The Mangatainoka River is a river that flows in the Wairarapa region of New Zealand's North Island. Its headwaters are on the eastern side of the Tararua Range to the southwest of the town of Eketahuna, and it roughly parallels the Wairarapa Line railway and State Highway 2 through the northern Wairarapa before meeting the Tiraumea River just before its confluence with the Manawatu River just south of Woodville. The railway's 162 metre bridge across the river between Newman and Hukanui is the longest on the entire line. and further to the north, the small settlement of Mangatainoka sits on the river's banks north of Pahiatua.
Prior to European settlement of New Zealand, the shallow river was thickly lined with matai, rimu, and totara trees that prevented serious flooding to the swampy lands alongside the river. However, large-scale removal of the forest and drainage of the fertile land for agriculture has caused the river to flow faster, deepening its channel, heightening erosion, and encouraging it to meander to try to return to its slower pace. This has resulted in devastating floods, including one in October 2000 that caused at least NZ$200,000-300,000 of damage to the regional council's scheme to stabilise the river and rehabilitate its environment.
The river is popular for fishing, especially for trout. Approximately 50 km of the river is fishable, and due to its proximity to State Highway 2 and major northern Wairarapa towns, access is easy.
- Geoffrey B. Churchman and Tony Hurst, The Railways of New Zealand: A Journey Through History (Auckland: HarperCollins, 1991), pg. 160.
- Horizons Regional Council, Mangatainoka River Scheme, pg. 1.
- Horizons Regional Council, 17 October 2000, Flood Damage Inspections Begin
- Fish and Game New Zealand, Wellington Region - Fishing: Access