Is the River Thames at Bourne End Clean? Why We Need Evidence, Not Opinions

The River Thames at Bourne End can look beautiful.

On a calm summer morning, the water reflects the trees, sailing boats move quietly across the reach and insects hover around the marginal plants. It is tempting to look at the scene and conclude that the river must be clean and healthy.

Equally, after heavy rain, when the water becomes brown and turbid or pieces of debris float downstream, it is easy to decide that the river is badly polluted.

Neither conclusion is properly scientific.

A river cannot simply be described as “good” or “bad”. Water quality is a collection of physical, chemical and biological measurements, all of which can change with the weather, the season, the flow of the river, the time of day and the precise location at which a sample is taken.

To understand the water quality of the Thames at Bourne End, we need evidence.

That is where A Level Biology becomes particularly valuable.

What Does the Official Evidence Tell Us?

The reach containing Bourne End forms part of the Environment Agency’s Thames “Reading to Cookham” water body.

The Environment Agency currently classifies this wider stretch as having moderate ecological status. However, the detail behind that single word is much more interesting. The biological quality elements were classed as good, the invertebrate classification was good and the macrophyte—or aquatic plant—classification was high. Dissolved oxygen was rated high, while phosphate and temperature were only moderate. The classification history also records concerns involving persistent chemical pollutants.

That already demonstrates the problem with asking whether the river is simply clean or dirty.

Some indicators suggest a river capable of supporting a healthy biological community. Others reveal nutrient, temperature, physical modification or chemical pressures.

More importantly, an Environment Agency classification for a 38-kilometre water body cannot tell us the exact condition of the water beside a particular pontoon at Bourne End on a particular morning.

For that, we need local measurements.

A River Is Constantly Changing

Water quality is not fixed.

A sample collected at 9 am may produce different results from one collected at 4 pm. A sample taken after several dry days may differ considerably from one taken after a thunderstorm. Water beside dense aquatic vegetation may contain different concentrations of dissolved gases from water in the centre of the channel.

Temperature, river flow, rainfall, photosynthesis, respiration, agricultural runoff and discharges into the river can all affect the results.

Thames Water provides a near-real-time map showing monitored storm-overflow activity, including the time and duration of recorded discharges. This is useful contextual evidence, although it does not replace direct sampling at Bourne End.

The scientific question should therefore not be:

“Is the Thames at Bourne End clean?”

A better question would be:

“How do the physical, chemical and biological indicators of water quality vary at Bourne End with location, depth, season, time of day and recent rainfall?”

That is a much more interesting investigation.

Dissolved Oxygen: Can Aquatic Organisms Breathe?

Dissolved oxygen is one of the most important measurements.

Fish, freshwater shrimp, insect larvae and many microorganisms need oxygen dissolved in the water for aerobic respiration. A river may look clear but still have an oxygen problem.

Oxygen enters the water through contact with the atmosphere, especially where the water is disturbed at weirs or around obstructions. Aquatic plants and algae also release oxygen during photosynthesis.

At the same time, respiration by plants, animals and microorganisms removes oxygen. Decomposers can consume particularly large quantities when breaking down sewage, dead algae or other organic material.

The Environment Agency’s real-time water-quality monitoring systems commonly measure dissolved oxygen alongside temperature, conductivity, pH, turbidity, ammonium, chlorophyll and nitrate.

For an A Level investigation, dissolved oxygen could be measured:

• near the bank and towards the main channel;

• beside dense plant growth and in more open water;

• at the surface and, where it can be done safely, at greater depth;

• early in the morning and later in the afternoon;

• before and after a period of heavy rain.

Morning and afternoon comparisons would be particularly interesting. Plants respire throughout the night but cannot photosynthesise without light, so dissolved oxygen may be lower around dawn. During a sunny day, photosynthesis may increase the oxygen concentration.

Temperature must be recorded at the same time because warm water holds less dissolved oxygen than cooler water.

One isolated dissolved-oxygen reading would tell us very little. A repeated pattern would be much more valuable.

Carbon Dioxide, pH and Photosynthesis

Carbon dioxide is closely linked to oxygen.

Respiration releases carbon dioxide, while photosynthesis removes it. As dissolved carbon dioxide increases, it can affect the pH of the water.

Directly measuring dissolved carbon dioxide in the field can be more difficult than measuring oxygen, but students could combine suitable dissolved-gas tests with pH measurements and observations of plant density.

The most useful investigation might compare:

• heavily vegetated water with open water;

• shaded areas with sunny areas;

• morning readings with afternoon readings;

• flowing water with sheltered areas near the bank.

This provides an excellent opportunity to connect ecology with the familiar A Level Biology equations:

carbon dioxide + water → glucose + oxygen

and

glucose + oxygen → carbon dioxide + water + energy

The equations are simple. Seeing their effects in an actual river makes them meaningful.

Turbidity: How Much Light Can Pass Through the Water?

Turbidity measures the cloudiness of water caused by suspended particles.

These particles may include clay, silt, organic matter, algae and microorganisms. Turbidity is normally measured using a turbidity meter in nephelometric turbidity units, although a turbidity tube can provide a simpler comparative measurement.

Heavy rainfall may wash soil and other material into the river. Boat movements, increased flow or disturbance of the riverbed may also raise suspended sediment.

High turbidity matters because it reduces the amount of light reaching submerged plants. This may lower photosynthesis and eventually affect dissolved oxygen.

Suspended particles can also settle on leaves, eggs and riverbed habitats.

However, cloudy water is not automatically polluted water, just as clear water is not automatically safe water. Turbidity is one piece of evidence that must be interpreted alongside the other results.

Temperature at Different Locations and Depths

Water temperature affects almost every part of a river ecosystem.

It affects metabolic rate, respiration, photosynthesis and the amount of oxygen that can remain dissolved in the water. It can also determine which species can survive in a particular habitat.

Students could lower a temperature probe to several depths, provided this can be done safely from a pontoon or boat. In a shallow, fast-moving section, the water may be well mixed and the differences small. In deeper or more sheltered areas, a temperature gradient may be found.

Measurements should also be taken:

• in sunlight and shade;

• near the bank and in the main channel;

• close to incoming streams or drainage channels;

• at several times during the day.

The result would be a temperature profile rather than one apparently precise but unrepresentative number.

Flow Rate: The Variable That Changes Everything

Flow rate influences nearly every other result.

Fast-flowing water is usually better aerated, while slow-moving water allows sediment to settle. Increased flow after rain may dilute some substances while simultaneously bringing additional sediment, nutrients, bacteria and organic material into the river.

A simple surface-flow investigation can be carried out by timing a floating object over a measured distance. Several repeats are needed, and the float must be recovered so that nothing is left in the river.

A flow meter would provide better local velocity measurements.

For a more ambitious investigation, students could measure the approximate cross-sectional area of the channel and combine this with average velocity:

discharge = cross-sectional area × mean velocity

At Bourne End, however, safety must take priority. There is no need for students to enter deep or fast-moving water merely to obtain another measurement. Sampling from the bank, pontoon or a properly supervised boat is much more appropriate.

Aquatic Plants and Marginal Vegetation

The plants growing in and beside the Thames are not merely scenery.

Aquatic plants provide habitats, refuge from predators, surfaces for eggs and feeding areas for many organisms. Their photosynthesis can also influence oxygen and carbon dioxide concentrations.

Students could establish a transect along the bank and record:

• the plant species present;

• percentage cover;

• water depth;

• distance from the bank;

• degree of shading;

• sediment type;

• evidence of grazing or physical disturbance.

Quadrats could be used for marginal plants, while photographs would create a permanent record that could be analysed later in the classroom.

Repeated photographs from the same points would reveal seasonal change much more effectively than a single visit.

The investigation should also distinguish between native plants, invasive species and filamentous algal growth. A large quantity of green material does not necessarily indicate a healthy ecosystem. Excessive nutrient concentrations can encourage rapid algal growth, which may later create an oxygen demand as the algae die and decompose.

Invertebrates: The River’s Living Record

Chemical measurements show the condition of the river at the moment the sample is taken. Invertebrates can reveal what conditions have been like over a longer period.

Some freshwater invertebrates are relatively tolerant of pollution or low oxygen. Others require well-oxygenated water and are much more sensitive.

A carefully controlled sweep or kick sample might reveal freshwater shrimp, snails, leeches, caddisfly larvae, mayfly nymphs, beetle larvae and other organisms.

Riverfly monitoring uses the types and numbers of freshwater invertebrates as an indicator of river health. It complements chemical testing because the organisms reflect the ecological effect of water conditions rather than merely the concentration of a substance on one day.

Students could calculate:

• species richness;

• total abundance;

• the relative abundance of indicator groups;

• a diversity index;

• differences between habitats.

Finding many organisms is not enough. A sample containing hundreds of individuals from one pollution-tolerant species may indicate a less balanced community than a smaller sample containing a wide variety of sensitive species.

Microbial Content: Clear Water Can Still Contain Bacteria

Microbiology is one of the most important—and most easily overlooked—parts of water-quality testing.

The water may appear completely clear while still containing microorganisms associated with faecal contamination.

For designated bathing waters, the Environment Agency tests for E. coli and intestinal enterococci. These are used as indicators of faecal pollution.

A proper microbial investigation at Bourne End would require carefully collected sterile samples and an appropriate laboratory method. Results should be expressed quantitatively, normally as the number of organisms or colony-forming units in a stated volume of water.

This work also requires particularly careful risk assessment.

Unknown environmental microorganisms should not be treated as harmless. School or tuition investigations should use approved procedures, sealed test systems or an accredited laboratory. Incubated cultures should not be reopened, and student results should never be used to declare the river safe for swimming.

The most revealing comparisons might be:

• after prolonged dry weather;

• after heavy rain;

• upstream and downstream of potential inputs;

• beside the bank and in the main flow;

• across several months.

One sample cannot establish microbial safety. Repeated, professionally controlled testing is needed.

Nutrients and Other Chemical Measurements

Although oxygen, carbon dioxide and turbidity are important, a fuller survey should include additional chemical variables.

Phosphate and nitrate are particularly relevant because they can stimulate excessive plant and algal growth. Ammonium may indicate organic pollution, while conductivity can reveal changes in the concentration of dissolved ions.

Useful measurements could include:

• pH;

• nitrate;

• phosphate;

• ammonium;

• conductivity;

• alkalinity;

• dissolved oxygen;

• water temperature.

These should not be investigated as unrelated numbers. Students should look for relationships.

Does turbidity increase after rain?

Does phosphate concentration rise at the same time?

Are warmer areas associated with lower dissolved oxygen?

Do areas with more aquatic plants show larger differences between morning and afternoon oxygen readings?

Does invertebrate diversity change between habitats?

These questions turn data collection into scientific analysis.

Designing a Reliable Bourne End Investigation

A credible study needs more than an impressive box of sensors.

To do an accurate investigation we need three sampling locations: one upstream, one beside the main area of interest at Bourne End and one farther downstream. Each location needs a clear description, photographs and, where appropriate, a grid reference.

At every location, the same measurements should be taken using the same method.

Each measurement should be repeated. Equipment should be calibrated, sampling containers labelled and the time, weather and recent rainfall recorded.

The investigation should also be repeated across the year. A river in February is not the same ecosystem as a river in August.

A useful programme might include:

• monthly baseline testing;

• morning and afternoon comparisons;

• additional sampling after heavy rainfall;

• seasonal plant surveys;

• regular invertebrate monitoring;

• occasional accredited microbial analysis.

This would gradually create a genuine local dataset.

What A Level Biology Students Would Learn

The value of this work extends far beyond learning how to operate a dissolved-oxygen probe.

Students would have to consider:

• independent, dependent and control variables;

• random and systematic error;

• repeatability and reproducibility;

• representative sampling;

• uncertainty;

• correlation and causation;

• risk assessment;

• statistical significance;

• ethical treatment of organisms;

• the limitations of their conclusions.

They would also discover that real biological data are rarely neat.

A sensor may drift. A sample may become contaminated. One site may be inaccessible. A plant may be difficult to identify. Results may contradict the original hypothesis.

That is not failed science.

That is science.

My View of the River Has Changed

When I look across the Thames at Bourne End, I see sailing water, a working navigation channel and an attractive part of the local landscape.

A biological investigation encourages me to see much more.

The river is a moving system of organisms, gases, nutrients, microorganisms, sediment, temperature changes and human influences. Every insect larva, patch of weed and dissolved-oxygen reading contributes another piece of evidence.

The official evidence suggests a mixed picture: a river supporting valuable biological communities but still affected by nutrient, chemical and physical pressures.

Our local measurements could reveal how that wider picture appears at Bourne End—and how it changes from one day to the next.

Conclusion: Replace Assumptions with Evidence

So, what is the water quality of the River Thames at Bourne End?

The honest answer is that it cannot be reduced to one word.

The wider Environment Agency classification is moderate, but several biological indicators are good or high. Other indicators reveal continuing pressures. Conditions at one precise location may also change rapidly with rainfall, temperature, river flow, plant activity and pollution events.

The only scientifically defensible approach is to measure, repeat, compare and analyse.

That is why this could become such a powerful A Level Biology project.

Students would not simply learn about ecosystems from a textbook. They would investigate a real river, collect evidence about their local environment and begin constructing a long-term record of its health.

The Thames may look peaceful from the bank.

The science beneath the surface is far more complicated—and far more interesting.