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The Waikato Plains (the alternative nameWaikato Basin is an ambiguous term as it can refer to the entire river catchment) form a large area of low-lying land in the northwest of theNorth Island ofNew Zealand. They are thealluvial plains of theWaikato River, the country's longest river with a length of 425 km (264 mi).[1]
The plains can be divided roughly into theMiddle Waikato Plain (alsoMiddle Waikato Basin[2]Hamilton Basin[3]), extending in all directions around the city ofHamilton, and theLower Waikato Plain, nearer the river's mouth.[4] The two are broken by the rough, low-lying hills of theHakarimata Range, betweenNgāruawāhia andHuntly, and theTaupiri Range.[3]
The region is heavily populated by New Zealand standards, with many living in Hamilton towards the center of the plains.[1]
The plains are an area of once swampy land, much of which was drained by the early settlers, such as the Morrin brothers in the late 18th century[5] and is now intensively farmed.Dairy cattle,sheep,grain andmaize are all farmed here, but it is dairy farming that is the staple of the local economy. The lower plain is also known forvineyards, and the middle plain has some of the southern hemisphere's topthoroughbred stables, notably around the towns ofCambridge andMatamata.
A considerable amount of the land ispeaty, and significant sections especially in the north east are still undrainedswamp.[2] Dozens of shallow riverine lakes lie at the central and southern end of the lower plain, notablyLake Waikare.[6]
This part of the Wailkato watershed is currently managed as the Middle Waikato Management Zone[7] and the Lower Waikato Management Zone.[4]
There is geological evidence that the Hamilton Basin portion was formed as arift valley.[8] The normal faulting associated with this may have become inactive before 350,000 years ago,[9] although in lake sediments there is evidence of significant local earthquake activity as recently as 7600 years ago, but epicenters may relate to known current active fault systems to the north east of the plains.[6]
The basement isWaipapa Morrinsville Terranegreywacke formed in theLate Jurassic toEarly Cretaceous (160-120Ma).
The Hamilton Basin originally formed under water so marinePliocene beds underlieQuaternary deposits that date from about 1.8 million years ago.[2] The oldest deposits are called the Puketoka Formation which containsclays,sands,breccias andignimbrite sheets frompyroclastic flow reaching the basin.[2] The younger Waerenga Gravels are composed of weathered greywacke debris presumably deposited in fans extending from the surrounding ranges.[2] Even younger, and so closer to the surface usually, is the Karapiro Formation withrhyolitic sands andgravels which can be weathered to clay.[2]
The Waikato River over, even the last 1800 years, has changed course many times in the basin. It has changed course even more spectacularly on at least four occasions in the last 100,000 years, flowing northeast from the region of the currentLake Karapiro, and exiting nearThames in theHauraki Gulf leaving drainage of the basin to theWaipa River which has not always been a tributary to the Waikato River.[2] Between 65,000 years to at most 25,000 years ago it drained the Waikato Plains but then drained through the Hauraki Plains for 6,000 years returning to drain the Waikato Plains from around 19,000 years ago.[2] So only in recent times,again, has it flowed northwest to empty into theTasman Sea nearPort Waikato.[2] This means the recent sedimentary deposits near the river are mainly altered volcanics washed down from theNorth Island Volcanic Plateau and broken down volcanic soils but there are layers oftephra/breccia from the many significant rhyolitic eruptions to the south where recent river flooding has not been disruptive.[6] Layers of the fifteen layer Kauroa Ash Formation are found within both the Puketoka and Karapiro Formation depending upon where you are in the basin. The ash beds can be many meters thick, although thin out north of Hamilton, and include the eight layer 3 to 5 m (9.8 to 16.4 ft) thick Hamilton Ash Formation deposited between 350,000 and 100,000 years ago.[2] There are also tephras derived from theandesiticstratovolcanoes of theTongariro andTaranaki regions, as well as fromMayor Island/Tuhua.[2] The airfall tephra layers of the last 50,000 years vary from between 0.5 to 1.5 m (1 ft 8 in to 4 ft 11 in) thick.[2]
37°40′11.79″S175°31′5.93″E / 37.6699417°S 175.5183139°E /-37.6699417; 175.5183139