Forestry link to river damage

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 the 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’’.

In the lower reaches, bank erosion was the major source, with hotspots around Neds Creek and Brook Stream, the report said. 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 subcatchment, produced almost 20 per cent of the sediment in the lower Maitai River, the report concluded.

Previous research on the river showed that fine sediment was smothering flora and fauna habitats, and was as a potential driver for toxic cyanobacteria blooms.

The 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 runoff. 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.

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’’.