Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia KATHRYN J. AMOS*, JAN ALEXANDER*, ANTHONY HORN  , GEOFF D. POCOCK à and CHRIS R. FIELDING§ 1 *School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK (E-mail: j.alexander@uea.ac.uk)  Department of Natural Resources and Mines, Indooroopilly, Queensland 4068, Australia àDepartment of Natural Resources and Mines, Ayr, Queensland 4807, Australia §Department of Earth Sciences, University of Queensland, Queensland 4072, Australia ABSTRACT Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high-flow events is not well documented. This paper presents an investigation into sediment transport processes in a short-duration, high-discharge event in the Burdekin River, a large sand- and gravel-bed river in the monsoon- and cyclone- influenced, semi-arid tropics of north Queensland. The Burdekin’s discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m 3 s )1 . Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley-Smith bedload and US P-61 suspended sediment samplers over 16 days of a 29-day discharge event in February and March 2000 (peak 11 155 m 3 s )1 ). About 3Æ7 · 10 6 tonnes of suspended sediment and 3 · 10 5 tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river. Keywords Bedload, discharge event, hydraulically limited, semi-arid, supply limited, suspended sediment. INTRODUCTION Sediment transport in many rivers is surprisingly poorly understood given its importance to reservoir siltation, sediment supply for beach replenishment and management of ecological conditions. Sand transport in rivers is often estimated by empirical equations (e.g. Ackers & White, 1973; Yang, 1973; Brownlie, 1981; Van Rijn, 1984a,b), which assume that the amount and grain size of sediment in transport depend on hydraulic parameters (e.g. bed shear stress, stream power or velocity) and local bed compo- sition. The total sediment in transport at a point, 1 Present address: Department of Geosciences, 214 Bessey Hall, University of Nebraska – Lincoln, NE 68588-0340, USA. Sedimentology (2004) 51, 145–162 doi: 10.1046/j.1365-3091.2003.00616.x Ó 2004 International Association of Sedimentologists 145