Dive Deeper: What phys-chem tells us about waterway health
- comms514
- Aug 4
- 6 min read
The annual Townsville Dry Tropics Waterways Report Card grades the health of our rivers, creeks, and coasts, by analysing water quality data, which measures things like nutrients and physical-chemical properties, known as phys-chem.
Environmental Data Analyst of the Healthy Waters Partnership, Adam Shand said measuring multiple parameters is essential for understanding the health of waterways.
“Together, they provide a powerful picture of our environment,” he continued. “Without these perspectives, we would be flying blind on the quality of our water.”
How did water quality measure up in the 2025 Report Card?
Water quality is made up of many moving parts and while an overall grade might stay the same from year to year, what’s happening beneath the surface can shift significantly. That’s why it’s important to go beyond the final score and dive into the details.
The 2025 Report Card revealed imbalances in some of our waterways, driven by changes, both up and down, in phys-chem indicators.
What are phys-chem indicators (and why should you care)?
Phys-chem indicators describe the physical and chemical characteristics of water. Though, not all are measured in every zone in the Report Card. Phys-chem include:
Dissolved Oxygen (measured in freshwater and estuarine zones)
Turbidity (measured across freshwater, estuarine and inshore marine zones)
Total Suspended Solids (TSS) (inshore marine zones only)
Secchi Depth (inshore marine zones only)
These indicators help scientists understand oxygen levels, water clarity, and the presence of suspended particles, which all tell a story in a waterway’s ability to support aquatic life.
Here’s a breakdown of each phys-chem indicator, how it can vary, and what it means for our waterways.
Dissolved Oxygen: the breath of life
Dissolved oxygen is the amount of oxygen available in the water. Just like humans need oxygen to breathe, so do fish, insects, and other aquatic animals. Even aquatic plants rely on it to survive. Levels that are too high or too low can harm aquatic life.
What causes oxygen levels to shift?
Temperature: As water temperatures go up, oxygen levels go down. Fish are cold-blooded and must adjust their body temperature with the surrounding water. So, when water gets warmer, fish speed up their metabolism and use more oxygen. The combination of less available oxygen, but more being needed can quickly cause problems. Cooler water on the other hand holds higher levels of oxygen. This means that oxygen levels in a water body will naturally vary during the day and night, as temperatures rise and fall.
Excess nutrients cause aquatic plants and algae to grow rapidly. These plants use oxygen as they grow and even more when they die and decompose. Bacteria that help break down dead plants use a lot of oxygen in the process, also lowering oxygen levels.
Invasive weeds: Unwanted aquatic plants can clog waterways and use oxygen needed by native plants, fish, and animals.
Several waterways recorded low levels of oxygen, including the Ross Basin freshwater zone which received a ‘poor’ grade. This was driven by ‘poor’ scores at each of the weirs in the Ross River and ‘very poor’ scores at both monitoring sites in the Bohle River. Without deeper investigation, such as measuring dissolved oxygen along with temperature throughout the day and night, it can be difficult to pinpoint the cause.
Turbidity: clearing things up
Turbidity refers to how clear or cloudy the water is. When water contains large amounts of tiny particles, like sediment, organic matter, plankton, microplastics, or even dissolved substances like dyes, it becomes murky. These particles can be so small they’re invisible to the naked eye, much like smoke in the air, which looks hazy even though you can’t see the individual particles.
High turbidity blocks sunlight from reaching underwater plants, which need it to grow. It can also affect fish and aquatic animals, making it harder for them to feed, breed, navigate or find shelter.
What influences turbidity?
Sediment sitting on the bottom of a waterway can be stirred up and suspended in the water when the area is disturbed. This can happen due to turbulence from wind, waves, rain, or closer to the coast, tides. Animals, such as wild pigs and horses, may also disturb sediment when they drink or forage near the water. Even bottom-feeding fish and other aquatic animals can stir up the river bottom.
Land use, erosion, and runoff all play a role in how much soil and other materials end up in the water.
In dry years, reduced rainfall means fewer sampling opportunities. But when rain finally arrives, turbidity may increase due to the disruption of dry soils.
The shape of the landscape influences how water and sediment moves through the catchment. Learn more about this in our StoryMap: Hydrology and Hydrodynamics of Cleveland Bay.
Water clarity improved in some rivers and creeks across the Townsville Dry Tropics, including the Bohle River, and Althaus and Sleeper Log Creeks in the Black Basin. However, both Crystal and Bluewater Creeks in their estuarine zone saw a drop in water clarity.
Total Suspended Solids (TSS): what’s floating around?
TSS includes a combination of inorganic particles, like sediment, sand, and silt, and organic solids such as algae, bacteria, decaying plant matter, animal waste, and microplastics that are suspended in the water.
Just like turbidity, high levels of suspended solids can reduce sunlight, which is essential for both seagrass and coral because they convert light energy into chemical energy, to produce food. As suspended solids sink and settle on the seafloor they can also smother these delicate habitats.
What’s the difference between turbidity and TSS? Turbidity measures how light is scattered and absorbed, whereas TSS measures the quantity of solids in the water.
Storm runoff, coastal development, dredging, and treated wastewater are all common sources of suspended solids in marine environments.
TSS is only measured in the inshore marine zones, which includes Halifax and Cleveland Bays, with the latter receiving a ‘good’ grade for TSS, rising from ‘moderate’ the year before.
Secchi: seeing into the blue
This is a simple way to measure water clarity. A black and white disk is lowered into the water, and the depth at which it disappears tells us how clear the water is.
Clearer water supports healthy marine ecosystems and shows how sediment and algae levels are changing.
Since the first Waterways Report Card, released in 2019, Halifax Bay has oscillated between ‘poor’ and ‘very poor’ grades each year.
Why it all matters
In the Report Card, a waterway might receive a ‘good’ overall rating, but when you dig deeper, results across nutrients and phys-chem could show imbalances that warrant a closer look.
Monitoring a range of indicators allows managers, scientists, and communities to make informed decisions and take action to improve the quality of water flowing to the Great Barrier Reef.
How you can help improve phys-chem properties
While phys-chem water quality indicators are often influenced by natural factors, our everyday actions still have a big impact, especially when it comes to reducing pollution and erosion.
Keep soil in place. Avoid clearing vegetation along creek banks or near waterways. Plants help stabilise soil and reduce erosion. In your garden, use mulch or ground cover to stop soil washing away during rain.
Keep an eye on runoff. Every time is rains, pollutants like chemicals, detergents, or paint, wash down storm drains and end up in our waterways.
Reduce use of fertilisers and pesticides. When synthetic fertilisers run off lawns and into waterways, the extra nutrients can fuel algal growth and reduce oxygen levels in the water. Choosing friendlier alternatives, like compost or natural options, are things we can do to lower the potential impacts of fertilisers and pesticides in our rivers and creeks.
Get involved. Trees, mangroves, shrubs, and grasses can be a waterway’s best defence against runoff. Many of our Partners are delivering on-ground projects to increase this natural defence system around Townsville’s waterways, and some could use an extra pair of hands. Check out the volunteer gigs to find a tree planting event happening soon.
Stay informed. Curious how your local rivers and creeks are doing? See how they’re tracking in the Waterways Report Card.
