pollution

There are many different types of pollution affecting our rivers.

Nutrient pollution / eutrophication

What causes eutrophication?

Eutrophication is the excess of nutrients in the water. Although this might sound like a good thing, that is not necessarily the case. In river ecosystems, there is a natural healthy balance of nutrients which can be used up in the food chains. These nutrients (such as nitrogen and phosphorous) naturally come from organic matter (such as fallen leaves) which are broken down by small animals to form the basis of a food web. But when humans interfere with nature, there can be an increase in nutrients and in this case there can be "too much of a good thing".

What are the effects of eutrophication?

Substances such as fertilisers used in back yards and on farms, and sewerage that has leaked out of septic tanks, add to the natural balance of nutrients, sometimes tipping it into oversupply. The algae in the water consume these nutrients and this causes them to grow in large amounts. When the algae die, the oxygen in the water is reduced and this is harmful to many plants and animals that live in the river. Find out more about this process by reading this website's section on algae.

Toxic or Chemical pollution

What causes chemical pollution?

Pesticides, heavy metals and other chemicals can either make their way into the river from "point" sources (a specific area which can be identified such as a factory) or "diffuse" sources (coming from a range of sources over a wider area, for example fertiliser waste from people's gardens across a number of suburbs).

What are the effects of chemical pollution?

There are many impacts from chemical pollution, including:

• Toxins in large amounts can cause the direct poisoning and killing of aquatic wildlife such as fish. This is often known as a fish kill. Hundreds of fish lying dead along a riverbank is not a pleasant thing to see, or smell!
• Toxins that enter animals' bodies without killing them gradually build up in the ecosystem, becoming more concentrated in the food webs as the larger animals eat the smaller animals. Think of it this way: if ten small fish have an amount of toxic chemical in their bodies, what would happen to the one bird that ate all ten of these fish? As time goes by, small signs of toxic poisoning appear such as skin damage on fish, the thinning of birds' egg shells, irritation of fish eyes and clogging of fish gills.

Pathogen pollution

What is a pathogen? It's something like a bacteria or a virus that causes illness or disease in animals, humans or plants.

Waste (yes, that is a polite way of saying poo) from animals that live and feed by the river can make its way into the water. In excessive amounts this could cause pathogen pollution. For example if a large number of feral pigs wallowed in the shallow waters of a river, their waste could accumulate in the pools nearby and damage the health of other animals. Nobody likes to swim in a poopy pool!

Pathogen pollution also occurs if septic tanks leak into groundwater or into runoff that reaches the river.

Physical pollution - litter

What causes litter damage?

The major cause of litter is laziness and carelessness. That might sound harsh but it's true. Littering is so easy to avoid!

Examples include:

• Litter such as plastic bags, drink bottles and cans dropped along the river's edge or dumped off the side of a boat.
• Old fishing line and tackle left behind by people fishing
• Oil containers, tyres and machinery in stream gullies, dumped by residents

What are the effects of litter?

Plastic bags and fishing line can entangle animals such as birds and fish and prevent them from moving or eating properly.

Animals such as fish and turtles may mistake litter for food and choke on the foreign materials.

Larger objects dumped into waterways can block the natural flow of the water and cause erosion and flood damage as the water is forced to move around the rubbish.

Even if rubbish ends up at tip sites, toxic chemicals in the rubbish, such as paint or oil can leach into the soils and into the groundwater when it rains.

Thermal pollution

What causes changes in water temperature?

Water temperature will change depending on the seasons, but nature is usually prepared for these natural variations. But what happens if a large quantity of hot water is dumped from a factory, or if a large shady tree which overhangs the river is suddenly chopped down? It's these sudden changes in temperature that can do the most damage.

What are the effects of changes in water temperature?

Increased temperature decreases the oxygen levels in the water, making survival difficult for the animals, especially those that are cold-blooded (ectothermic) and cannot regulate their own body temperature.

Increased temperatures in the water can promote algal blooms which can cause a lot of damage to river ecosystems.

Warmer temperatures enable more salt to be dissolved in the water, so warmer water can become too saline (salty) for the animals that usually live there.

Increased turbidity can result in higher water temperatures as the sun's rays warm the dirt in the water.

If the temperature of the water decreases, then the breeding cycles of aquatic animals such as fish and macro invertebrates can be affected.