Coach House Geography

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Fieldwork in Wales Day 2

Posted by Lindy on October 27, 2009

I could not really say what I expected until I got there. But here is the general theory about each variable and what I expected. Tomorrow, I will tell what we actually found!


Water is slower to warm and slower to cool than the land. The air temperature above the land is affected by the land temperature below it.

What I am expecting:

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

So given this was a pleasant mild day, I would anticipate that the water temperature would be a bit lower than the air temperature. There is no other sources of warmth along Nant Gwynllyn so I would not expect any peaks or troughs anywhere along the river. So all sites would probably be the about the same as each other and would be a few degrees below the air temperature.

Dissolved Oxygen (DO)*

The lower the temperature the more oxygen can dissolve in the water before it becomes saturated.

Dissolved oxygen needs to be high enough to support animal life. The amount of oxygen in a sample can be affected by a number of different circumstances

Certain water bodies, like swamps, naturally have low levels of dissolved in the water. This is because decaying matter uses a lot of oxygen during decomposition – or more accurately the micro-organisms that break down the decaying matter use up much of the oxygen in the process. Many rivers do not have a single source, but ooze out of boggy areas, which can lead lower dissolved oxygen levels.
Much of the oxygen in water comes from plants during photosynthesis so the presence of plants would indicate higher oxygen levels. The amount plants add vary with the time of day. During the day they are adding to the DO but once the sun goes down, photosynthesis stops and respiration takes over as the main plant process, so they like the animals use up the dissolved oxygen.

Oxygen from air as wind blows across the water’s surface and can add to the overall level as can the presence of ‘white water’ as the air bubbles mix with water as it tumbles over the rapids and waterfalls.

What I am expecting

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

There was little wind that day, so no site would have oxygen entering from the air by this method.
Site 1 was just below the place where the river came off a plateau which contained a bog from which much of the water was sources. There was little plant life there. So the dissolved oxygen would probably be lower.

Site 2 had lots of white water from so probably this site more than any other will have a higher dissolved oxygen.

Site 3 & 4 were both under the trees, so did not get much sunlight, but there were few water plants in either. Neither had white water although in both the water had currents in and shallow slow flowing parts, so their dissolved oxygen would likely be lower than site 2.

pH (acidity)
The expected values are 6.5–8.5, from slightly acid to neutral to slightly alkaline.

There is some potential sources of acid in this river, from the acid given off by peat or other rotting material around the sources and also from animal pollution. But given this is on a site to the west of the UK, as most of the rain bearing clouds come across the Atlantic and so do not have acid in the clouds from industrial processes.

What I am expecting

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

Site 1 is near the boggy sources, so this is likely to be more acid.

By the time the water reaches Site 2, the water may be slightly less acid.

At site 3, sheep have access to the river – there were some in the field that day and this is their only source of water, so pollution of urine (acid) and faeces is likely

Site 4 is has no particular sources, so is likely to be close to neutral


Turbidity refers to how cloudy the water is due to the solids carried in it in suspension. High turbidity indicates a lot of suspended material while low turbidity means that it can be seen through quite clearly.

The turbidity determines how much light gets into the water and how deep it goes. Excess soil erosion, dissolved solids or excess growth of micro-organisms can cause high turbidity on a long term basis. All of these can block light. Without light, plants die. Fewer plants mean less dissolved oxygen. Dead plants also increase the organic debris, which micro-organisms feed on. This will further reduce the dissolved oxygen. No dissolved oxygen means other aquatic life forms cannot live in the water.

What I am expecting

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

The water is stained dark brown from its sources in the bogs. The bottom can be seen as if through a brown stained glass window. I am unsure as to whether this means it has high turbidity. Will have to wait until the test to see the answer to that one.
The velocity of the river:

In general, the steeper the slope, the higher the velocity. But there are other factors involved in the exact velocity of a particular place. Some parts of a river, for example on the outside of a bend the velocity is likely to be faster, while on the inside slower than the average. In mountain streams, large boulders can create currents and eddies that may either speed up or slow down the speed of the water.

What I am expecting

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

I would expect from the general rule, the velocity would decrease down stream, getting slower as you proceed from site 1 to site 4. But it would also seem likely that  site 2 would have a higher velocity that the others. However, given that the river was divided and very rocky on site 2, the velocity here could not be measured.

The width, depth and cross-section of a river:

The width, depth and cross-section all change according the rainfall that has occurred in the recent past. All of these will increase if recent heavy rainfall has occurred.

In general as more water is added to a river from tributaries you would expect it to get wider and deeper. However, in some places a river might become much wider but shallower, if the river banks were gentle. Also if it passed through a narrow gap, the river might become much deeper but narrower.  So it is the cross-section that has some consistency, and that should increase as the river passes down stream.

What I am expecting

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


Is the amount of water passing a particular point in cumecs (cubic metres per second) and can be found by multiplying the velocity in metres by second by the cross-sectional area in square metres. As more water is added, then in general, you would expect the discharge to increase down stream.

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

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