Manawatu River


The Manawatu River, often spelled Manawatū in New Zealand English, is a major river of the lower North Island of New Zealand.

Name

The river, along with the more northern Whanganui River, gives its name to the Manawatū-Whanganui region. The name of the river was given by the tohunga Haupipi-a-Nanaia, a descendant of Haunui-a-paparangi. Haupipi-a-Nanaia or Hau, travelled down the west coast in pursuit of his wife Wairaka, who had eloped. When Hau reached what is now known as the Manawatū River, he is said to have stopped and clutched his chest, horrified at the prospect of crossing so mighty an expanse of water. Therefore the river's name comes from the Māori words manawa and tū. In this context, when said together, the interpretation is ‘heart standing still’ to represent how Hau felt when he first saw the river. However cross he did, and a few kilometres south of Paekakariki, Hau overtook the fugitives and changed Wairaka into a rock.

Geography

The Manawatu River has its headwaters northwest of Norsewood, on the eastern slopes of the Ruahine Range on the North Island of New Zealand. It flows initially eastward before turning south-west near Ormondville, flowing before turning north-west near Woodville. At this point it enters the Manawatu Gorge, between the Ruahine and Tararua Ranges. Beyond the gorge it joins with the Pohangina River at Ashhurst and turns south-west, flowing through the city of Palmerston North. At this stage the river is still flowing swiftly and carrying gravel from the mountains.
After Opiki, it slows and has a lower gradient, meandering over the Manawatu Plains; its bed at this point is mud and silty sand. In its meandering and frequent shifting of course it has created oxbow lakes, lagoons, and swamps. Sediment deposited along its course has created levees, higher than the surrounding plain; when the river is in flood it overflows these and creates wetlands. The Manawatu reaches the Tasman Sea at Foxton Beach, on the west coast of the North Island, creating the Manawatu Estuary.
Major tributaries of the river include the Makakahi, Mangahao, Pohangina and Oroua Rivers. The Manawatu's total length is, making it only the 12th-longest in the country, but at it is one of New Zealand's greatest rivers in terms of flow, and second only to the Waikato River among North Island rivers.

Bridges

The river is crossed by 18 road, 3 rail bridges and He Ara Kotahi walk/cycleway. The lowest and longest, Whirokino Trestle and Manawatū River Bridge, carry SH1 over the Moutoa Floodway and over the river. It replaced the 1938 and 1942 bridges in February 2020, at a cost of $70m.

Geology

The Manawatu is unique among New Zealand rivers in that it crosses a mountain range. The river has formed a "water gap" across the mountains because it is older than the Ruahine and Tararua Ranges. Most rivers arise from an already-existing range of mountains or hills, but beginning about 3 million years ago the central North Island mountain ranges began to uplift across the Manawatu's current course. Because it drained a large catchment, the river had sufficient flow to keep pace with and erode the rising mountains, eventually forming the Manawatu Gorge; other rivers were unable to and were diverted into the Manawatu instead.
After exiting the Manawatu Gorge, the river carries rock and sediment down from the mountains. During glacial times, with the prevalence of ice, snow, and bare mountains, this erosion increases and forms a stony elevated terrace. During an interglacial, while the mountains are forested, gravel outwash is reduced and the river cuts down into the terrace, forming a gorge. This cycle has created four distinct terraces between the Manawatu Gorge and Palmerston North.
The Manawatu Plain was seabed 5 to 6 million years ago, and as it was raised above water by the action of the Australian and Pacific Plates it buckled, forming five long and low ridges parallel to the mountains, which impede the flow of the Manawatu, Rangitikei, and Oroua Rivers, forcing them to flow southwest rather than directly into the Tasman Sea.

Vegetation

At the time of human arrival, the Manawatu Plains were covered with forest. Towards the foothills and the Manawatu Gorge grew black beech, turning into tawa forest at lower altitudes. Along the plains and terraces the forest was mixed podocarp and tōtara, changing to mixed tawa, tītoki, and māhoe in the sand dunes.
Across the flood plain of the Manawatu and on the low-lying land bordering the river, the predominant vegetation was semi-swamp forest, mostly kahikatea and pukatea. Three major swamps bordered the Manawatu: Moutoa Swamp north of the river, towards the mouth, known as the "Great Swamp" in the 19th century; Makerua Swamp further inland, to the south of the Manawatu and north of the Tokomaru River, which covered 22000 acres; and Taonui Swamp on the north side of the river. The main vegetation in the swamps was harakeke and raupō: harakeke in the drier parts, raupō in the wetter.
The Manawatu river mouth and estuary was listed under the Ramsar Convention as a Wetland of International Importance in 2005.

Flooding

The Manawatu River flooded in February 2004, displacing over 3000 people and damaging over 1000 Manawatu farms. The cost of the flood in terms of insurance payouts was NZ$122 million. Further damage was prevented by the opening of the Moutoa floodgates, which intercept the river between Foxton and Shannon.

Water quality

In 2018 a case study on water quality in the Manawatū-Whanganui region was jointly commissioned by and the Ministry for the Environment, the study was conducted by and the results were reviewed by the National Institute of Water and Atmospheric Research and StatsNZ. Results showed that water quality for sediment and E. coli have improved over the past seven to ten years in the Manawatū-Whanganui region. The report found strong statistical evidence of a connection between regional scale water quality improvements and local scale interventions.
In 2006 Fonterra were criticised for an application to discharge of wastewater into the Manawatu River. In 2009, the Cawthron Institute found that the river had the highest gross primary production compared to 300 rivers and streams in the Western world. High GPP rates are an indication of poor ecological health and can lead to various environmental issues. In 2011, the Horizons Regional Council laid blame with the Palmerston North City Council for "considerable" and "sustained" breaches of one of its discharge consents, and some degree of non-compliance with two others. A report by the Ministry for the Environment ranked 76 New Zealand sites for water clarity and E. coli levels. Using those measures, they found four other New Zealand rivers rate worse than the Manawatu.