New Zealand Journal of Forestry (1978) 23(1): 21–48
©New Zealand Institute of Forestry
Erosion in the south-eastern Ruahine Range: Its implications for downstream river control
Erosion rates in the south-eastern Ruahine Range are estimated, by three different methods, to be about 20 m3/ha/yr. Erosion has been periodic in nature; a period of above-average erosion was apparently initiated in the late 1930s as a result of a period of increased storminess, resulting erosion rates being no more than double the long-term average. Deterioration of the forest cover, which appears to be fundamentally natural but perhaps exacerbated by introduced animals, may have been a factor; the introduced animals have probably been responsible primarily for prolonging the period of above-average erosion by retarding recovery of the forest cover.
Past discussions of the impact of mountainland erosion upon downstream values have tended to overlook the fact that sediment supply rates to the lower catchments are generally less than rates of erosion in the upper catchments, because of sediment storage in the channel systems. Sediment supply and storage rates may vary over time, and may therefore affect downstream values.
Sediment supply rates to the piedmont stream channels have probably been substantially increased by removal of the forest cover from the valley throats and alluvial fans at the foot of the range. This has permitted stream channel entrenchment and bank erosion, which have provided an additional major source of sediment and have removed the natural "throttling" effect upon sediment transport from the range. In addition, removal of the forest from the banks of the piedmont streams has increased bank erosion and, channel instability.
The stream catchments are integrated systems which can be managed at several different locations, using different techniques, to achieve the ultimate objective of ensuring stability of the piedmont stream channels. It is therefore necessary to view specific techniques, such as mountainland revegetation, in a wider context, and to choose the most effective and cheapest combination of available control options.