Overbank floods in modified lowland rivers often inundate a mosaic of different land uses (e.g. forests, crops and pastures) on the floodplain. We used a glasshouse experiment to investigate dissolved organic carbon (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 Australia. Six replicate samples of six intact soil and groundcover treatments were collected during summer; three from a forest (bare soil, wallaby grass and leaf litter) and three from an adjacent paddock (bare soil, wheat and 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 during summer floods can be a major source of DOC and a major contributor to DO depletion.