Ecohydrology of Surface and Groundwater Dependent Systems: Concepts, Methods and Recent Developments (Proc. of JS.1 at the Joint IAHS & IAH Convention, Hyderabad, India, September 2009). IAHS Publ. 328, 2009. 103 The potential effect of re-snagging on hydraulic habitat MARK SOUTHWELL 1 , MARTIN THOMS 1 & CRAIG BOYS 2 1 Riverine Landscapes Research Laboratory, Resource and Environmental Sciences, University of Canberra, Australian Capital Territory 2601, Australia mark.southwell@canberra.edu.au 2 New South Wales Department of Primary Industries, Port Stephens Fisheries Centre, Private Bag 1, Nelson Bay, New South Wales 2315, Australia Abstract Within riverine ecosystems, physical diversity facilitates biodiversity. In particular, large wood within river channels influences the distribution of hydraulic patches and their character, within the river landscape. We surveyed 30 reaches along the Barwon–Darling River in southeastern Australia to describe their hydraulic character, before and after the reintroduction of large wood (re-snagging). We found considerable hydraulic diversity within the reaches, but there was little difference between reach types (control, reference or re-snagged) and survey times. No significant differences were observed between the re-snagged reaches at the two survey times when all reaches were considered together, although differences were identified in some individual hydraulic patch variables. These results may be due to the low-flow conditions during and between survey runs. Successive surveys at a range of flow levels, both before and after the occurrence of larger flow events (which may encourage scour around re-introduced large wood), will be required to clarify this. Key words large wood; acoustic doppler profiler; Barwon-Darling River; hydraulic diversity INTRODUCTION Physical diversity is a key component of the biodiversity of riverine landscapes (Poff et al., 1997; Thorp et al., 2008). At the reach scale, physical diversity can be described as variations in hydraulic characteristics, such as water depth, flow velocity and turbulence, which create distinct hydraulic patches or areas of similar hydraulic character (Thoms et al., 2006). The pattern or configuration of hydraulic patches at the reach scale is influenced by the morphology of the river bed and banks, and obstacles that may be present within the river channel (Dyer & Thoms, 2006). In many inland rivers, large pieces of wood form major in-channel obstacles to flow. The influence of large wood on hydraulic character depends on placement within the channel, and the size and shape of each piece (Gippel et al., 1996). Large wood has been shown to increase hydraulic diversity within river channels (Hughes et al., 2007), and promote bed scour through deepening and pool formation on the downstream sides (Crook & Robertson, 1999). Large wood is an important type of physical habitat for riverine fishes in many large lowland river systems (Crook & Robertson 1999; Gehrke & Harris 2004; Boys, 2007). For example, the presence and abundance of Murray cod (Maccullochella peelii peelii) and golden perch (Macquaria ambigua), two iconic native Australian fish, have been found to be strongly associated with the diversity of large wood in the mid reaches of the Barwon–Darling River, New South Wales (Boys, 2007). However, large wood has been removed from many large Australian lowland rivers in the Murray–Darling Basin to improve navigation (Gippel et al., 1996; Crook & Robertson, 1999). This has had a detrimental effect on native fish communities within the basin (Gehrke & Harris, 2004). Currently, native fish numbers are estimated to be 10% of pre-European settlement levels (MDBC, 2003). Iconic species, such as freshwater catfish (Tandanus tandanus) and silver perch (Bidyanus bidyanus), are also showing evidence of declining abundance and distribution (Schiller et al., 1997; Gehrke & Harris 2004). The re-introduction of large wood to rivers (re-snagging) is an important restoration strategy in many countries (Crook & Robertson 1999; Piegay et al., 2005; Lester & Wright 2008). In the Murray–Darling Basin, several demonstration reaches have been established to test the effectiveness of the re-introduction of large wood to rivers at the reach scale (Barrett, 2004). One demonstration reach is located between the townships of Bourke and Brewarrina on the Barwon–Darling River, within the upper section of the Murray–Darling Basin in New South Wales. Large wood was re- introduced at selected trial reaches within the larger demonstration reach. The biological response of Copyright © 2009 IAHS Press