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Waikato River

The Waikato River is the longest river in New Zealand. It starts its journey in the central North Island volcanic zone and flows into Lake Taupo. From there it continues its journey north where it flows into the Tasman Sea at Port Waikato. The catchment area is approximately 14456 square kilometres. The main stem of the river is 336km long, with about 22478km of tributaries. The catchment consists of moderately steep through to undulating topography and the river passes through predominantly pasture, with areas of indigenous forest and plantation forest. 

The dominant underlying geology is a mixture of tephra, pumice, alluvial and greywacke. Several aspects of the river’s water quality are considered to be excellent for sustaining aquatic life (e.g. dissolved oxygen, total ammonia, pH).

Other aspects are good or better in the upper river and the hydro-lakes, but are poorer further downstream (e.g. clarity, e. coli and nitrogen)—although even then, conditions are generally better than in rivers in many other developed countries. However, concentrations of geothermal contaminants tend to be close to ecological guideline values in the upper river, but further downstream they are diluted somewhat (e.g. arsenic and boron).

Over the past 20 years there have been both improvements and deteriorations in water quality throughout the river. Continued improvements in wastewater treatment have meant that concentrations of some contaminants have declined (e.g. total ammonia and BOD). But at the same time, concentrations of nitrogen in the river have slowly increased, reflecting the ongoing increases in the many tributaries that drain areas of farmland.

At present, the main cause of concern about the water quality of the Waikato River over the next few decades is the prediction that increasing loads of nitrogen and phosphorus entering the river from larger and more intensively-used areas of farmland will increase the risk of harmful algal blooms occurring in the river.     

Waikato Regional Council and Waikato and Waipa River iwi are partners on Healthy Rivers: Plan for Change/Wai Ora: He Rautaki Whakapaipai. This project worked with stakeholders to develop changes to the Regional Plan to help restore and protect the health of the Waikato and Waipa rivers. The plan change will help to reduce sediment, bacteria, nitrogen and phosphorus entering water bodies, including groundwater, in the Waikato and Waipa River catchments. More information is available at

Sites 128

Monitored sites in the Waikato River catchment

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