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Ruamāhanga River

The Ruamāhanga River is the largest river in the Wairarapa.  With headwaters in the Tararua Ranges, the Ruamāhanga River flows through the Wairarapa Valley for about 130 kilometres. It passes through several townships and extensive farmland before discharging into Lake Onoke which opens into the sea at Palliser Bay. 

The River is a popular area for a variety of recreational activities including swimming, fishing, and other water based activities which include jet boating, rafting, kayaking and canoeing.

Greater Wellington Regional Council monitors the environmental health of the river at 23 locations, and recreational water quality at 10 locations within this catchment.

Agricultural landuse dominates the catchment and is the main pressure on catchment water quality, as well as treated sewage discharges from the five main Wairarapa townships. Water quality is monitored at four locations along the Ruamāhanga River and results show a marked decline with distance down the valley.

The environmental health of the river is also monitored on many of the main tributaries, including the Kopuaranga, Waipoua, Whangaehu, Waingawa, Waiohine, Taueru, and Huangarua rivers. Generally, water quality is excellent in the western tributaries, which originate in the bushclad Tararua Range, and poorer in the eastern tributaries.

Lake Wairarapa is located within the lower Ruamāhanga catchment. The Tauherenikau and Waiorongomai rivers discharge into this lake – both of these rivers have excellent water quality.

Sites 30

Monitored sites in the Ruamāhanga River catchment

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Scientific Indicators
Scientific data for this catchment

This dashboard shows information on the data collected by the regional councils for water quality indicators, analysed as state and trend.

The state for the catchment is represented by the median concentration for the parameter across all sites within the catchment and then compares that value to the quartiles for all monitored sites in New Zealand.

Click on the parameters state icons to compare this catchment with others in the region.

  • State

    State shows how the median of samples from this site compares to other sites

  • Trend

    Trend shows how the quality of water is changing over time. Depending on the sampling history duration, five and ten year timescales are available:

Can I trust this data?

The Cawthron Institute has worked alongside regional councils to verify the processes and methods used for data collection, laboratory analysis of samples collected and the statistical analysis and interpretation of the results presented.

If all Cawthron ticks are green, then you can trust this data. However, if one or more ticks are orange, then conclusions should be treated with some caution.

For more details on each tick, see our 'Can I Trust This Data?' Factsheet.

Can I trust the data?
Data Collection

All samples were collected using approved field protocols and have been analysed in accredited laboratories. Therefore the data shown here has been collected and analysed following best practice.


All samples were collected using approved field protocols and have been analysed in accredited laboratories.

Trend - Adjustment for flow

This data is not flow adjusted. National guidelines suggest that flow-sensitive variables are flow 'adjusted' before trend analysis. Therefore, any trends shown here may be affected by variations in flow across sampling occasions.


E. coli



Black disc





Total Nitrogen


Total Oxidised Nitrogen


Ammoniacal Nitrogen



Dissolved Reactive Phosphorus


Total Phosphorus

Download Data

.ZIP file of indicator readings for all sites in New Zealand

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