1. Forest responses to changes in drought frequency is a critical unknown for the future of Amazon forests under climate change, but equally important is the much less studied response to large floods, which may also increase tree mortality and change forest functionality. Further, forest vulnerability to flood is being exacerbated by large hydroelectric dams on Amazon rivers that put upland environments not adapted to flood at unique risk. 2. To address this critical knowledge gap, we evaluated the synergistic effects of the extreme 2014 rainfall coupled with the newly constructed Jirau hydroelectric dam on tree survival and forest functionality, in the upper Madeira River basin. We used surveys of campinarana white-sand forests (stems >1 cm in seven 1 ha plots) conducted before and after the extreme flood to test trait-based ecological theory predictions of the impact of drought on overall community function. 3. We found that flooding increased mortality by nearly five-fold (from 3.2% to 15.1%), mostly in smaller trees. This large mortality induced significant and consistent shifts in community function, towards species with conservative life strategies: direct comparison of trait differences between surviving and dying trees showed that survivors had smaller, high density stomata, and higher leaf dry matter content, wood density and root tissue density. Size and density of stomata and root tissue density were the most important predictors of species mortality rates. 4. Synthesis and applications. Although focused on a single event in one type of forest, this work highlights the general importance, and need for further study, of synergism between climate change and development in Amazon forests. In particular, we expect that continued expansion of hydroelectric dams in Amazonia will likely intensify the impact of large floods on forests made newly vulnerable by these dams, with substantial effect on future forest functionality in expanded floodplain areas across the Basin.16-Aug-2019