Stream clean-up aims to ‘tempt’ trout back
12 December 2016

Sixty-five tonnes of accumulated silt has been blasted out of a Christchurch stream in an effort to entice trout back to breed, and improve the habitat for other aquatic life.

Recently North Canterbury Fish & Game staff spent two weeks co-ordinating the removal of sediment from the Wairarapa Stream in western Christchurch.

The stream is one of the Avon River’s most important tributaries.

The operation removed silt which had accumulated over the years from storm-water drains flowing into the once-pristine stream. The goal was to make the stream suitable for trout to spawn in, and habitable by other aquatic life.

“Silt layers on top of the stream gravels had smothered the invertebrates that live in the stream bed, and by removing the sediment aquatic life has the chance to prosper again,” says Fish & Game Officer Emily Arthur-Moore.

In total, around 540 square metres of streambed near the Jellie Park Recreation and Sport Centre in Ilam were cleaned. That equates to 120 kilograms of silt per square metre cleaned.

The project had planned to be carried out over a longer reach but the techniques took longer than expected.

“Various techniques were used during the two weeks, including water jet blasters, diggers and a specialised piece of equipment called a ‘sand wand,’ Ms Arthur-Moore says.

Support was given to the project by the Christchurch City Council, Environment Canterbury and Nelson-based dredging and water treatment consultants Southwater, who also provided various pumps, hoses and dosing equipment ‘gratis’.

The silt-laden streambed was agitated using either the water jet blasters or the sand wand and the disturbed silt was then captured and then removed to a de-watering site.

After a process to remove the silt, cleaned water was returned to the stream and sediment remaining was taken to a landfill for disposal.
Monitoring was carried out throughout the project to assess levels of sediment going downstream.

“Large amounts of rubbish were also removed during the process, mainly broken glass and crockery but also items of footwear, batteries and plastic.”
Ms Arthur-Moore says that the twelve volunteers who answered the call to help out are to be congratulated.

“They joined staff from Southwater, CityCare and Fish & Game during the fortnight’s work.

“I’m overwhelmed by the anglers and others who volunteered and who put in many long hours make the project work” Ms Arthur-Moore says.

One volunteer was Tony Findlay, who grew up in the area 50 years ago “He recalled being able to drink the stream water from the three springs that once fed the stream bed that he was working in, ‘not that you would do that now,’ he told us.”

The stream will be monitored over the coming years to see if the improved habitat increases the diversity of invertebrates, which trout predate on. Also of interest is the degree that silt may return to cover the gravels.

Ms Arthur-Moore says it’s hoped the techniques and knowledge gained from the project can be applied in other enhancement projects around the region.

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