Water
quality
KEY FINDINGS FROM OUR 2025 REPORT CARD
Townsville’s two main basins, the Ross and the Black, both maintained ‘good’ water quality in their freshwater (upstream) and estuarine (downstream) areas, as they have done since the first Townsville Dry Tropics Report Card.
However, water quality is made up of many moving parts, and while the overall grade might appear steady from year to year, what’s happening beneath the surface could be shifting. That’s why it’s important to look at the details in the data, and not just the final mark.
Beneath the surface in the Ross Basin Freshwater zone
In the 2025 Report Card, the Ross Basin freshwater zone receives an overall ‘good’ grade for water quality. On the plus side, nutrient levels improved in both the Upper and Lower Ross. But this was offset by a drop in low dissolved oxygen at each of the weirs in the Lower Ross, where grades fell from ‘good’ or ‘moderate’ to ‘poor.’
Over in the Bohle River freshwater sub-basin, nutrient levels were still high, and although turbidity improved from ‘good’ to ‘very good,’ it was counterbalanced by low dissolved oxygen dropping from ‘poor’ to ‘very poor.’ This resulted in the Bohle sub-basin maintaining a ‘poor’ water quality grade.
Diving into the Ross Basin Estuarine zone
Closer to the coast in the Ross Basin estuarine zone, water quality also remained ‘good,’ but the overall score dipped slightly.
Total phosphorus declined from ‘very good’ to ‘good’ in the Bohle River, as well as Sandfly and Alligator Creeks. The largest drop in nutrients, however, was seen in Louisa Creek, where dissolved inorganic nitrogen fell from ‘good’ to ‘very poor.’ Combined with already low scores for total phosphorus and low dissolved oxygen, Louisa Creek is out of balance and needs a closer look to understand the drivers.
The decrease in low dissolved oxygen scores seen in the freshwater zone, was also seen in the estuarine zone, with grades dropping from ‘very good’ to either ‘good’ or ‘poor’ across monitoring sites. The movement of water in Cleveland Bay, such as tides and waves, could be playing a role.
Short on air, why are oxygen levels low?
Oxygen in the water is vital for fish, plants and other aquatic life. When levels become too low, it impacts their survival. Many things can influence low levels of oxygen, such as temperature, algal blooms and weeds.
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.
Excessive nutrients can 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.
Unwanted, invasive aquatic weeds can also choke creeks and use oxygen needed by native plants, fish, and animals.
Without measuring additional indicators, such as weeds and algae, or taking a deep dive into other parameters like temperature, it’s difficult to pinpoint what’s causing low dissolved oxygen levels.
Getting to the bottom of the Bohle’s water quality grades
This is the sixth year in a row that the Bohle River freshwater sub-basin has received a ‘poor’ water quality grade, whilst grades in the Bohle estuarine sub-basin remained ‘moderate’ for the third consecutive year.
A lot of Townsville’s housing, industry and agricultural growth has occurred in the Bohle sub-basin over the past decade, putting a significant urban load on the catchment. Sustainable urban development practices, such as water sensitive urban design, could alleviate some of this load.
With urban development continuing, it’s important to keep a close watch on how this impacts the entire Bohle sub-basin, which includes Louisa Creek, Stony Creek, Saunders Creek, and the Little and Middle Bohle. Expanding the number of sampling locations would help uncover the range of influences behind the ‘poor’ and ‘moderate’ water quality grades. We are working with our Partners on this issue and welcome new partners to the table.
Behind the scenes of the Black Basin Freshwater zone
​For the sixth year in a row, the Black Basin freshwater zone has held onto its ‘good’ grade for water quality and received its highest water quality score since the Report Card begun.
There were noticeable improvements in turbidity in the Black River, Rollingstone and Bluewater Creek sub-basins, with the biggest swing at Althaus Creek which jumped from ‘very poor’ to ‘moderate.’ This increase is after several years of low scores and will need to be monitored for consistency.
Bluewater sub-basin also showed positive gains in low dissolved oxygen levels. However, Ollera Creek retains a ‘very poor’ grade for the third year running.
Without further investigation, it is unclear what is influencing these fluctuations.
Behind the scenes of the Black Basin Freshwater zone
The estuarine (downstream) areas of the Black Basin are still sitting at a ‘good’ overall grade, but saw a slight dip in score this year.
The only estuary to decline, from ‘good’ to ‘moderate,’ was Crystal Creek. This was largely driven by elevated nutrients and changes in turbidity.
In the Rollingstone downstream environment, which includes Camp Oven, Saltwater and Rollingstone Creeks, all saw a drop in scores for dissolved inorganic nitrogen but also improved water clarity. The most influential decrease in this estuary was in Camp Oven Creek, where low dissolved oxygen fell from ‘moderate’ to ‘very poor.’
Around Bluewater estuary, the Althaus Creek turbidity indicator shows ongoing low scores and a ‘very poor’ grade, whilst both Bluewater and Sleeper Log Creeks dropped to ‘poor.’
Clearing up turbidity
Turbidity refers to the clarity of the water. The higher the score, the clearer the water. When water is cloudy, due to things like sediment, silt, or organic matter, it can block sunlight, affecting the growth of aquatic plants and aquatic animal’s ability to navigate.
Lower rainfall totals may have contributed to the drop in turbidity grades across the Black Basin estuarine zone. In drier years, sampling opportunities are limited because creek beds are often dry. When rain finally arrives, it can stir up fine sediments from the creek bed, causing turbidity measures to increase.
Pesticides
Pesticides, which also includes insecticides and herbicides, are used to manage pests and unwanted plants. In the Townsville Dry Tropics, there are only two sites, one in each basin, that are monitored. This year, the Ross sample site saw a slight rise is pesticide risk, from ‘very low’ to ‘low.’ Sometimes just detecting a pesticide, regardless of the concentration, can change the score due to its toxicity.
Compared to the 2024 Report Card, two new pesticides were recorded, while one from last year didn’t show up this time around. Because different pesticides affect aquatic life differently, knowing what’s in the water and at what levels, helps us better understand impacts and what it could mean for the health of the ecosystem.
We can also help reduce pesticide risk, starting in our backyard. Everyday products that we use on our garden can end up in our waterways, if we’re not careful. Choosing natural options and following label instructions, are things we can do to lower the potential impacts of pesticides in our rivers and creeks.
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Pesticides could be a wider problem in the Dry Tropics, but it is difficult to determine without increased sampling effort.
