Abstract The amount of water stored and moving through the Amazon floodplain is not known, yet is necessary for understanding the role of the wetland in the regional carbon balance and the sediment and nutrient exchanged with the main river channel. Here, we separate the Amazon floodplain into six 330 km × 330 km areas and use gravimetric and imaging satellite methods (i.e., GRACE, SRTM, GPCP and JERS-1) to estimate the amounts of water filling and draining from the mainstem Amazon floodplain. We show that the amount of water stored on and subsequently drained from the mainstem Amazon floodplain each year represents about 5% of the total volume of water discharged from the Amazon River into the Atlantic Ocean. Despite a five-fold increase in mainstem discharge from upstream to downstream, the floodplain water volume exchanged with the river only doubles (based on all six 330 km reach lengths). Rates of exchange along the 330 km reach lengths vary from 5500 m 3/s during floodplain infilling to −7500 m 3/s during drainage. The contribution to the floodplain from local upland runoff represents less than 20% of the floodplain water volume for any given time.