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.”
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”.
Click here to be taken to the full article