Broken River Restoration Project: Phase 1 – Scoping Report

"December 2005".

Project Number: Broken River Restoration Project - M/BUS/69-2.

MDFRC item.

Phase 1 of Broken River Restoration Project (see Phase 1- Site selection report) and (Phase 1 – Scoping Report), plus Phase 2 of Broken River Rehabilitation Project (see Summary survey of fish and macroinvertebrates June 2006 to May 2007) and (final report 2008), plus fish monitoring phase (see Summary report 2009) and (Final report 2009).

Large woody debris is an important habitat feature in most lowland rivers in Australia. Its importance to the sustainability of riverine ecosystems is now recognised and the removal of wood from rivers is now listed as a threatening process under state legislation. In addition to the removal of woody debris, the clearing of native riparian zones has added to the degradation of riverine habitats. The clearing of native riparian zones has been listed as a threatening process to aquatic ecosystems under federal legislation. The objective of this report is to review the available literature on large woody debris and assess the options available for the re-snagging of the Broken River. "River improvement" works were carried out in the Broken River between 1960 and 1975 in an effort to decrease the frequency of flooding. Works undertaken included the construction of levees and removal of woody debris. In addition, much of the adjacent riparian zone has been extensively modified by clearing and grazing. It is now known that woody debris is one of the major drivers in lowland rivers. The addition of woody debris increases the complexity of the channel morphology by scouring and the creation of pools and slackwater habitats. These slackwater areas are important nursery areas for larval and juvenile native fish and support a high abundance and diversity of invertebrates. The woody debris itself also supports a diverse invertebrate community and is an important habitat for many larger native fish such as Murray cod. Indeed, the number of fish in a river can be related to the quality of habitat available and the habitat requirements of a particular species may change through the stages of its life history. Fish communities in streams with a poor diversity of habitat are usually dominated by fish species that are tolerant of a wide variety of habitats and these generalists are most often introduced species. It is now known that the impact of woody debris in rivers when flows are at bank full is minimal and does not increase the risk or impact of flooding on the adjacent floodplain and the presence of woody debris does much to stabilise banks and prevent erosion. In general, this is because most woody debris is relatively small compared to the river channel. To effectively rehabilitate a system such as the Broken River it will be necessary to have in place long-term strategies to facilitate restorations. Simply placing LWD back into the river is a short-term measure, which would need repeating in the future. Effective restoration will require the implementation of a program to restore the riparian vegetation, which in the future will naturally replenish LWD. As instream wood is largely derived from riparian vegetation, an intact riparian zone of native vegetation is very important. A healthy riparian zone should eventually reduce the need for artificial re-snagging of the river. The recommended density for wood reintroduced into a river is approximately 1m3 per 100 m2. The Broken River is a relatively linear system compared to rivers with high sinuosity where the natural fall and location of material is likely to be strongly associated with meanders. The Broken River also has a relatively low velocity, subsequently reintroduced wood may be aligned upstream, downstream or perpendicular to the bank and depending on the size of the wood, it is unlikely that there will be a need to physically anchor the wood to the bank. Wood should be added as aggregations as well as individual pieces creating a diverse and complex instream habitat. The length of reaches to be restored and the density at which wood is added will be dependent upon the number of trees available. As a minimum requirement, a reach should be at least 100m in length and be re-snagged at a minimum density of 1m3 per 100m2. In a reach of 100m long and 30m wide this would require 15 trees with the approximate dimensions of 10 meters high and 0.5m diameter. Once the wood is reintroduced, monitoring of fish and invertebrate communities, and changes to the channel morphology will be undertaken.