Forestry linked to damaging sediment in Nelson river
Monday, 30 April 2018
Pine forest plantations are 'probably' contributing a disproportionate amount of damaging fine sediment into Nelson city's Maitai River, new research says.
Recently harvested or replanted forestry land was a substantial source of sediment in the Maitai and tributaries in its upper and middle reaches, the NIWA research carried out for Nelson City Council showed.
Pine sediment accounted for 80 per cent of sediment below the Maitai Dam.
Sediment sources in the middle reaches were dominated by gorse and broom soil sources, which was 'almost always associated with pine forest that had not been replanted following harvest, or had been replanted, but had yet to achieve canopy closure.'
**READ MORE:
* [Failure to change forestry practices harming river ecology, group says
*](https://www.stuff.co.nz/environment/103005708/failure-to-change-forestry-practices-harming-river-ecology-group-says) Study finds pine forest link to sediment in Waimea, Moutere estuaries
* Toxic algae levels on the rise in Maitai River
* Fine sediment contamination of waterways always negative, says TDC scientist
* Tasman farmer adds to calls for tougher controls on forestry
* Forestry under fire as communities left in shock from ex-cyclone Gita**
In the lower reaches, bank erosion was the major source, with hotspots around Neds Creek and Brook Stream.
Bank sediment was 'likely to have been derived from soils that were previously from pine forest.'
Deep scarring, associated with the pine harvest on steep hill slopes in the Brook Stream sub-catchment, produced almost 20 per cent of the sediment in the lower Maitai River, the report concluded.
Previous research on the river showed fine sediment was smothering flora and fauna habitats, and was as a potential driver for toxic cyanobacteria blooms.
The Nelson City Council had refused to release the research earlier this month, amid calls for stronger controls on forestry practices along the Maitai River to reduce sediment run-off.
At the time it said the report was part of wider research that wasn't complete.
But on Monday, the report was published on the council website.
Council chief executive Pat Dougherty said in a letter advising Stuff about Monday's publication that while the report provided 'useful guidance on key sediment sources' for the river, it didn't 'give any indication of trends in sediment quantity or sources over time'.
The 'snapshot' research was carried out in December 2016.
Pine forest was estimated to occupy 26 per cent of the watershed at the time, with mature pine forest plantations producing very little sediment.
Replanting of forests in some areas had not occurred, or had been delayed, with the ground colonised by gorse and broom.
Meanwhile pasture-derived sediment was likely to have elevated phosphorus content, which supported the growth of cyanobacteria, or blue-green algae.
The study recommended management strategies to reduce sediment yield during harvesting, to replant harvest pine forest land, and to identify and mitigate the sources of bank and pastoral sediment.
Dougherty said the council was in the process of retiring its forestry blocks which were 'not well-suited to plantation forestry, and may have contributed to sediment issues in the past'.
The council was considering other uses for the land and developing management strategies based on slope, soil type and erodibility.
It had shared the research with forestry industry representatives, and 'active engagement' was continuing with forestry companies through the Nelson plan process.
Dougherty said national environment standards on plantation forestry, which come into force tomorrow , would be more stringent than the rules local forestry operators had been required to abide by in the Nelson Resource Management plan.
Earlier this month, residents' group, Friends of the Maitai, expressed concerns the new standards wouldn't apply to tributaries of the Maitai; where it said most of the river's sediment came from.