A major fish kill occurred in the Richmond River estuary in January 2008 due to oxygen depletion following extensive overbank flooding. This paper examines spatial and temporal changes in the chemistry of main channel waters, thereby identifying the primary sources of deoxygenating water. Over 40 km of the mid- to lower estuary main channel was deoxygenated within seven days of the flood peak. Hypoxia was confined to downstream of the confluences with mid-estuary backswamp basins and occurred during the later phase of the flood recession. Water chemistry at key locations in the estuary indicated elevated concentrations of redox sensitive species associated with acid sulfate soils (ASS) during the hypoxic period. Peak concentrations of Fe
Iron-sulfide minerals in benthic sediments may sequester potentially toxic trace elements that are introduced to estuaries from natural and anthropogenic sources (Chapman et al., 1998, Morse and Luther, 1999, Simpson et al., 2002 and Teasdale et al., 2003). Understanding iron-sulfide formation in benthic sediments is therefore central to assessing the risk posed by sedimentary trace elements (Machado et al., 2004, Burton et al., 2005a and Burton et al., 2006a). This report provides a baseline description of sedimentary iron-sulfide and trace element behaviour in Coombabah Lake – a sub-tropical estuarine lake in southern Moreton Bay, Australia (Fig. 1).