The distribution and properties of soils in relation to erosion in a selected catchment of the southern Ruahine Range, North Island, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Soil Science at Massey University

Abstract

The soils of a selected subcatchment of the Southern Ruahine Range have been mapped at a scale of 1:5,000. The soil mapping units have been further characterised by measurement of a number of soil physical and chemical properties, together with an investigation of their sand and clay mineralogies. The erosion history since 20,000 yrs B.P. when the Aokautere Ash was deposited in the West Tamaki River catchment, has been partially reconstructed for this catchment. It is one of erosive periods and resulting aggradational gravel deposits, alternating with more stable periods with soil development and vegetation growth. Studies of a histosol (organic soil) on the summit plateau of the Southern Ruahine Range, at the head of the catchment, suggests that this soil is approximately 4600 years old, and prior to this time the summit plateau was stripped by erosion. Present erosion occurs predominantly: (1) on convex creep slopes, just below the summit plateau, and (2) on the steep valley-sides. In the former zone, where Takapari hill soils exist, deep-seated creep and mass movements occur. In the latter zone, where Ruahine steepland soils exist, superficial soil and rock slips are more common. An investigation of the soil-water relationships for each soil mapping unit indicates that a number of factors render the Takapari hill soils and Ruahine steepland soils particularly susceptible to erosion. A comparison of soil properties which affect the erosion susceptibilities of each soil mapping unit has enabled an ordering of the units with respect to erosion risk. Thus, areas of high, medium and low risk to erosion in the West Tamaki River catchment have been delineated. Many of the deep-seated erosion surfaces occur in The high risk area. Thus, if stabilisation of these sites is possible, by intensive revegetation programmes, the result will be a decrease in the amount of gravels carried out of the mountainland by rivers onto the surrounding fertile floodplains.