The geomorphic effect of vegetation on streams: a comparative study on the effect of grass, Australian native trees and introduced willows on stream channel width in southeastern Australia

The effect of vegetation on river channel width has received considerable attention in the previous literature. However, despite numerous studies no clear pattern has emerged as to whether the presence of dense woody vegetation creates narrower or wider channels compared to sites with grass. By measuring the bankfull and base width of 35 river reaches, of different sizes, lined with three different types of vegetation (grass, native trees and introduced willows), this study set out to determine the cause of the conflicting results presented by previous researchers. As well as testing whether the presence or absence of trees is important for determining channel width, this study also asked whether the effect of vegetation is dependent on the type of vegetation in the riparian zone. By sampling rivers draining a range of catchment areas, this study also tested whether the effect of vegetation is mediated by the size of the river. In this study, the presence or absence of trees was unable to account for the variation in channel widths observed between field sites. However, there was a significant correlation between the bankfull and base width of channels, and the type of vegetation in the riparian zone. The presence of introduced willow trees was associated with significantly wider channels at both bankfull and base width than sites with either native trees or grass, at small catchment areas. There was no significant difference in the bankfull and base width of channels lined with native trees and sites with grass. The association between vegetation type and channel width appeared to be mediated by catchment area. The differences in bankfull width between sites lined with willows and sites with grass or native trees were greatest at sites with small catchment areas, while differences were not significant at sites with catchment areas above 80km2 (50km2 for base width). The presence of a large number of inchannel trees at sites with willows appears to be the factor most likely to account for the wider channels at these sites. These in-channel trees create significant blockages, and appear to generate bank scour leading to channel widening. The findings of this study highlight the importance of the individual characteristics of different types of vegetation. The classification of woody vegetation as present, or absent, does not take into account the substantial variation between different types of vegetation. Where a particular type of tree settles and grows in relation to the channel appears to be a major factor in determining how it affects channel width. In future, studies on the effect of vegetation on channel dimensions should consider the individual characteristics of different vegetation types. They should also take issues of catchment scale into account as the effect of vegetation on channel width appears to be different on rivers of different sizes