The removal of Instream woody habitat (IWH) has been identified as a major contributing factor in the decline of many freshwater fish populations. To identify and prioritise areas for river restoration where IWH needs protection and augmentation, baseline information on the level of IWH in rivers and its condition is required.
Field assessments of natural IWH densities were undertaken in ‘pristine’ river reaches throughout Victoria. This information was used in a predictive modelling approach to determine natural IWH densities in rivers across Victoria. Current densities of IWH river reaches across Victoria were calculated using a combination of high resolution aerial photographs, field assessments and measures of riparian overhang. A simple comparison of the predicted natural IWH densities and the current IWH densities enabled researchers to assess the condition of IWH densities in streams throughout Victoria.
The predicted natural IWH densities in Victorian rivers were on average 0.03 m3m-2 and varied according to slope, stream width and climatic variables. Current densities of IWH in river reaches across Victoria are on average 0.01 m3m-2 which equates to an average reduction of ~41% below estimated natural IWH levels. Over 20,000 Victorian river reaches, equalling ~17,000 km, have severely or highly depleted IWH densities. IWH densities more than 80% below natural levels were estimated for 30% of river reaches.
Maps and IWH loads of this broad-scale assessment will help managers identify areas in most need of rehabilitation activities. Modelled IWH loads also will aid research into to IWH dynamics and its ecological role.