hypothesis was obviously false. There are 3 possible causes of this:

(a) Experimental error. See below for more detail on this in the evaluation.

(b) The theory is that the slope of the river bed is steeper the nearer the source you are. It is the slope that determines the velocity of the river. So on average, the course of the river tend to be steeper the nearer you are to the source.Longit section

However, there are a number of factors to be born in mind when applying this idea. The Nant Gwynllyn is in no way uniform as the diagram implies

Gradiat anom

(c) The other factor that needs to be taken into account is the bedload. Large angular rocks and hard resistance channel sides and base force the water to go around the obstructions. These create friction with the water, both of which slow the water down. This was demonstrated clearly by the path of the orange that was rejected in site 4!

Hypothesis 6: The estimated cross-section will increase the further the site is from the source

Hypothesis 7: The estimated discharge will increase the further the site is from the source.

Both these hypothesises were shown to be just about true from the data. Although (see in the evaluation) how valid the figures were leaves is open to question. The data also showed that the width and depth can vary quite a lot, both reducing and increasing over the distance examined, but it their combination that shows some consistent increase, the further downstream the site is.


Water quality:

The kit was complete and simple to use. The instructions were clear so as to make the best possible use of the test available. However, the lack of sensitivity of the dissolved oxygen test means that tests over one water course is unlikely to show any great variation as there is only one ‘moderate’ value (4ppm).

What else I could have done: I could have tested more sites, although given the similarity of the river chosen, I am not sure if this would tell me anything. However, what I can and will do, is to retest those same sites when the environmental conditions change. For example, we had had little rain in the preceding 4 weeks and the weather was unusually mild for the time of year. So I will retest after heavy rain when it is colder and also in high summer, when the water is even lower.

River Feature

These tests could have been carried out with more care as I do not feel that I got accurate enough results to be sure of the conclusions.

3 oranges placed is a fairly random fashion did not give clear enough results. Next time (on the retest – see above) I will carefully measure every 20 cm and drop another one in. (This would only be possible on a small stream like Nant Gwynllyn. On larger rivers you could do 50cm gaps for example)

The measurement of maximum depth in one place only and its use to estimate the cross-section was not good enough.

All in all I enjoyed the experience and have learnt lots about how to do it better next time.


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