Sediment Dynamics and the Hydromorphology of Fluvial Systems (Proceedings of a symposium held in Dundee, UK, July 2006). IAHS Publ. 306, 2006. 1 Flows that form: the hydromorphology of concave-bank bench formation in the Ovens River, Australia G. J. VIETZ, M. J. STEWARDSON & B. L. FINLAYSON eWater Cooperative Research Centre, School of Anthropology, Geography and Environmental Studies, The University of Melbourne, Australia g.vietz@pgrad.unimelb.edu.au Abstract Ecological processes associated with in-channel benches have become a key focus of environmental flow studies in Australia. In this paper we present an initial investigation into the relationship between the morphology of mature benches and the flow regime responsible for their maintenance. We define benches as depositional features resulting from the vertical accretion of suspended sediment within a river channel. A two- dimensional hydrodynamic model (River2D) was used to represent the hydrodynamic conditions over a concave-bank bench on the Ovens River, southeast Australia, and deposited material was analysed. For stages higher than the bench surface elevation a large low-velocity and generally reverse- flow eddy is evident over the bench with velocities less than 0.2 m s -1 allowing for the deposition of silt and fine sand. Our results indicate that deposition on the bench is greatest during large in-channel flows and a depositional environment is still present at near-bankfull flows. These findings identify the importance of in-channel high-flow events for the maintenance of natural channel morphology. Key words concave bench; deposition; hydrodynamic; reverse flow; vertical accretion INTRODUCTION In-channel benches are an important morphological and ecological component in the meandering rivers of southeast Australia, yet the presence of benches is reduced by flow regulation (Walker & Thoms, 1993). The key aim in this research is to identify the range of flows which maintain in-channel benches through deposition. We have defined a bench as “a bank-attached, planar and narrow, fine grained sediment deposit occurring at elevations between the river bed and the floodplain” (Vietz et al., 2004). This definition is an extension of that put forward by Erskine & Livingstone (1999) and by focusing on “fine grained” sediment <0.5 mm (Blott & Pye, 2001) and considering processes of formation we can distinguish benches from bars, terms that are commonly used interchangeably. We describe benches as being predominantly the result of deposition of fine grained suspended-load sediment by vertical accretion whereas bars are predominantly the result of deposition of coarser bed-load material by lateral accretion. This distinction was mooted by Woodyer et al. (1979) and the continuum of form elucidated, with bars often forming the nuclei for benches. It is the process of vertical accretion associated with lower velocities in flow expansion zones that we have observed to form benches and this is the focus of our research. Six distinct bench types have been identified in the Ovens River, southeast