In a prealpine river particulate organic matter (POM) and sediment community respiration within the uppermost 40 cm of the bed sediment were measured from April to July 1992. Tubes filled with sorted sediment (4-8 mm) were incubated for 6 weeks. Respiration rates were detected using a closed chamber system with internal water circulation. Total POM content decreased from 4.3 kg m -2 to 1.5 kg m -2 during a five week period of low flow and increased to 8.0 kg m -2 after several sediment-moving floods. These variations probably reflect the enhanced mobility of POM particles after such floods. Sediment community respiration showed a similar temporal pattern. It declined from 0.5 g to 0.3 g O 2 m -2 d -1 and reached 0.7 g O 2 m -2 d -1 at the end of the study.