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Mahurangi action plan

Mahurangi Harbour research and monitoring

The Mahurangi Action Plan is a joint ARC and Rodney District Council project.

Mahurangi Harbour research and monitoring

Why conduct research in Mahurangi?

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The Mahurangi Action Plan (MAP) was launched as a proactive response to increasing sedimentation in the Mahurangi Harbour.

Environmental monitoring showed that sedimentation was causing a decline in the overall health of the Harbour. Sediment loss from rural land use activities was considered to be the dominant source of sediment entering the estuary, with forestry activities considered to be an important secondary source.

Research, on the sources of sediment, both natural and human induced, and to quantify the relative contribution various sources make has been a key focus to date.

ARC engaged NIWA to undertake sediment source mapping to identify the sources of catchment soil contributing to sediment disposition in the Harbour.

NIWA also mapped the distribution of the soil sources throughout Mahurangi Harbour from pasture, native forest and exotic pine forest.

The results of this study are contained in the ARC technical publication 321: Sediment Source Mapping in Mahurangi Harbour (June 2006).

The findings include:

  • pastoral farming comprises 70% of the catchment area, contributes 15 - 55% of soil in the river delta and 10 - 30% across much of the rest of the Harbour
  • urban development makes up approximately 4% of the catchment area and contributes little to sediment load
  • native forests which make up approximately 20% of the catchment area also contribute sediment. This source contributes <30% in the river delta and generally less than 10% elsewhere
  • forestry comprises 8% of the catchment area. It contributes a locally high proportion of sediment in the river delta (45 - 80%) and about 14% of the total sediment load across the whole Mahurangi Harbour
  • on average, exotic pine forest land use contributes almost twice as much soil per unit area of catchment as pasture and native forest land. However most of the sediment load comes from pasture and native forest in the small sub-catchments along the side of the harbour
  • if forestry harvesting on steep land exposes bare soil to a storm event, the proportions of sediment from pine forestry will increase dramatically
  • most of the sediment load on Mahurangi Harbour is delivered in a small number of storm events each year.

Joint monitoring

The ARC and RDC are developing a monitoring strategy for the catchment. Existing monitoring and new initiatives underway include:

  • quantifying sediment generation
  • quantifying changes in land use
  • climatic monitoring
  • state of the Environment (‘outcome') monitoring
  • intervention / ‘output' monitoring.

Further investigations undertaken under the MAP umbrella include:

  • developing a field based system for documenting freshwater and saltwater riparian margins, and the condition of these margins
  • a mangrove study which compares aerial images of the catchment from 1960 with 2004 to quantify the rate and extent of mangrove colonisation.