a flat coral with brown fine sediment on it


Queensland’s rivers naturally deliver sand, silt and clay to inshore waters of the Great Barrier Reef

Sand is heavy and generally sinks out of the water column near river mouths, while silt and clay can be transported further out.

Heavy rain, usually between October and May, results in increased sediment and nutrients in the rivers. This can make them look muddy, and delivers large amounts of sediment to the reef.

Our research

AIMS has been monitoring water quality in the Great Barrier for four decades. Since 2005, we have sampled sediment in the reef’s inshore waters through the Great Barrier Reef Marine Monitoring Program. The results are used to track progress against targets in the Reef 2050 Water Quality Improvement Plan and the Reef 2050 Long Term Sustainability Plan.

We supplement this inshore water quality monitoring with regional models, including eReefs, to predict how water quality will respond to climate change and improvements in catchment management.

AIMS also uses satellite remote sensing images to study year-to-year changes in water quality, and determine how changes in light affect corals and seagrasses near the sea floor.

Condition / status

Erosion from large-scale clearing and livestock disturbance is causing elevated levels of sediment and nutrients to discharge into the reef’s coastal waters.

This is supported by evidence from:

  • sediment tracing research highlighting distinct sources of erosion
  • changes in sediment river loads informed by catchment modelling and river monitoring
  • reef water quality monitoring and satellite remote sensing images
  • sediment signatures in cores of reef sediment and corals.

Catchment modelling suggests fine sediment loads delivered to the Great Barrier Reef are 3 to 8 times higher than pre-development.


Suspended sediment plays an important role in marine processes and food webs. But excessive sedimentation and turbidity can damage coastal ecosystems by:

  • making seabed surfaces unsuitable for coral larvae to settle and grow 
  • limiting the light available for seagrasses, symbiotic coral microalgae and other reef organisms, which reduces their ability to feed, grow, and reproduce 
  • reducing the growth rate of adult corals and their ability to recover from other stresses such as bleaching, crown-of-thorns starfish and tropical cyclones
  • causing coral disease and mortality, and
  • reducing the likelihood of larval fish settling onto coral reefs and negatively impacting feeding behaviour of juvenile reef fish

This leads to coastal ecosystems being dominated by algae and losing their coral diversity.

Sediment effects are managed by reducing end-of-catchment sediment loads and assessing concentrations against relevant water quality guideline values.