Impacts of agricultural expansion on floodplain water and sediment budgets in the Mekong River

•Impacts of agricultural expansion on floodplain sediment budget is studied.•While water level decreased in river, the floodplain water budget increased.•Floodplain sediment input has not changed since 1980, while the budget has decreased.•These were enabled by the agricultural expansion that decreased terrain roughness.•Floodplain not anymore acts as sediment sink thus cannot reach equilibrium.