| Manawatū River | |
|---|---|
 | |
 The Manawatū River system | |
| Native name | Manawatū (Māori) |
| Location | |
| Country | New Zealand |
| Region | Manawatū-Whanganui |
| Cities | Palmerston North |
| Physical characteristics | |
| Source | Ruahine Ranges |
| • coordinates | 40°0′53″S176°7′1″E / 40.01472°S 176.11694°E /-40.01472; 176.11694 |
| Mouth | Tasman Sea |
• location | Manawatu Estuary,Foxton Beach |
• coordinates | 40°28′S175°13′E / 40.467°S 175.217°E /-40.467; 175.217 |
• elevation | Sea level |
| Length | 180 km (110 mi) |
| Basin size | 5,899 km2 (2,278 sq mi) |
| Discharge | |
| • average | 102 m3/s (3,600 cu ft/s) |
| Basin features | |
| Landmarks | Manawatū Gorge,Manawatu Estuary |
| Tributaries | |
| • left | Tamaki River,Pohangina River,Oroua River |
| • right | Mangatewainui River,Mangatoro River,Tiraumea River,Mangahao,Tokomaru River |
TheManawatū River is a major river of the lowerNorth Island of New Zealand. The river flows from theRuahine Ranges, through both theManawatū Gorge and the city ofPalmerston North, and across the Manawatū Plains to the Tasman Sea atFoxton.
The river, along with the more northernWhanganui River, gives its name to theManawatū-Whanganui region. The name of the river was given by thetohunga 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 wordsmanawa (heart) andtū (stand still). 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 Paekākāriki, Hau overtook the fugitives and changed Wairaka into a rock.[1]
The Manawatū River has its headwaters northwest ofNorsewood in theTararua District, on the eastern slopes of theRuahine Range on the North Island of New Zealand. It flows initially eastward before turning south-west nearOrmondville, flowing 40 kilometres (25 mi) before turning north-west nearWoodville. At this point it enters theManawatū Gorge, between the Ruahine andTararua Ranges.[2] Beyond the gorge it joins with thePohangina River atAshhurst and turns south-west, flowing through the city ofPalmerston North. At this stage the river is still flowing swiftly and carrying gravel from the mountains.
AfterOpiki, it slows and has a lower gradient,meandering over theManawatu Plains; its bed at this point is mud and silty sand. In its meandering and frequent shifting of course it has createdoxbow lakes,lagoons, andswamps. 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.[2] The Manawatū reaches theTasman Sea atFoxton Beach, on the west coast of the North Island, creating theManawatu Estuary.[3]
Major tributaries of the river include theMākākahi,Mangahao,Pohangina andOroua Rivers. The Manawatū's total length is 180 kilometres (110 mi), making it only the 12th-longest in the country, but at 102 cubic metres per second (3,600 cu ft/s) it is one of New Zealand's greatest rivers in terms of flow, and second only to theWaikato River among North Island rivers.
The river is crossed by 18 road bridges (not including the under-construction Parahaki Island Bridge), 3 rail bridges[4] andHe Ara Kotahi walk/cycleway.[5] The lowest and longest, Whirokino Trestle and Manawatū River Bridge, carrySH1 1.1 km (0.68 mi) over the Moutoa Floodway and 180 m (590 ft) over the river. It replaced the 1938 and 1942 bridges[6] in February 2020, at a cost of $70m.[7]
The Manawatū 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 andTararua 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 Manawatū'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 Manawatū Gorge; other rivers were unable to and were diverted into the Manawatū instead.[8]
After exiting the Manawatū 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 Manawatū Gorge and Palmerston North.[2]
The Manawatū 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 (oranticlines) parallel to the mountains, which impede the flow of the Manawatū,Rangitikei, and Oroua Rivers, forcing them to flow southwest rather than directly into the Tasman Sea.[2]
At the time of human arrival, the Manawatū Plains were covered with forest. Towards the foothills and the Manawatū Gorge grewblack beech, turning intotawa forest at lower altitudes. Along the plains and terraces the forest was mixedpodocarp andtōtara, changing to mixed tawa,tītoki, andmāhoe in the sand dunes.[9]
Across the flood plain of the Manawatū and on the low-lying land bordering the river, the predominant vegetation was semi-swamp forest, mostlykahikatea andpukatea.[9] Three major swamps bordered the Manawatū: 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 Manawatū and north of the Tokomaru River, which covered 22000 acres; and Taonui Swamp on the north side of the river.[10] The main vegetation in the swamps washarakeke andraupō: harakeke in the drier parts, raupō in the wetter.[11]
TheManawatū river mouth and estuary was listed under theRamsar Convention as a Wetland of International Importance in 2005.
The Manawatū River flooded in February 2004, displacing over 3000 people (primarily fromMarton andFeilding) and damaging over 1000 Manawatū farms. The cost of the flood in terms of insurance payouts was NZ$122 million. Further damage was prevented by the opening of the Moutoafloodgates, which intercept the river betweenFoxton andShannon.
In 2018 a case study on water quality in theManawatū-Whanganui region was jointly commissioned byHorizons Regional Council and theMinistry for the Environment, the study was conducted byLand Water People (LWP) and the results were reviewed by theNational Institute of Water and Atmospheric Research (NIWA) andStatsNZ. Results showed that water quality forsediment andE. coli had improved over the previous 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.[12]
In 2006Fonterra were criticised for an application to discharge 8,500 cubic metres (300,000 cu ft) of wastewater into the Manawatū River.[13] In 2009, theCawthron Institute found that the river had the highestgross primary production (GPP) compared to 300 rivers and streams in the Western world.[14] High GPP rates are an indication of poorecological health and can lead to variousenvironmental 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.[15] A report by theMinistry for the Environment ranked 76 New Zealand sites for water clarity andE. coli levels. Using those measures, they found only four other New Zealand rivers rate worse than the Manawatū (theWaitara,Whanganui,Waipā andRangitīkei).[16]
