When the Federal Government approved the bulldozing of nearly 2,000 hectares of forest on Cape York's Kingvale Station this month, critics warned that the clearing would add to the sediment load running onto the Great Barrier Reef.
Many said the approval undermined the Government's $500 million budget commitment to protect the reef, and was at odds with Great Barrier Reef Marine Park Authority (GBRMPA) advice, which lists land-based run-off as a significant threat.
So how does land clearing affect sediment runoff, and what implications can this have for downstream marine environments like the Great Barrier Reef?

Sediment flowing onto reef has increased four-fold

Large-scale clearing is usually done by dragging 100 metres or more of ship-anchor chain between two bulldozers.
The desired effect is that everything between the dozers is mown flat.
Typically this type of clearing is done to make way for cropping, or to enhance grass cover for cattle grazing.
Denuding the landscape in this way has a double-barrel effect on erosion, according to Professor Jon Olley from the Australian Rivers Institute at Griffith University.
Removing the tree canopy exposes the soil to erosion from direct rainfall impact, and destroying the tree roots removes the "reinforcing" that holds the soil together.
"Studies have shown that runoff in subtropical regions increases by a factor of about four when trees are removed and replaced with paddocks," Dr Olley said.
In total, the Great Barrier Reef receives run-off from 35 basins, and river discharge is the biggest source of nutrients to inshore waters.
GBRMPA estimates that sediment inflow into the Great Barrier Reef has increased more than four-fold in the last 150 years, and by as much as 10 times in more disturbed catchments.

Tropical rains compound erosion

Rain events in the tropics can be severe.
Tully, in the wet tropics south of Cairns, averages more than 4 metres of rain annually, with most of this falling between November and April.
These intense downpours increase the potential sediment load that can be washed away, and also mean the sediment can be carried further downstream to the reef, according to Dr Olley.
"Removing vegetation increases the landscape's vulnerability to those couple of large storm events that provide most of the runoff in the year," he said.
That material can often include nutrients like nitrogen and phosphorus from fertiliser, and also pesticides, if the runoff is coming from agricultural land.
Some pesticides can have direct toxic effects to marine organisms, while the nutrients can be taken up by phytoplankton to form large algal blooms.
Although the sediment is eventually washed into the ocean, increased runoff into river systems can also be problematic.
Channels are filled in, and tidal flow can be affected.
"You end up with less complexity in the channel system, and so less diverse habitat, and so the whole ecology of the system is affected," Dr Olley said.
Toward the river mouths, mangroves require a balanced sediment load, though they are more resilient than reefs and seagrass according to Dr Frederieke Kroon, a coastal ecologist at the Australian Institute of Marine Science (AIMS).
"Mangroves actually need sediments, to a certain extent, to trap and increase the area where they can grow into," Dr Kroon said.
"But you don't want too much sediment … if the mud layer around mangroves gets too high, then you don't get that tidal movement anymore, which mangroves need to survive."

Too much sediment can kill coral, seagrass

Declining water quality from land-based runoff is listed as "one of the most significant threats" to the reef by the GBRMPA.
Similarly, fragile seagrass ecosystems can suffer due to declining water quality, according to Dr Kroon.
Seagrass, like all plants, requires sunlight to produce food through photosynthesis, as do corals containing symbiotic algae in their tissues that provide them with food.
The light is reduced even more if there are algal blooms from increased nutrients, and after too long without sunlight, corals and seagrass will die.
In turn, animals like dugong which rely on these habitats for food also die or are forced to relocate.
As well as blocking sunlight, sediments also smother corals — young and old — stunting or completely stopping their growth, Dr Kroon said.
"Corals can deal with being covered in sediments to a certain extent by removing particles on them with their tentacles. But if you get too much sediment they can get overwhelmed, and eventually die," she said.
Sediments can cover all the surfaces on a reef, including bare rock, which is the preferred landing spot for coral larvae to settle and grow.
"When hard surfaces in coastal waters are covered by sediment, it makes it difficult for these larval corals to recruit to, which means there's no reef recovery," Dr Kroon said.

'Brushing and scraping' fish unable to keep up

On a healthy coral reef, one group of fish plays an important role in removing and cycling sediments.
These fish unwittingly ingest sediment that is attached to the algae they feed on, according to reef-fish ecologist Sterling Tebbett from James Cook University (JCU).
"Some fish have specialised teeth that act like brushes, and they brush the sediment and their food from the algae, and they've got little toilet sites off the reef where they deposit this," he said.
"Others, like parrotfish, scrape the entire reef surface of algae.
Without the brushing and scraping fish, the algae lawns they normally feed on go from being neatly mown to longer and laden with sediments.
"When more algae grows it limits the reef's ability to recover," Mr Tebbett said.
"Corals can't recruit to surfaces dominated by these longer algae turfs, and it is harder for the reef to regenerate following a disturbance like bleaching."
The sediment-removing fish help keep reef processes working, but too much sediment and they can no longer do their jobs.
While the worst impacts of sediments are felt on inshore reefs, storms, cyclones and flooding can cause the footprint to spread much further out from the coast, Dr Kroon said.
"During really big floods the plumes can make it out to the mid-shelf reefs, and sometimes to the outer reef depending where you are," she said.
"Around the wet tropics where the outer reef is only 50km away from the coast, it can definitely happen, and it has happened before."