RPL WRRDC’S RIPARIAN LANDSRP MANAGEMENT NEWSLETTER A COMPONENTia OF THE RIVER RESTORATION AND MANAGEMENT PROGRAM Managing snags and large woody debris MANAGING Snags and large woody debris (LWD) are the sticks, branches, trunks and whole trees that fall into rivers snags and and streams. LWD is important in streams and rivers from both an ecological and a geomorphic/hydraulic viewpoint. LWD provides important in-stream habitat arge oody for aquatic animals, as well as stable sites for the processing of carbon and nutrients.Through its impact LW on channel structure and flow, LWD also assists in the ebris formation of habitat (such as scour pools). This latter D process has led to the misguided belief that LWD also causes significant channel erosion. Another false belief is that snags significantly reduce channel capacity, leading to overflowing of banks during flood events. These misunderstandings about the effects of LWD on erosion and flooding, has meant that snag removal programs have continued throughout Australia, even after the initial rationale for snag removal (safer river transport) had ceased to be relevant.The problems that exist in managing large woody debris are, therefore, not so much its negative impact, but the long and widely- held perceptions of its impact. continued page 3 EDITION 16, 2000 CONtents Theme: Managing snags and Large Woody Debris 1 and 3 Research notes 1: The importance of large woody debris surfaces for algal growth in lowland rivers 6 Getting a Grip: Resnagging the River Murray 9 This publication is managed by Research notes 2: Some observations on the amount and the Land and Water Resources distribution of large woody debris in Australian streams 12 Research and Development The riparian adventures of Eco-man and Dr Earth 18 Corporation (LWRRDC), GPO Box 2182, Canberra It’s a Wrap: News from around Australia 21 ACT 2601 LWRRDC’s mission is to provide national leadership in utilising R&D to improve the long-term productive capacity, sustainable use, management and conservation of Australia’s land, water and vegetation resources. The Corporation will establish directed, integrated and focused programs where there is clear RIP rian lands: justification for additional public WHERE LAND AND WATER MEET funding to expand or enhance a the contribution of R&D to From the Editor sustainable management April 2000 already (!) and the Riparian Lands Program Phase One is of natural resources. wrapping up in time for Phase Two to begin in June 2000. I am pleased to LWRRDC’s Home Page is: announce that I have been appointed as the Program Coordinator for www.lwrrdc.gov.au Phase Two, with Phil Price also working with me in this role over the next five years.We are thrilled to be able to continue working on a program that Edition 16, April 2000 is committed to integrating science and practical experience in ways that RipRap is published four times a promote and assist the improved management of rivers and riparian lands year. Contributions and comments across Australia. Negotiations are now underway with many different are welcomed and should be groups about the priorities for investment, and opportunities for partner- addressed to the Editor. ship in Phase Two of the Program. Before we can move into Phase Two, however, there remains a great Editor: Dr Siwan Lovett deal of work to be done in Phase One. The next few months will be spent ensuring that the results of the research undertaken in Phase One are Feedback and comments to: converted into useful and relevant products for a range of different end- Dr Siwan Lovett LWRRDC Program Coordinator users. One of these products is RipRap,with this edition focusing on the River Restoration issue of snag and large woody debris management. This topic was and Riparian Lands requested by quite a few readers, and we have tracked down the most LWRRDC, GPO Box 2182 recent research being undertaken across the country and combined it with Canberra ACT 2601 information from the Riparian Land Tel: 02 6257 3379 Management Technical Guidelines Fax: 02 6257 3420 to provide a good overview of the Email: [email protected] management and R&D issues. I Website: www.rivers.gov.au hope you enjoy this edition and, in particular, the cartoon that is Designed by: Angel Ink thinking laterally about ways to Printed by: Goanna Print better manage our riparian lands!!! ISSN 1324-6941 2 THEME CASE STUDY GETTING A GRIP IT’S A WRAP INFORMATION MANAGING snags and arge oody ebris continued from page 1 L WD Extract from Simon Treadwell (ed.), 1999, ‘Managing snags and large woody debris’ in WHY IS LWD IMPORTANT? S. Lovett and P. Price (eds), Riparian Land Management Technical Guidelines, Volume two: LWD is very significant in the ecology of On-ground Management Tools and Techniques, Land and Water Resources Research and streams and, by reason of the linkages between Development Corporation, Canberra, pp. 15–22. water and land, in other ecosystems. It is now apparent that de-snagging has had a Large woody debris as habitat for fish significant negative environmental impact on Woody debris provides important habitat for stream ecosystems. Major effects include the direct use by a number of aquatic and terrestrial loss of habitat for fish and other aquatic and organisms. Such uses include shelter from high terrestrial organisms, to the point where some current velocities, shade, feeding sites, spawning native species are threatened or locally extinct. sites, nursery areas for larvae and juvenile fish, The removal of snags has also had a significant territory markers and refuge from predation. impact on channel morphology. De-snagged Snags are most effective as habitat if they have a rivers typically become uniform drainage complex structure providing a number of channels, with fewer channel features such as different-sized spaces, including hollows within scour holes and bars that retain, or act as the debris piece and spaces between branches. substrates for the processing of carbon and Branches extending into the water column and nutrients by instream organisms. Furthermore, above the water surface provide habitat at the extensive research on the hydraulic effects of different water levels required by different fish snags has indicated that snags, especially in species. Single large trees that fall into a river can large rivers, have little adverse impact on often provide the full range of complex spaces channel capacity and snag removal does little required. to reduce the height of major floods. The challenge in achieving ‘best practice’ LWD management lies in maximising the Snags as habitat for other organisms positive contribution of LWD in both of its In general, the types of snags that provide major roles; including, where appropriate, the habitat for fish also provide habitat for other restoration of snags in de-snagged rivers. aquatic and terrestrial organisms. Submerged Fortunately, this challenge has been made wood, with a complex surface structure of easier by recent research that confirms the real grooves, splits and hollows, provides space for (as opposed to the perceived) impact of LWD colonisation by a range of invertebrates, on streams, rivers and riparian land (see later microbes and algae. Some invertebrates feed articles in RipRap for details). directly on the wood while others graze the biofilm (that is, the combined microbe and algal community). The species composition within the biofilm community depends on the position of the wood substrate within the water column. The shallower the water in which the substrate occurs, the higher the density of algal species compared with substrate located deeper in the water column where light does not reach. Species composition of both biofilm and inver- tebrates also depends on the substrate type. Willows and other introduced tree species appear to have a less diverse invertebrate community compared with native/indigenous tree species. Similarly, community composition Natural snags provide a range of habitat types suitable for fish and other aquatic and terrestrial organisms, Ovens River, varies according to the type of substrate (for Victoria. Photo by Simon Treadwell. example, wood compared with concrete pipes). THEME CASE STUDY GETTING A GRIP IT’S A WRAP INFORMATION 3 MANAGING snags andLW arge oody D ebris Birds, reptiles and mammals also use woody of food for invertebrates and fish. The algal debris for resting, foraging and lookout sites. component of biofilm may also produce a signif- Birds commonly use the exposed branches of icant amount of carbon and, hence, food through snags as perch sites, while turtles often climb out photosynthesis. Many invertebrate species and of the water using snag surfaces. Snags spanning some fish eat the algae that are growing on wood the channel may also be used by mammals and surfaces. In sandy, turbid rivers where woody reptiles as stream crossing points. Many aquatic substrate may be the only hard substrate avail- invertebrates have a terrestrial adult stage and able for colonisation, or in rivers that have been require snags extending above the water surface isolated from floodplain carbon inputs by river to provide sites for emergence from their larval regulation and clearing, most of the food for to adult stages. aquatic animals is found on snags. In upland streams, debris dams (large accumulations of woody debris that often span Snags as sites for carbon the entire channel) retain large amounts of and nutrient processing particulate organic material. This material Another important, but often overlooked decomposes into smaller pieces and is then function of snags is their role in carbon and transported downstream. (As stream size Above: Channel habitat formed by nutrient processing. Snags provide important increases, large debris dams become less snags parallel to flow, Warren River, substrate for the development of biofilms. The common and the ability of woody debris to retain Western Australia.