Where along a river's length will vegetation most effectively stabilise stream banks?

Abstract

Riparian vegetation has different impacts on stream processes depending upon its position in a catchment. Native riparian vegetation is increasingly becoming the favoured stream management tool but managers need to locate revegetation schemes where they will most effectively achieve ecological, geomorphological, or other, project goals. Using the Latrobe River in SE Australia as an example, this paper illustrates a structured decision-making approach for assessing the role of vegetation in stream bank erosion at different points throughout a catchment. Three bank-erosion process groups are identified: subaerial preparation, fluvial entrainment, and mass failure. Although these processes act on banks throughout the catchment there exists spatial zoning in the dominance of each process group over the others. Bank erosion in upper reaches is dominated by subaerial preparation, in mid-basin reaches by fluvial entrainment, and in the lower reaches by mass failure. We find that in upper reaches, windthrown trees are responsible for most bank sediment transfer to the flow. Where direct fluvial entrainment of bank material is the dominant erosion process, flow resistance due to vegetation becomes crucial. In reaches where bank slumping is the dominant erosion process, increased bank shear strength due to root reinforcement is the major role of vegetation in stabilising banks. Other effects, such as tree surcharge, and altered bank hydrology appear to exert only minor influences on the slumping process. Considering the above variables we are able to define a critical zone in which revegetation will be most effective in reducing bank erosion. On the Latrobe River, this zone occurs in that portion of the river where it first leaves the mountain front and meanders across a broad floodplain. This reach occupies the second quarter of the river's length. This information, combined with other scale analyses (e.g. ecological, hydrological), will assist river managers to plan physically based riparian revegetation strategies.