Land-use effects on channel morphology in streams in the Moutere gravels, Nelson, New Zealand

Abstract

Land-use can have significant effects on channel morphology, especially in smaller-sized catchments (<5ha). Pasture streams in these small catchments are usually narrower than forested streams. It is hypothesized that the sediment trapping and retention ability of grass sod is responsible for the narrower channels in pasture streams. It is suggested that the coarser root structure in forested streams is less effective in armouring bank material against fluvial erosive processes. This, along with the influence of woody debris obstructions in diverting and channelising stream flow is thought to be the reason behind the wider and more variable stream widths in forested streams.

In the Hakarimata Ranges, Waikato New Zealand, it appears that streams in pine plantations that were planted onto pastureland 15 years previously are in the process of widening back to a forested channel morphology, releasing the sediment retained in the banks by the grass sod. As the majority of new plantings in pine plantations are occurring on pastureland or reverting pastureland the possibility of increased sedimentation in streams during the conversion process may be an issue in some areas.

The purpose of this thesis was to assess the influence of land-use on channel morphology in another area of New Zealand. The Moutere Gravels in Nelson provided an area of contrasting geology, hydrology and climate to that in the Hakarimata Ranges. The study compared channel morphology characteristics in 15 streams in small-sized catchments 5 streams each in pasture, pine plantation and native forest. Channel morphology measurements were made along a representative 100 m section of stream reach in each catchment. Woody debris was measured in each of the pine plantation and native streams to assess its influence on channel morphology.

There was no significant difference in bankfull and channel widths between the three land uses in the Moutere Gravels. Width variability was less in the forested streams compared to the pasture streams. The presence of large woody debris (LWD) in the pine and native streams did not appear to be influencing channel width. There were no significant differences between the three land-uses in channel depth and cross-sectional area.

Width-to-depth ratios were significantly higher in the pasture streams in comparison to the forested streams. The higher number of bank undercuts and lower width-to depth ratios in the forested streams indicated that the tree roots were assisting in stabilising and retaining the channel bank material. Bank disturbance was low in all streams regardless of land-use ranging from 1-3%. There were more fines in the streambeds of the pasture sites but higher levels in one site influenced this. The median particle size was significantly lower in the pasture and pine sites in comparison to the native sites.

The presence of LWD in the pine and native streams increased the number and variety of pools and influenced sediment storage in the stream channel. The volume of LWD in these streams was low in comparison to streams in similar temperate forests in the Pacific Northwest of the USA.

The results of this study differ from similar studies in New Zealand and overseas. It is suggested that low sedimentation rates, low frequency of floods of sufficient magnitude to influence channel morphology and the cohesive structure of the channel bank material in the Moutere Gravels, may provide some explanation for the lack of land-use effects on channel morphology in these small catchments.

While the results of land-use effects on small streams in the Moutere Gravels are the exception when compared to other similar studies it does demonstrate that in some circumstances factors other than land-use can exert a dominant influence on channel morphology. When assessing the possible implications of converting pastureland to pine plantations, the influence of local hydrology, geology and climate need to be considered.