Overbank floods in modified lowland rivers often inundate a mosaic of different land uses (e.g. forests, crops and pastures) on the floodplain. One of the possible outcomes of extensive floods is that high concentrations of dissolved organic carbon (DOC) derived from the floodplain may enter the river, resulting in hypoxic blackwater events. We used a glasshouse experiment to investigate DOC and nutrient (TP, NH4, NOx) releases, chemical oxygen demand (COD) and dissolved oxygen (DO) depletion in water following inundation of soil and vegetation from a lowland river floodplain in southern Murray-Darling Basin, Australia. Six replicate samples of six intact soil and groundcover treatments were collected in the Edward-Wakool River system during summer; three from a forest (forest soil (bare soil), forest soil+wallaby grass and forest soil+leaf litter) and three from an adjacent paddock (paddock soil (bare soil), paddock soil+wheat and paddock soil+ryegrass). Samples were placed in pots, inundated with river water over 16 days, and their leachates were compared with a river water control. All vegetated groundcover treatments had significantly higher DOC and COD and significantly less DO at both Day 1 and Day 16 than did the soil only treatments or the control. Leachates from paddock treatments were less coloured than those from forest treatments, despite having similar concentrations of DOC. Our findings imply that the inundation of any vegetation (native vegetation, native pastureland, improved pasture or crops) during summer floods can be a major source of DOC and a major contributor to DO depletion that this challenges the current paradigm and community perception that flooding of native forests is responsible for most of the DOC input to rivers during flood.