Assessing the Distribution and Changes of Instream Woody Habitat in South-Eastern Australian Rivers

Abstract

Managers and communities are now artificially reintroducing instream woody habitat (IWH) to rivers following historic large-scale removal. Riverscape-scale datasets that quantify existing habitat conditions are fundamental to setting the priorities and allocating resources for such programs. Unfortunately, such datasets are rare, primarily because existing assessment approaches are limited in their accuracy (remote sensing) or are costly and labour intensive (field assessments). This study used both field assessments and aerial data to improve the accuracy of remotely sensed measures of IWH and estimate current IWH volumes and subsequent condition (compared with reference levels) across approximately 28 000 km of stream in the south-eastern Australian state of Victoria. We found that aerial measures, when used in conjunction with measures of stream size and riparian overhang, produced significantly better estimates of IWH loads than using aerial data alone. The statewide assessment indicated that streams currently have IWH volumes, on average, 41% lower than reference levels that represented an average reduction of 0.0207 m3 m−2. The degree of IWH condition was highly variable across regions (20–95% reductions from reference levels), a likely reflection of regional variation in land use practices and past river work activities. This scale of IWH reduction may pose major negative impacts on the ecological integrity of these streams. Whilst the approach used during this study has temporal and spatial limitations, it was designed as a generalised, rapid and relatively inexpensive method to measure stream condition and assist with priority setting at state and regional levels. Copyright © 2016 John Wiley & Sons, Ltd.