Situated in the East Cape region of New Zealand‘s North Island, the Waipaoa is the largest of three catchments draining to the east of the Raukumara Ranges. The Waipaoa catchment extends from Poverty Bay at the eastern boundary and exit point for the Waipaoa River to the catchment headwaters high in the Raukumara Ranges in the west (Hicks et al., 2000). As discussed in more detail later this chapter, for this study the greater Waipaoa catchment includes the smaller Waimata catchment. The individual catchment‘s outlines are shown in figure 3.11 and the two catchments cover a combined area of 2500 km² (Marden et al., 2008b).
Rugged and deeply dissected, the Raukumara Ranges reach a maximum elevation of 1752m at Mt. Hikurangi summit and comprise a zone of ranges and basins which are associated with the forearc crumple zone of the Hikurangi subduction margin (Litchfield & Berryman, 2005). The Raukumara Ranges may be divided into two subgroups, firstly lying in a northeast-southwest direction and forming the backbone of the peninsula are the bedrock frontal ridge ranges, with an average elevation of 1300m. This group of ranges forms the north-eastern extension of the North Island‘s axial range and provide a major topographic divide between the east and west coast. The second subgroup and the main group for this study are the lower elevation eastern ranges from which the Waipaoa River descends. It is the central and southern section of the East Cape peninsula which contains the Waipaoa catchment and the eastern ranges of the second sub group have peaks which rarely rise above the 1000 m mean elevation (Mazengarb & Spaden, 2000; Litchfield & Berryman, 2005). Roughly round
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in shape, the catchment of the Waipaoa has major tributaries of the Mangatu, Waingaromia, Wharekopae, Waihuka, Waihora and Te Arai Rivers which drain the ranges and coastal lowland hills. The Te Arai under the Waipaoa catchment‘s current configuration is the southern most tributary joining at the southern end of the Poverty Bay flats as the Waipaoa River reaches the sea.
Figure 3.11; Key features for the Waipaoa sedimentary system. This figure shows the locational setting for the Waipaoa catchment and illustrates key feature of the Waipaoa sedimentary system.
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The back country of the East Cape today is densely covered in native podocarp- broadleaf rainforest and it is these forests which would have been dominant for the last 15 thousand years (ka) as suggested in pollen evidence such as that by Allsop (1973). The podocarp-broadleaf forests would have also covered the lower coastal ranges and even the coastal plains during the warmest periods of the Holocene and prior to human settlement (Eden et al., 2001). While the region‘s climate has had a large effect on vegetation cover, with shrub and grasslands being dominant in cold glacial conditions and full forest cover during the Holocene optimum (McGlone et al, 1984), recent human land use changes have had a larger influence on vegetation cover. It was the coastal and lowland regions that were those first to be influenced by anthropogenic activities. Evidence for the clearance of forest was firstly associated with the Polynesian arrivals which have been estimated to have occurred from as early as 1200 AD (Higham and Hogg, 1997). Maori are known to have cleared large sections of the native lowland podocarp forest for hunting and cropping purposes and to have maintained these clearings as bracken and scrub through periodic burnings (McGlone and Wilmshurst, 1999). These forest clearances would have been principally located close to the main centres of habitation (McGlone, 1989), which were dominantly in the south-west and lower reaches of the catchment (Jones, 1988). Later, clearance of forest is indicated to have occurred in the inland ranges and high back country. Evidence of these clearances is shown in pollen records taken from Lake Waikaremoana by Newnham et al. (1998) and may have been coincident with a warmer period during the Holocene. There has been no evidence in the Waipaoa of inland and high country clearances and coastal hill country clearances were probably not maintained.
Following the periodic slash and burn clearance of relatively small areas of forested land by Maori were the large scale clearance practices associated with European settlement. Clearance of the lower reaches of the Waipaoa and the coastal plains and hills first occurred around the 1820s (McKay, 1982). The basin wide clearance of forest and conversion into pasture for agricultural farming occurred between 1880- 1920. These forest clearances left only an estimated 2.5% of native forest cover on land that was steep or remote (Gomez et al., 1999). This wholesale land clearance produced an immediate geomorphic response, with increases in erosion (discussed later this chapter) and flood peak response of rivers. This landscape response was of
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great concern to those farming on the Poverty Bay Flats and also local iwi, due to not only the high levels of investment in both agriculture and settlement in the lowland areas, but also the rates of change that were occurring in the landscape. These landscape changes in response to the land clearances became the focus of many studies on erosion and catchment dynamics to provide strategies to mitigate these adverse effects. It was thanks largely to the decline in the economic viability of pastoral farming in this high erosion prone landscape, and the downstream effects from flooding and sedimentation on the Poverty Bay Flats, that there has been a reconversion of land to plantation forest in more recent years. The conversion of steep hill country pasture to plantation forestry was begun by the New Zealand Forest Service during the 1960s and head water areas were replanted to help control erosion. Plantation forests now cover 20% of the catchment and indigenous forest 6%, with the remainder remaining in pasture (Page and Trustrum, 1997). Gisborne is the major settlement of the region, located on the north-eastern Poverty Bay Flats and provides the infrastructural support for the region‘s industry which consists of mostly agricultural and forestry exports.