Port Hills fires knock health of waterways
The Port Hills fires have added to the pressures already facing some of Christchurch’s polluted waterways.
In the days after the fires, parts of the Cashmere Stream turned a chocolatebrown colour from sediment washing down from the hills.
Erosion from the hills has long been an issue for the waterways beneath, but large areas of burnt vegetation had exposed already loose soil to the elements.
Heavy rain risked washing the soil into waterways, smothering some of the last remaining aquatic life in the city’s urban streams.
The Cashmere Stream is the major contributor to the Heathcote River and is its primary source of sediment, studies have shown.
Despite having sediment issues dating back decades, the stream remains a stronghold for some freshwater species in Christchurch, such as freshwater crayfish (koura) and freshwater mussels.
More sediment going into the stream risked further damage to their already polluted habitat.
‘‘It’s a bit of a death by a thousand cuts-type scenario,’’ said Shelley McMurtrie, principal aquatic ecology scientist at EOS Ecology.
‘‘It does show you that our waterways are resilient and they can bounce back from these things, but there is that ongoing aspect that this is just another nail in the coffin.’’
‘‘It’s the equivalent of us living in a smoggy city constantly.’’
Sediment was one of the primary factors in reduced waterway health, she said.
Revegetation should begin immediately along with other short-term erosion control measures.
‘‘When you have very dirty-looking water and it’s dirty for a continuous period of time, it affects the ability of the animals living in that water to properly breathe and to feed.
‘‘It’s the equivalent of us living in a smoggy city constantly.’’
Much of the stream passes through private land, meaning any control measures would likely require their cooperation.
The Cashmere Stream care group has lobbied authorities to act urgently before heavy rain arrives.
‘‘Already we’re approaching the fiveweek mark and we’ve actually done nothing,’’ said chairman Ken Rouse.
‘‘I appreciate that there’s a lot to be done in terms of assessing it, but we’ve not put any concrete plans in front of private landowners and so forth on what leadership the council can offer.’’
The most urgent issue were the firebreaks, he said, which would act as funnels for soil washing into waterways.
Recent erosion control trials on the hills had been successful, and there were opportunities to build infrastructure to deal with the problem more permanently, but provisional work needed to start quickly.
‘‘The gentle rain we’ve had has not really produced any erosion, but that’s more good luck than anything.
‘‘There are solutions out there. They’re not bullet-proof but they certainly will improve outcomes dramatically, Rouse said.
‘‘There does need to be a serious vote to make sure that this is effective, because it has the potential to be a real disaster.’’
The Christchurch City Council said assessment work regarding run-off would begin on Monday.
It would be funded from existing budgets and likely involve putting in place silt traps, such as hay bales, along with revegetation.
‘‘As a council one of our priorities is to protect and enhance our waterways and the decision to approve this work allows us to work with landowners to implement short-term solutions,’’ said mayor Lianne Dalziel.
More long-term work may involve creating settlement ponds and strategic replanting to create natural fire breaks, said Christchurch Recovery Manager Murray Sinclair.
The council had spoken to 10 of the largest landowners in the area as well as the Christchurch Adventure Park.
There would also be a focus on reducing ash and sediment making its way to Lyttelton Harbour, he said.