River Pollution

Rivers are an important part of our landscape; they distribute water all over the country and provide a habitat for wildlife. Over history they have been an essential factor in the survival and growth of man, providing water, food, transport, power and more recently for disposing of waste! Without the presence of rivers our country would be a very different place.

Today they are highly valued for their beauty and attract much tourism. Pollution threatens to spoil and damage the rivers and streams that have been such a large resource to us. Pollution can occur in rivers from various sources. It could come about from direct disposal, surface run off, groundwater drainage, side streams, other water sources and acid rain. All these fall into two general types of pollution direct and indirect. Direct comes from pollution being directly deposited into the water and indirect entering the water from soils and rain runoff from land.

Agriculture has a large role to play in river pollution and as it covers a large part of the country has a lot of influence on how clean rivers can be kept. Farmers will often keep livestock on fields near rivers, of which cattle are most relevant regarding water pollution. The manure passed from these animals washes of the surface of the soil and into the river, as the manure breaks down nutrients leach into the river through ground water. This manure or “silage” is a strong pollutant that can have a massive effect on the quality of the surrounding water and give rise to environmental issues.

Manure produces certain chemicals, some of which are essential to life in a river. The quantities of these chemicals need to be a certain level to sustain a healthy ecosystem; the excessive amounts of these nutrients that manure provides are far too much for a river to cope with.

Nitrates, phosphates and sulphates are all released from manure, they are essential plant nutrients and once in the water will soon cause an abundance of growth in aquatic plants. This excessive growth causes problems for wildlife species that use the river for food, shelter etc. The flow of the river is reduced and as organic matter dies the bed becomes shallower increasing the chances of flood.

In these situations biodiversity is reduced but the overall biomass of the water can be increased this is because only a few species dominate but they are extremely abundant and numerous.

The chemicals not on only increase vegetation but effect species of fish, invertebrates and aquatic mammals that live in or around the river. Each species has its own tolerance levels of nutrients in water, but the numbers of species that can survive in a polluted river decreases as these nutrient levels continue to increase.


Chemical pesticides can have a more direct relationship with pollution simply from the fact that they have direct killing power, this means that certain species who absorb or consume the pesticide will die. This is different to chemicals altering the properties of water, which results in a change of habitat and life in the river. The sudden death of any organisms within the river ecosystem will have a direct impact on other species in the community, through loss of shelter or food. The use of these pesticides also increases the nutrient levels in the water with the same effect as with manure.

Disposal of domestic waste combines both these pollutants with human excrement and grey water from washing machines, baths, sinks etc. Although these waste products are treated there is still high levels of pollutants in the water that is released into rivers by sewage companies. Industry waste dumps massive amounts of chemicals and poisons into rivers regularly which poison and destroy aquatic life.

When fossil fuels are burnt they release sulphur dioxide into the atmosphere, which are absorbed by clouds and falls to the earth as rain. The rain enters the river directly and through soil run off and leaching. The sudden decrease in pH can kill fish and invertebrates and cause the water to be unsuitable for many species. There are few species that tolerate water with a pH lower than 5.

Once the pollution has entered the river it is hard to control, it quickly spreads with the movement of the water to other rivers, streams lakes and ponds. This movement, despite spreading the pollution to other areas is what helps rivers to cleanse themselves. Because of the constant distribution of fresh water the pollution gets diluted and dispersed, lessening its impact on particular areas. Rivers with water falls get a lot more oxygen into the water supply, the oxygen is used by aerobic bacteria to break down dead organic matter, this helps the river to disperse nutrients and clutter on the bed.

If the same levels of pollution were deposited into a still body of water the nutrient levels would rise continuously and provide a habitat suitable only for the most hardy of creatures, such as rat tailed maggots and sludge worms if any life at all.

Ponds and rivers have very different reactions to the same pollutants, pond require a much higher degree of management in relation to pollution due to their ‘closed’ nature, pollution can only be diluted by rainfall or from directed ditches and channels. It cannot cleanse itself through water flow.

Pond and lakes are a much more stable environment because of the lack of flow, plants and animal life can establish and develop easily. In a fast flowing river aquatic life can be reduced to bottom dwelling creatures living in the silt and stones because the force on anything higher is too much for many plants to take root. For this reason changes in nutrient level from pollution can be far more devastating in ponds than in rivers and streams.

In polluted pond water the vegetation ‘blooms’ chocking off oxygen to other species in the water, this unbalances the ecosystem and reduces the ability for diversity, algae blooms dominate shutting out light and consuming lots of oxygen, the growth gets to a point where the water can no longer sustain it and the algae production collapses, this damages the ecosystem even more by sinking to the bottom and building up a layer of organic matter. The reduced oxygen in the water means bacteria and fungi cannot break down the matter and the depth of the pond decreases. Constant pollution accelerates eutrophication resulting in the succession to land with no aquatic life left.

Algae blooms cannot build up in rivers due to the flow, but can in slower parts of the river or small pool off the river, when the matter dies it is more likely to be washed away and distributed more evenly over a wider scale. Lessening the impact of silt build up.

Ponds and rivers can also be affected by oil runoff from roads; the oil creates a film over the surface of the water. In a pond this layer will sit and spread evenly over the surface preventing oxygen dependant species from surfacing to breath resulting in death and reduced diversity. Larva living in the water will not be able to emerge reducing populations for the season; this will affect other species that feed on them by reducing a resource. This causes whole population to dip and without water cleansing disappear from the site all together. In rivers the oil would flow down the course and be deposited in pools and ponds attached creating problems in unique habitats that survive along side the river. The main body of flowing water often recovers well from pollution but at the cost of other habitats.

Another area where flow helps to cleanse pollution is where suspended soils hang in the water, in ponds where water remains generally still, organic matter and clays from surface runoff sit in the water reducing light to vegetation and damaging animals by clogging and chocking breathing and feeding mechanisms. In rivers these soils cannot be suspended but move with the flow and are deposited along the bed and banks when the water slows.

Generally the water in a river can cope well with pollution but the vegetation and land around the river suffers as pollutants are deposited there. These bank sides are vital habitats for many birds, insects and aquatic mammals. When the soil around the water is rich in nutrients, it tends to be dominated by only a few invasive species such as Himalayan balsam, which quickly spreads along the watercourse, out competing species more valuable to wildlife.

The presence of waterfalls and increased gradients along river help with the cleansing process through increased oxygen. I measured the cleanliness of water in three areas along a water course, at one point a water treatment facility deposited treated water back into the river. Even though the water had been treated there are still visible traces of increased nutrients in the water. Several test were done, measuring the cleanliness by chemical tests and species of invertebrates found. The results showed that the water quality decreased immediately after the input of treated water, with higher nutrient values and decreased diversity of invertebrates.

Further down the river after a small waterfall, the water had become cleaner and more abundant. This shows the rivers ability to recover from the effect of pollution through self-cleansing. The oxygen gained from waterfalls and changing gradients helps to defend the river from the effects of plant growth and oxygen reduction that occurs with it. These features also help to increase the speed of the flow dispersing pollutants faster.