RIVER RESEARCH AND APPLICATIONS River Res. Applic. 21: 567–578 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rra.828 THE EFFECT OF ALTERED FLOW REGIME ON THE FREQUENCY AND DURATION OF BANKFULL DISCHARGE: MURRUMBIDGEE RIVER, AUSTRALIA KEN PAGE, a * ARTHUR READ, a PAUL FRAZIER b and NICK MOUNT c a School of Science and Technology, Charles Sturt University, Wagga Wagga, NSW 2678, Australia b Department of Ecosystem Management, School of Environmental Sciences and Natural Resources Management, University of New England, Armidale, NSW 2351, Australia c School of Geography, Birkbeck College, University of London, London WC1E 7HX, UK ABSTRACT On meandering rivers with well-developed floodplains, bankfull stage has geomorphological and ecological significance because it approximates the level of connection between the channel and the floodplain. As a river rises to bankfull stage, sedi- ment begins to be deposited on the floodplain, wetlands are progressively inundated and organisms migrate between the channel and floodplain habitats. On many rivers large headwater dams have reduced the frequency and duration of floodplain inundation downstream. However, the lack of reliable pre-regulation flow data has made it difficult to quantify the effects of river regula- tion. This study used historical regulated and modelled natural flow data to determine the effects of regulation on the frequency and duration of bankfull flows on the Murrumbidgee River, one of Australia’s largest and most heavily regulated rivers. In combination with floodplain surveys the flow data show that regulation has halved the frequency and duration of bankfull flows. This reduction in channel–floodplain connection has implications for the ecological health of the Murrumbidgee River. Copyright # 2005 John Wiley & Sons, Ltd. key words: Australia; bankfull discharge; river regulation; channel–floodplain connectivity INTRODUCTION Bankfull stage is reached when water fills a river channel and begins to spill out onto the adjacent floodplain sur- face (Wolman and Leopold, 1957). For rivers with well-defined and horizontally extensive floodplains, bankfull stage can be defined as mean floodplain elevation and typically varies little at a reach scale (Page, 1988). Because bankfull approximates the stage at which the channel and floodplain become connected it is of interest to both geomorphologists and ecologists. River geomorphologists are particularly interested in bankfull discharge because it is thought to control gross channel geometry and to be responsible for the bulk of a river’s sediment transport (Wolman and Leopold, 1957; Wolman and Miller, 1960; Knighton, 1998). Moreover, the apparently consistent frequency (return period) of bankfull discharge on floodplain rivers in diverse geographic areas (Wolman and Leopold, 1957; Leopold et al., 1964) suggests that it is of particular significance as an index of a river’s flow regime. For river ecologists, bankfull stage approximates the level of connection between the channel and adjacent floodplain at a river reach scale. Channel to wetland connection often commences at lower stages but progressive inundation results in the majority of wetlands being connected to the channel when it reaches bankfull (Frazier et al., 2003). Flood peaks (pulses) that exceed bankfull stage are now seen to be the major force controlling biota in river–floodplain systems (Bayley, 1991). Junk et al. (1989) argue that in large river systems with floodplains, the overwhelming bulk of riverine animal biomass is derived directly or indirectly from production within the flood- plains and not from the downstream transport of organic matter produced elsewhere in the basin. Received 14 December 2003 Revised 21 April 2004 Copyright # 2005 John Wiley & Sons, Ltd. Accepted 11 June 2004 *Correspondence to: Ken Page,School of Science and Technology, Charles Sturt University, Wagga Wagga, NSW 2678, Australia. E-mail: kpage@csu.edu.au