Effects of riparian grazing and channelisation on streams in Southland, New Zealand. 1. Channel form and stability

Abstract

The effects of mixed sheep and cattle grazing of stream margins, channelisation, and the benefits of riparian retirement were assessed through a survey of five streams in Southland, New Zealand. Morphological and vegetation data affecting erosion processes and aquatic habitat were compared among grazed, channelised, and retired reaches. There is no evidence that grazing streambanks in floodplain streams of northern Southland will lead to rapid and severe deterioration of channel form, except in small streams (< 2 m wide) under intensive grazing of wet streamside soils. Generally, the dominant erosion mechanism—the undercutting of banks—is largely unaffected by grazing stream margins. In contrast, channelisation has led to severe streambank and streambed erosion in two of the three streams examined. The major factor in this degradation appears to be straightening and deepening the channel so that underlying uncohesive shingle is exposed to high flows. Riparian retirement had variable effects depending on the stability of the stream channel. On smaller, relatively inactive channels, it reduced localised bank erosion from livestock trampling, especially at cattle crossings. However, this damage (which sometimes can be quite visible) did not lead to significant change in average channel form or width in the 7–15 years since the land has been converted to intensive agriculture from extensively grazed tussock. Retirement also increased vegetation overhang. On the larger channels that were more actively meandering, retirement had comparatively little benefit because any retirement or grazing effects were rapidly overtaken by channel migration.