Instream woody habitat (IWH), commonly referred to as 'snags', consists of trees, branches and logs that fall or are washed into rivers and streams. IWH has a range of functions essential for maintaining the health of a waterway, and in turn supports recreational fisheries and other social and cultural values. In the past, IWH has been removed from many Victorian rivers for boating purposes, property protection and to reduce flooding. Research has since shown that the removal of IWH has minimal impact on flood mitigation, and that such works impair river stability and degrade river health. The removal of IWH has also been identified as a major factor in the decline of many freshwater fish populations. River restoration programs often involve the re-introduction of IWH, however more information on current levels of IWH are needed to help focus efforts.

ARI has been involved in a collaborative project to investigate past and current IWH densities in Victorian rivers. Field assessments of IWH densities were undertaken in 'pristine' rivers using GPS and underwater sonar technology. Predictive modelling of this data was then used to estimate natural IWH densities in rivers across Victoria. Current densities of IWH were calculated using field assessments and high resolution aerial photographs. A comparison of the predicted natural and current IWH densities enabled researchers to assess the overall condition of IWH densities throughout Victorian rivers.Close to 27,700 km of rivers were mapped for IWH densities. On average, current IWH densities were 41% below estimates of natural IWH levels. Nearly 17,000 km (62%) of rivers were classified as having IWH densities severely (80% below natural levels) or highly depleted (60% below natural levels).

Catchment managers can use this broad-scale assessment of IWH to identify areas in most need of rehabilitation activities. ARI is continuing to investigate what IWH levels are needed to maximise benefits for particular fish. This project was a collaboration with other DELWP divisions, Fisheries Victoria, Catchment Management Authorities, the University of Melbourne and Melbourne Water, with funding from the Victorian Investment Framework. Establishing the Fish Habitat Network in Victoria and encouraging communities to participate in river restoration programs have also been key parts of this project.

The branches and logs in this stream will help create a healthy waterway

Australian Trout Foundation members plant trees on a stream bank to enhance Barred Galaxias habitat (image: Angus Bell)

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The following fact sheets are available:

The following journal articles are available: