THE IMPACT OF SMALL AND LARGE IMPOUNDMENTS ON FRESHWATER MUSSEL DISTRIBUTION IN THE HAWKESBURY-NEPEAN RIVER, SOUTHEASTERN AUSTRALIA MEREDITH BRAINWOOD, a SHELLEY BURGIN a * and MARIA BYRNE b a College of Health and Science, University of Western Sydney, Richmond Campus, Locked Bag 1797, South Penrith DC, NSW 1797, Australia b Department of Anatomy and Histology, F-13, University of Sydney, Camperdown, NSW 2006, Australia ABSTRACT The influence of weirs on the distribution of freshwater mussels was investigated in the Hawkesbury-Nepean River, Australia. Distribution of species and densities of size classes were strongly correlated with catchment level factors (e.g. location around a major impoundment, stream order). At catchment scale, weir height, presence of a fish barrier, fish ladder type and position above or below small weirs did not influence the presence/absence of mussel populations. Lower mussel densities in the upper catchment may therefore reflect inhibition of host fish migration. Where present, weir height and geomorphic reach type were linked to differences in densities among species. Geomorphic reach-based differences were reflected by the Hyridella species, but not Velesunio ambiguus. When population structure was described by size class distribution, there were significant differences between densities of small and medium mussels from weirs above, compared to weirs below, a major impoundment, but not for large mussels. Upstream populations may therefore be functionally extinct. Distribution of mussel size classes differed among geomorphic reach types with highest densities for each class found in the least human-impacted reaches. Small mussels were almost invariably found below the major impoundment, most frequently below weirs. Distribution patterns were inconsistent across species, suggesting habitat preference. V. ambiguus and Hyridella australis were most abundant in shale reaches, where assemblages were influenced by fish ladder type. Hyridella depressa and H. australis dominated in sandstone gorges and straights with assemblage density related to weir height. In upper catchment sandstone reaches, mussel assemblages comprising predominantly V. ambiguus were influenced by fish ladder type and weir height. While multiple factors defined localized distribution, large impoundments were linked with reduced population densities. The probable mechanism is the restriction of host fish movement and resulting lack of recruitment. In the Hawkesbury-Nepean River, smaller weirs also seriously impacted recruitment. Copyright # 2008 John Wiley & Sons, Ltd. key words: Hyridella depressa; Hyridella australis; Velesunio ambiguus; impoundment; recruitment; population structure Received 23 August 2007; Accepted 18 October 2007 INTRODUCTION There is little doubt that dams disrupt longitudinal linkages within streams. As well, they impede the lateral movement of fauna and decrease the availability and diversity of habitats (Grubbs and Taylor, 2004). The local ecology of flora and fauna, such as freshwater mussels, both within and below impoundments, is affected mainly by altering flow regimes and reducing connectivity among species (Walker et al., 1995). Focusing on smaller dams without fish passages, Watters (1996) found distinct differences in species distribution and density among mussel assemblages above and below impoundments, and attributed these to restriction of movement by fish hosts. The impact of small dams on the density and species richness is twofold for freshwater mussels (Watters, 1996). The most obvious is the alteration of habitat in a manner that favours some species over others, resulting in a decline in species richness (Blalock and Sickel, 1996). Mussel species that have a low tolerance for sediment deposition are particularly affected by the bank and channel modification that accompany construction of weirs and dams (Box RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 24: 1325–1342 (2008) Published online 9 May 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1087 *Correspondence to: Associate Professor Shelley Burgin, Dean’s Office of Research, University of Western Sydney, Richmond Campus, Locked Bag 1797, South Penrith DC, NSW 1797, Australia. Copyright # 2008 John Wiley & Sons, Ltd.