Widespread hill slope erosion associated with grazing and agriculture has long been assumed to be the primary cause of damaging terrestrial run-off onto the Great Barrier Reef, but the surprise findings of a Griffith University research project may overturn that.

Princess Charlotte Bay study

Fluvial geomorphologist, Dr Andrew Brooks from Griffith’s Australian Rivers Institute has been leading a four year study funded by the Federal Government’s Reef Rescue Program, into the source of sediment to Princess Charlotte Bay in Cape York, an area adjacent to the northern-most section of the Great Barrier Reef.

“We have effectively re-written the story on where people thought sediment was coming from in the Normanby Basin, which potentially has implications for the rest of the Great Barrier Reef,” Dr Brooks said.

The key reason for this is that instead of relying on desktop modeling, which is what the previous understanding was largely based on, we have expended huge effort in going out and actually measuring these processes in the catchment and then building a new model based on these data,” he said.

Sediment sources

“The evidence suggests that rather than all of the slopes across the catchment contributing the sediment, it is instead highly concentrated in certain parts of the landscape. The primary culprits are river bank and gully and erosion — particularly from within certain parts of the floodplain and the river margins.

“Our research shows that around 60% of the sediment that makes its way from the upper Normanby catchment to Princess Charlotte Bay comes from these areas.

“We have also identified the coastal plain as a major source and one which is even greater than the upper catchment sediment sources; a process no one has taken into account before.

“While it might seem esoteric to say erosion is coming from gully and river bank erosion rather than just off hillsides everywhere — this fundamentally changes how we should be managing the landscape if we want to reduce erosion.”

And there is much at stake in getting the management right because $100s of millions of dollars are being expended by the Qld and Federal Governments in trying to improve water quality flowing onto to the Reef.

Dr Brooks said in order to effectively utilise these resources it is critical to fully understand what is going on in the landscape.

“This field data driven approach has highlighted some concerning issues with the models that could well have implications management prioritisation across all of the Great Barrier Reef catchments”.

“The reason we have this false impression of what was going on in the Cape York landscape is because the models were massively over-predicting hill slope erosion and completely ignoring other processes — which are the real source of the problem”.

The Crown of Thornsstarfish

The northern part of the Great Barrier Reef has also been shown to have largely been spared the dramatic coral decline evident elsewhere on the Great Barrier Reef over the past 30 to 40 years., but there are some worrying signs coming from this and other research that the northern part of the Reef is not immune from impacts such as the Reef’s old nemesis, the Crown of Thorns starfish, outbreaks of which are now known to be driven by poor water quality.

“Given that the northern part of the reef is really the Jewel in the Crown of the Great Barrier Reef it has never been more important to fully understand the impacts of terrestrial runoff on this part of the Reef, and what needs to be done to ensure there is no further decline in the water quality in this important area”, said Dr Brooks.