Winter flooding of bottomland hardwood (BLH) floodplains in the Mississippi Alluvial Valley (MAV) causes dynamic availability of resources to wintering mallards ( Anas platyrhynchos ). The effect of changing resource availability on mallard body condition and timing of life-cycle events are important considerations for waterfowl habitat conservation planning in the MAV. During a study of mallards wintering in the Mingo Basin of southeastern Missouri, USA, I collected data on population size, habitat use, behavior, food habits, body composition, and chronology of the prebasic molt during 2 major flood events in 1982. I also analyzed winter (Dec–Feb) hydrological data for 14 rivers in the MAV from 1939–1940 to 1998–1999 to provide a perspective on variation of winter flooding in this ecosystem. Winter floods in the Mingo Basin precipitated ecological events that benefited mallards. During floods, mallards redistributed to shallowly flooded (<50cm) live forest dominated by red oaks ( Quercus spp.), increased daily foraging time by up to 8×, consumed 170–222 g dry weight of food/day, increased consumption of animal matter by up to 14×, gained fresh body and lipid mass, and initiated the prebasic molt. Winter flooding of major rivers in the MAV during 1939–1940 to 1998–1999 was highly variable among locations and years. An average of 6.7 ± 2.1 (±SE) and 5.1 ± 1.9 of 17 river gage stations were flooded >5 and >10 days/winter, respectively. Mallards increased daily food consumption by 33–39% over daily existence energy (DEE) levels during floods. These data suggest that previous estimates of foraging carrying capacity in MAV habitats (and other wintering and migration areas where significant fat deposition occurs) using only DEE-based daily food consumption estimates may be overestimated. Consequently, habitat and acre goals set by North American Waterfowl Management Plan Joint Ventures for these areas may be greatly underestimated. The evolutionary adaptations of mallards seem influenced by timing, duration, and extent of winter flooding in the MAV. Efforts to protect the integrity of MAV rivers, associated floodplain habitats, and their winter flow and flooding regimes are critical for sustaining local, regional, and continental mallard populations.