Key drivers of
water quality

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Did you know:

  • Dams can lead to lower concentrations of sediments, nutrients and salts downstream, as they are trapped in the dam. This will interrupt the regular supply of sediment and nutrients downstream.
  • Bushfires can cause higher suspended sediment stores in the catchment as well as more erosion because plant cover has been lost. Concentrations of small dirt particles and turbidity generally increase in the first stormflow after fires.
  • Fertiliser application can influence water quality by increasing nutrient levels.
  • Vegetated buffer strips along the margins of streams can reduce the delivery of suspended solids and dissolved nutrients into streams.

Water quality of rivers and streams within and between catchments can change over time.

Factors that influence water quality include: land cover (the amount, type and arrangement of native plant life in the catchment and along waterways), land use, land management practices (like animal stocking rates, application rates of fertilisers and other chemicals), catchment geology, climate and rainfall, topology and catchment hydrology.

We know how certain landscape features affect riverine water quality, but we only have limited understanding about their level of importance and how they interact with each other.

So it’s difficult to predict what the water quality will be at certain places at any specific time.

Did you know?

Climate has a big influence on water quality. Seasons, rainfall, runoff and changes in climate all affect water quality.

Different climatic conditions and seasonality can significantly affect water quality.

Large scale weather patterns like El Niño and La Niña can influence water quality.

This is because they alter rainfall patterns and intensity, and runoff.

Wet and dry periods can have a big impact on water quality.

What is blue-green algae?

Blue green algae is a type of bacteria also known as cyanobacteria. It grows in water under certain conditions like when the ratio of nitrogen to phosphorous concentration is low, where there are low flow conditions and the weather is warm. Algal blooms occur when there are large numbers of algae in the waterbody and this can harm water ecosystems and people.

Victorian rivers usually have

low flows in summer

Some aspects of water quality like sediments and nutrients improve in dry conditions as less material washes in from the catchment.

Water temperatures are warmer and water clarity can improve from the settling of suspended solids onto the stream bed.

Dissolved oxygen levels are lower because of warmer water.

Electrical conductivity (salinity) can increase where saline groundwater has intruded.

Concentrations of nutrients can increase as water levels drop, leading to poorer water quality. These conditions can lead to algal blooms.

What is blue-green algae?

What is a blackwater event?

Blackwater events can be a natural feature of lowland river systems and can occur when organic material, such as leaves and trees, are washed off the floodplain and into the river channel.

Normally, exporting organic material from floodplains to a river channel produces good results by nourishing lowland river ecosystems with carbon, which is a source of food for bacteria, fish and macroinvertebrates.

But if the rate of oxygen consumption by bacteria and microbes eating the leaf litter is faster than oxygen can be replenished from the atmosphere, it can deplete oxygen in the water column, leading to the death of fish and other aquatic life.

The black appearance of the water is caused by the release of carbon compounds that include tannins as the organic matter decays.

What happens when a catchment is already wet or dry?

If a catchment is dry, rainfall soaks into the ground and will not create surface water run off to nearby rivers. But when the catchment is wet, more rainfall can’t be absorbed and the water will flow overland to the nearest river. Under these conditions, this overland flow can pick up nutrients and sediments and carry them into the rivers.

Victorian rivers usually have

higher flows in the winter months

Rainfall in the catchment washes nutrients and carbon in leaf litter into the rivers and this can lead to poorer water quality.

Increased rainfall can also dilute nutrient concentrations in waterways to improve water quality.

Runoff from the catchment can produce high turbidity and suspended solids and stir up bed sediments.

Dissolved and particulate substances like eroded soils, nutrients, salts, toxins, pathogens and other contaminants are carried from the surrounding catchment into water systems after rainfall.

What is a blackwater event?

Catchment erosion can increase greatly with heavy rainfall and the resultant run off. This is called the 90:10 rule, where 90% of contaminants are delivered to water systems 10% of the time (heavy rainfall events).

What happens when a catchment is already wet or dry?

Water quality of urban streams varies greatly.

Changes to the hydrologic regime, or flow patterns, can affect water quality. Large areas of water-resistant surfaces, such as roads and roofs, can lead to faster rates of run off, erosion and moving pollutants into waterways. Surface run off, known as stormwater, usually carries a variety of chemical and biological materials into waterways.

Some human activities affect
water quality.

These include:

  • Icon to depict Cleaning of catchment vegetation

    Cleaning of catchment vegetation

  • Icon to depict Mining activities

    Mining activities

  • Icon to depict Discharges of wastewater

    Discharges of wastewater

  • Icon to depict Forestry operations

    Forestry operations

  • Icon to depict Stormwater flows

    Stormwater flows

  • Icon to depict River regulation and dams

    River regulation and dams

  • Icon to depict Use of pesticides, herbicides and fertilisers

    Use of pesticides, herbicides and fertilisers

  • Icon to depict Impervious or water-resistant services, such as roads and roofs, especially in urban areas

    Impervious or water-resistant services, such as roads and roofs, especially in urban areas

The Victorian Government recently identified actions giving special attention to human activities that impact on water quality.

These are:

Introducing targets to lower the amount of pollutants in our waterways

Introducing targets to lower the amount of pollutants in our waterways

Improving how we manage urban stormwater and sewerage

Improving how we manage urban stormwater and sewerage

Improving how we manage discharge of wastewater

Improving how we manage discharge of wastewater

Improving how we manage household wastewater onsite

Improving how we manage household wastewater onsite

There is more information in Chapter 3 on Waterway and Catchment Health in Water for Victoria.

Read this chapter on

The Government has updated Victoria’s State Environment Protection Policy (Waters).

This policy helps focus on water quality requirements across the state by setting out the beneficial uses of water and the environmental quality standards and aims to protect them. We then report against achieving the aims of the policy to get a picture of water quality condition.