More needs to be done to protect rivers and lakes from stormwater runoff, Wanaka residents say.
Anna Simmonds, of Wanaka, says she has observed damage in the Hikuwai Reserve and Rockabilly Gully that appears to be caused by stormwater runoff from new subdivisions.
Photos she took after heavy rain on Sunday showed surges of water running through the reserve, causing damage and depositing large amounts of sediment into the Upper Clutha.
“Every significant heavy rain we get, it is racing through the reserve and really gouging out quite a bit of that.”
Chris Arbuckle, of Wanaka, had spent more than 20 years in the freshwater industry, including five years at the Ministry for Primary Industries as a senior policy analyst.
He founded the Touchstone Project as a “direct-action initiative to support those concerned about the Lake Wanaka water catchment”.
When former farmland was developed into new subdivisions there was a lot more runoff, as water was no longer absorbed, he said.
“When you change an area that was previously paddocks into pavements, water runs off with more ferocity, more speed, and that is what I see happening with the Hikuwai situation.”
A keen mountain biker, he had been through Hikuwai Reserve only a few days ago and had seen a lot more water flowing down through the reserve.
“There is nothing to soak up the water, basically,” Mr Arbuckle said.
Prof Gerry Closs is director of Catchments Otago, a research theme based at the University of Otago.
“I think there is just a general lack of awareness in New Zealand on the negative impact of sediments on rivers,” Prof Closs said.
Sediment had “long-lasting, negative effects”.
“A river like the Upper Clutha, because it drains out of a lake, those rivers are almost always sediment-free, so they are actually very sensitive to any impact of sediment.”
He hoped to have have a research programme running next year that would look at the impact of sediment runoff on Lake Wanaka and its tributaries, and could also include the Upper Clutha.
Prof Closs said there should be “due care and diligence” wherever there was new development, but places like Lake Wanaka and Lake Wakatipu were “special places”.
The extent of the land-clearing on some development sites was a concern for him.
Queenstown Lakes District Council (QLDC) general manager of planning Tony Avery said the council acknowledged there had been concern raised over the quality of water being discharged into the river.
“The requirement for developers to meet the approved site management plans is something that QLDC planning and development and regulatory teams have been actively engaged with.
“Both of these teams work closely with developers to monitor the implementation of interim (during development) and final (post-development) management systems in their ability to limit discharge to no more than pre-development volumes.
Otago Regional Counil monitoring showed that “the water being discharged, whilst discoloured after events such as significant rainfall, is not silt-laden and therefore meets their water quality standards”, he said.