The fires may have gone, and the drought eased, but the much needed rain that pelted the Mid Coast earlier this year (in combination with the previous two factors) has caused a flow-on effect that is damaging and changing our environment and may take decades to see in its fullness.
Aquatic ecologist, Dr Keith Bishop has been deeply concerned by the severe impacts of sedimentation on rivers and steams since the early 1990s.
He is even more concerned now that he is witnessing changes on his home patch of the Mid North Coast of NSW, following the worst drought in the area in recorded history, and the bushfire catastrophe of 2019/2020.
Sedimentation is the overloading of our waterways by mud, sand, gravel and rocks, settling to the bottom of waterways and subsequently filling up deeper water holes where our valuable aquatic fauna take refuge in periods of drought.
"I have encountered this in hideous large scale in mine-affected rivers in Papua New Guinea, and more locally in the Hastings River, where long term farmers lament the transformation over the decades of the lower river about the tidal limit from a string of deep waterholes into shallow gravel 'races' which dry out in even a mild drought," Dr Bishop said.
"At a smaller scale I have seen magnificent sparkling jungle-clad streams in the Solomon Islands transformed by poor forestry practices into sluggish muddy swamps which are a haven for malaria mosquitoes.
"And again more locally in the Hastings catchment, where a beautiful forest-clad creek, loaded with spiny crayfish, was transformed by poorly planned dam construction activities into a lifeless 'quicksand' trap overwhelmed by sticky yellow mud metres deep."
Impacts at home
The Tarbuck Bay fire in late 2019 brought major sedimentation impacts to the creeks that run through Dr Bishop's property.
The impact was due to a combination of factors - the fire, the drought that devastated much of the vegetation that binds the land together, steep country, and the flooding rains of February.
The rain broke up and dislodged soil and land and sent huge quantities of rocks, gravels, sand and mud into the once moss-enclosed creeks.
"No longer green and mossy - the creeks now look more like quarrying areas," Dr Bishop said.
The rocks ended up in the steeper sections, while the lighter materials flowed down to flatter areas, where gravel, sand and charcoal was deposited up to half a metre depth.
"In a small farm dam on one creek about 20 tonnes of gravel was deposited, half filling it," Dr Bishop said.
"Over time much of this material is bound for Smiths Lake, being progressively pushed downstream by runoff from major rainfall events.
"There is little doubt that most of the very fine material has already reached the lake, making it more sticky and unpleasant to wade into, and given the nutrients that always come with such loads of sediment, opening up the possibility of algal blooms and unusual plant growths in the future," Dr Bishop said.
Impacts in the Manning Valley
Dr Bishop regularly surveys the Manning River and has done for many years.
"Sedimentation may lead to the filling-in of deep pools which are important drought refuges for fish, platypus, turtles and the like," Dr Bishop said.
"One such refuge pool in the Manning River upstream of Mount George, where a tributary stream draining a largely cleared catchment, is feeding in large quantities of rock, sand and gravel, so much so that a delta of this material is forming in the middle of the pool. Another such delta occurs where Dingo Creek joins the Manning River," Dr Bishop said.
The Manning catchment received the same flooding rains as the Great Lakes in February 2020.
"Although not hearing of any 'black porridge' events associated with first flows from fire affected tributary streams, as were reported in the neighbouring Hunter River catchment, I did expect the river to be carrying exceptional loads of sediments from steeper country given the drought and the fires," Dr Bishop said.
The Nowendoc River at Rocks Crossing had enough data for Dr Bishop to do "sensible comparisons" of turbidity (river dirtiness) between the February 2020 floods and similar previous floods.
Dr Bishop found that river turbidity in February 2020 reached levels about four times as high as experienced in the previous similar floods, meaning exceptional loads of sediment were present.
"Even this may be an underestimate as it appeared that the meter reading 'topped-out', not being able to read extremely high levels of turbidity," Dr Bishop said.
Two weeks after the peak of the February floods, Dr Bishop was shocked to see the Bight Bridge at Wingham was still very heavily loaded with sediment, and, more surprisingly, the amount of sediment that had been deposited on the Wingham Foreshore Brush Reserve.
"The amount of sediment strongly appeared to be disproportionately high given the moderate size of the flood," he said.
Dr Bishop also visited the headwaters of Dingo Creek, where out of control fires burnt through very steep country.
Again there was evidence of large loads of sediments either having entered the creek system or poised to enter with follow-up heavy rains.
A waiting game
Dr Bishop says given the visible evidence of sediment loads, the record-breaking nature of the drought, the extensiveness and power of the fires across the Manning catchment, and the intensity of the February rains, there are undoubtedly huge amounts of sediments either in the upper reaches of tributaries, or poised to enter those tributaries with the next heavy rains.
"For the further downstream deepwater drought refuges, that kept our valuable aquatic fauna alive in the recent tough times, there is a waiting game here," Dr Bishop said.
"It can take multiple strong rainfall events, over years, for the sediments to work their way downstream filling channels."
Dr Bishop fears a grim future for our drought refuges, with climate change meaning a possible future of more intense droughts, more intense rainfall events, and more devastating fires.
Human activities of clearing steep land and regular burning of the landscape will make the situation worse, Dr Bishop said.
"Let's hope that does not play out. Perhaps extremely strong rainfall events will flush out material from our rivers, but where will it go?
"Obviously the more sluggish-flowing estuaries and then, step by step over the decades, eventually stalling at the ocean mouth."
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