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Spectrum was a long-running weekly radio documentary series which captured the essence of New Zealand from 1972 to 2016. Alwyn Owen and Jack Perkins produced the series for many years, creating a valuable library of New Zealand oral history.
The once bush-covered hills which form the catchment for the Waipaoa and Mangatū rivers north of Gisborne, were cleared for farmland largely between the 1880s-1910s which has since accelerated the area’s well-established natural pattern of erosion. Jack Perkins looks at this moving land.
Ken Spence, former manager of Waipaoa Station and Deputy Chairman of the Catchment Board began his fight to save this land over thirty years ago and describes what measures he’s employed in an attempt to salvage it.
After Catchment Board geologist Robin Black made a survey of the area, he revealed that the beginning of the problem actually stemmed from over 10,000 years ago. He found evidence of old river beds covered in volcanic ash, sitting at 300 foot above the current river level which indicated a natural erosion cycle produced by a down-cutting of the river system.
The removal of bush from about a hundred years ago then contributed to a secondary erosion cycle and has created problems unprecedented in New Zealand. The move to regenerate planting in the back country was prompted in the 1930s and 40s by the urgency to protect coastal plains around Gisborne from disastrous flooding, as silt carried downstream was being deposited in the lower reaches of the river.
Black explains how the threat of flooding in the Waipaoa catchment area competes on a global scale, repeating every two to three years. The Poverty Bay Catchment Board initiated an engineering and protection works response to the excessive sediment flows that were recorded between the 1940s and 1960s.
Former soil conservator Grant McKee explains how they had to develop a method of planting more trees based on their observations of the old farmers as there was no scientific reference at that time, nor any appropriate study comparisons available from overseas.
The Forest Service planted radiata pine widely in addition to the earlier poplar and willow plantings, low grazing and debris dam initiatives. However, Black clarifies that the ineffectiveness of planting trees was just simply not understood in relation to the long history of natural erosion.
Spence shows Perkins the largest example of migrating hillside in the country, the infamous Tarndale slip in Mangatū which no amount of tree planting will stabilize now.
Spence feels a sense of shame for the way the land has been over grazed and under-planted by some who were leasing the land and therefore not concerned so much with its long-term sustainability.
Black provides a geological background to the causes behind the shifting land which are accentuated by winter rains. Spence tells how heavy periods of rain always presented problems with bogged sheep and altered the course of tracks.
Black describes the issues he had driving his motorbike up the Tarndale slip, his observations and conditions whilst traversing the landscape and how important it was for planters to have the appropriate work vehicles to cope with the specific terrain.
Ted Mottle of Mangatū Forest headquarters remembers working on the Tarndale slip in 1965. Land Rovers took them most of the way but they walked through a lot of mud. In those days they focused on planting poplars and willows in an attempt to ‘hold’ the slip but it never helped. He explains why pines are better and how planting on steep slopes occurs.
Bruce Willis of the Forest Service in Gisborne says a variety of conifers have been tried and tested in the past but pines always come out on top. More recently they have tried the acacia shrub which tolerates the winter wet and extreme dry soil soil conditions of the summer.
The programme concludes with a prognosis from Black who says although Mangatū and Tarndale will not change, the future looks good; the main pine planted areas are doing their job of preventing further erosion and reducing water volumes, and overall assisting in the reduction of floods in the Gisborne flats.