Hydrology and stream sediments in a mountain catchment

Abstract

New Zealand attitudes to soil and water conservation have their origins in Europe and North America where legislation for the conservation of soil and water resources preceded research by 20-30 years. In the 1930's and 40's much New Zealand land was in a depleted and eroded condition. Those who first advocated soil conservation saw a clear need for remedial action in preference to research. North American attitudes policies and research findings became the bases for New Zealand policies and programmes. Most surveys and investigations made in New Zealand mountain land were predicted on North American concern for soil surface conditions and Horton's concept of overland flow.

Some of the findings of this study have important implications for the future management of the Torlesse Stream Catchment. The application of these findings to other areas of New Zealand high country is untested. Flood control, water yields, low flows and sediment yields are found to be unrealistic objectives of land management. If it was necessary, most sediments could be prevented from entering stream channels by the effective treatment of less than one per cent of the total catchment. However, in consequence, the channel would degrade its channel and riparian lands. This may give rise to more serious problems than were “solved”. Stream channels are found to be more important to land stability than has been generally recognised. Strategic management of riparian lands may be the most cost effective method of inducing upper slope stability. Findings from this study support the view that land management proposals should avoid slopes which might fail as a consequence of land use. Such failure may initiate a cycle of erosion which could be impossible to arrest. This study makes a major contribution to our understanding about the revegetation of eroded lands, It establishes that because the area of land in need of treatment is much less than is generally believed, (from the point of view of stream sediment supplies) the cost per unit area is less important than was formerly thought. Two issues relevant to resource use planning in mountain lands are discussed and some directions for future research are outlined.