Desnagging to resnagging: new directions in river rehabilitation in southeastern Australia

Abstract

Extensive desnagging (removal of large woody debris and living riparian vegetation) and associated river improvement works were conducted in rivers of southeastern Australia (Victoria and New South Wales) between at least 1886 and 1995. Swamp drainage, large woody debris removal and vegetation clearing were strongly supported by legislation, government funding and institutional arrangements in both states. As a result, large amounts of large woody debris were removed from rivers, regenerating indigenous vegetation was cleared from within designed alignment widths and, ironically, huge numbers of exotic trees, especially willows, were planted. The environmental impacts of desnagging have only been documented on a few impacted rivers but have included increased flow velocity, spatially extensive bed degradation, massive channel enlargement and loss of fish habitat. Recognition of the need for more integrated land and water management, and new research on the hydraulic, geomorphic, biogeographic and ecological significance of large woody debris and the values of indigenous riparian vegetation during the 1980s led to a major shift in river rehabilitation. We have drawn on our own and other published research to further develop a set of guidelines for the incorporation of large woody debris into river rehabilitation plans. Our guidelines extend those recently prepared for southeastern Australia and address site selection, where to place timber, the amount to be introduced, how to distribute it, techniques of introduction and woody debris sources. However, in the long term, riparian vegetation rehabilitation within the potential recruitment zone is essential to supply large woody debris. Given that our results demonstrate that very large woody debris makes a significant contribution to the total loading, it will be a very long time (>100 years) before natural recruitment can be recreated.

Landscape genetics informs mesohabitat preference and conservation priorities for a surrogate indicator species in a highly fragmented river system

“…In addition, detailed spatial mapping of riparian vegetation, small in-stream structures and the presence of introduced species are not available across the entire MDB. These variables are likely to influence the dynamics and demography of a range of native freshwater species (Jackson and Williams, 1980;Growns et al, 1998;Erskine and Webb, 2003). These issues, in combination with the relatively low levels of variation found in our microsatellite data set and the small size of some population samples, could potentially affect our ability to detect landscape genetic patterns.…”

How natural capital delivers ecosystem services: A typology derived from a systematic review

“…Most of the links cited in the literature have a beneficial effect, but three studies find that species abundance has a negative impact on flood protection as a result of invasive species (mangrove, willow or tamarisk) reducing river channel capacity and trapping sediment (Lee and Shih, 2004;Erskine and Webb, 2003;Zavaleta, 2000).…”

Wood placement in river restoration: fact, fiction, and future direction

“…As a society, we have spent over 100 years removing wood from channels to improve navigation, water conveyance, and transportation (Collins et al 2002;Erskine and Webb 2003;Montgomery 1997). Natural resource managers, fisherman, and whitewater rafters have routinely removed wood from rivers that may pose a safety hazard, and often oppose wood placement because of the potential safety hazards.…”