An extensive flood plume caused by the recent severe weather event in northern Queensland is pushing vast amounts of river discharge to cover about 50,000 km2 of the Great Barrier Reef from Cairns to Mackay – stretching across inshore, mid-shelf, and outer reefs.
James Cook University's TropWATER water quality expert, Jane Waterhouse, says analysis of satellite imagery shows major flooding from more than 10 river basins has merged to form extensive flood plumes, extending more than 700km along the coastline and 100km offshore in some places.
"What we are seeing here is very large and prolonged flood plumes spreading across inshore, midshelf and outer reefs, seagrass meadows and other marine ecosystems," she said.
"Outer reefs are rarely exposed to flood plumes due to their distance from river mouths. The water may not be as turbid as inshore areas, but they are still receiving terrestrial runoff."
Flood plumes reduce light to coral reefs and seagrass, slowing their growth. Prolonged low light and sediment buildup can smother seagrass and weaken corals, increasing their vulnerability to bleaching and disease.
"People often think flood plumes are just freshwater. But our modern landscape of urban development, agriculture and grazing lands means higher levels of sediment, nutrients and contaminants can runoff during flooding from gullies, farms, and urban landscapes into catchments and out to the Reef," said Jane Waterhouse.
"Elevated nutrients entering the marine environment often lead to higher levels of macroalgae overgrowth on coral reefs, lower coral coverage, and less new coral growth. Sediments delivered by the plume can remain active for months after the flood event and can cause prolonged reductions in water clarity.
"Good water quality helps marine ecosystems thrive and bounce back from threats like mass bleaching. But flood plumes can put them under additional pressure, and their impact depends on how long they last, how intense they are, and how resilient the ecosystem is."
Tracking flood plumes across river basins in the Great Barrier Reef
TropWATER remote sensing scientist Caroline Petus said satellite images has been a critical tool under the Marine Monitoring Program to track flood plumes from each river basin.
"Analysing satellite images gives us a birds-eye view of the situation and helps us guide field teams to the right locations to collect water samples to assess water quality."
Major to moderate flooding was recorded across nearly every river basin from Cairns to Mackay, creating widespread flood plumes, with significant discharges from the Burdekin, Haughton, Ross, Black, Herbert, Murray, Tully, Johnstone, and Russell-Mulgrave rivers.
TropWATER's Stephen Lewis said the Burdekin River, one of the largest contributors to flood plumes on the reef, recorded its biggest peak flood discharge since 2009.
"In just 14 days, the Burdekin River discharged 15.6 million ML of water, which is enough to fill Sydney Harbour more than 31 times," he said.
"At its peak, nearly 1.6 million ML per day flowed from the river. This is the highest since 2009 and larger than the 2019 flood event."
Analysis of coral cores shows that the size of large Burdekin River floods has almost doubled compared to floods occurring 150-350 years ago.
"These larger floods are carrying more sediments and nutrients in floodwaters due to increased water volumes coupled with land use changes," Dr Lewis said.
"What's most concerning is that these floods are prolonged with elevated discharge occurring for over a week. This means that the offshore marine areas are exposed to poor water quality for longer periods.
"We're now seeing offshore reefs being affected more frequently from flooding, and we don't yet fully understand the long-term consequences of that exposure, in combination with other disturbances."
Managing agricultural runoff
Sugarcane farming is the largest agricultural industry along the Great Barrier Reef coast. During the wet season, fertilisers and pesticides are more likely to run off paddocks, as this period aligns closely with the preceding crop harvest and fertiliser application period.
TropWATER's Dr Aaron Davis said while many growers adapt their practices around seasonal rainfall conditions to reduce fertiliser and pesticide runoff, extreme floods like this are beyond management control.
"This level of flooding is devastating – entire crops in the Ingham region have been significantly impacted," he said.
"Events of this scale don't happen often, are difficult to plan for, and highlight the challenges of farming in the tropics."
Dr Davis said helping affected farmers get back on their feet is the first step in minimising longer term environmental impacts from such a major event.
The floods also provide an opportunity for scientists to assess how well remediation efforts to reduce gully and streambank erosion have held up and how much sediment has been lost from catchments into the Great Barrier Reef.
Water quality monitoring is part of the Marine Monitoring Program, coordinated by the Great Barrier Reef Marine Park Authority, in partnership with JCU TropWATER, Cape York Water Monitoring Partnership, Australian Institute of Marine Science and the University of Queensland.
Detailed assessments of impacts on seagrass meadows and coral reefs will be undertaken in the coming months by JCU TropWATER and the Australian Institute of Marine Science.
