More than 90 per cent of sediment in the Tauranga Harbour is made up of erosion off stream banks, landslides and earthworks, a comprehensive report has found.

Tauranga Harbour. Photo: File.

The National Institute of Water and Atmospheric Research (NIWA) released its findings on how much of the sediment run-off into Tauranga Harbour is from stream bank erosion to the Bay of Plenty Regional Council last week.

The project commissioned by the regional council last year is part of efforts to care for the harbour, says Bay of Plenty Regional Council direction and delivery committee chair Paula Thompson.

To fill the knowledge gap, NIWA undertook a case-study in the Kopurererua River catchment.

Research concludes that at least 95 per cent of the sediment that washes from that sub-catchment into the Tauranga Harbour comes from subsurface erosion off stream banks, landslides and earthworks.

Stream bank erosion was identified as the single most important source.

'Sediment is a concern in the harbour because it covers over sandy areas, encourages mangrove growth and has the potential to smother kaimoana (shellfish). It can also increase the amount of phosphorous in the harbour which may contribute to blooms of algae.”

'NIWA provided some good data on sediment flows into the Tauranga Harbour in 2010. It told us that pastoral areas contributed the most sediment, but we didn't know what proportion of that was coming from land run-off such as from farm tracks or cultivated land, versus stream bank erosion.”

In the last two years council has helped landowners to fence and plant of more than 100 kilometres of river and stream margins on waterways that feed into Tauranga Harbour.

'We've always known that riparian restoration was important for improving water quality but we couldn't quantify how much difference it was making to harbour sedimentation.

'The NIWA study has confirmed it's a worthwhile investment that we should continue to support.”

Further information for landowners or Maori Land Trusts about how the regional council can help them to care for their land and waterways is available here or by calling a land management officer on 0800 884 880.