Northern Wairoa River

The Mangahahuru Stream joins the Wairua River in the southern part of the Hikurangi Swamp. The upper reaches of the Mangahahuru are dominated by pine forestry with the lower reaches pastoral land. The productive Hikurangi Swamp is dominated by dairy farms and has an extensive drainage scheme. A dairy factory and the Hikurangi oxidation ponds both have consent to discharge waste into the Wairua River catchment.

The Wairua and Mangakahia Rivers join to form the Wairoa River just north of Tangiteroria. The Mangakahia River is 104 km long and has a total catchment area of 810 square km. The upper reaches of the Mangakahia are a mixture of plantation forestry and sheep and beef farming.
The Opouteke River is a major tributary of the Mangakahia River and is 31 km long. It joins the Mangakahia River at Pakotai and drains a predominately pine forest catchment.

The Manganui River begins along the western fringes of the Marertu Forest and flows northwest to eventually join the Wairoa River to the east of Dargaville. The river is 91 km long and drains a significant catchment area.

Another major tributary of the Wairoa River is the Kaihu River which originates in native forest to the west of Trounson Kauri Park and flows south, where it joins the Wairoa to the east of Dargaville. The catchment is a mix of exotic and native forest and agricultural land use.

Water quality in the Wairoa River is degraded largely due to pastoral farming which dominates this catchment. Water quality monitoring sites surrounded by pastoral land use (Mangere, Manganui and Whakapara) typically have elevated nutrient levels, particularly phosphorus and turbidity. Sites situated in native and/or pine forest (upper Mangakahia and Opouteke) have lower nutrient levels and generally good clarity (prior to forest harvesting).